Abstract
The article presents a conceptual approach for the spatiotemporal distribution pattern of principal lake types in the context of the glaciation history in the Cordillera Blanca. The tropical mountain range hosts one of the main concentrations of proglacial lakes in high-mountain settings worldwide, which have formed as a result of the dominant trend of modern glacier retreat. In the 20th century, glacial lake outbursts have severely affected large settlement areas in the Rio Santa Basin. Additionally to the striking newly emerged lakes, geomorphological evidence of paleolakes is found throughout the middle and lower valley sections. Based on empirical data from field research in over 20 valleys and the analysis of air and satellite images, the study provides a genetic classification of major lake types and a generalized model for the distribution of the present lakes and paleolakes. The origin of the lakes and their recurrent distribution pattern are associated with the individual stages of the Pleistocene to modern glaciation and their corresponding geomorphological landforms. Apart from the individual lake, the focus is put on the spatial arrangement of the lakes to each other based on a holistic landscape assessment. Implications are drawn for the hazard potential, in particular in terms of outburst cascades involving two or more lakes. On a supraregional scale, a clustering of certain lake types occurs in different mountain ranges of the Andes according to their specific topographical and glaciological settings. Even though the glaciated areas have all been subject to major ice losses, only some mountain regions are prone to form moraine-dammed lakes such as in the Cordillera Blanca. The key controlling factors for their formation are highlighted from a glacial-geomorphological point of view. The distribution of principal types of glacial lakes is outlined in a N–S profile along the Andes.
Keywords
I Introduction
In the Cordillera Blanca (8°30’–10°10’S/77–78°W; Figure 1), a remarkable transition of the glaciated mountain terrain has occurred in the course of the general trend of modern glacier retreat in the 20th century (Rabatel et al., 2013). Proglacial lakes up to 1–2 km in length have developed in the majority of the glaciated high-mountain valleys and become a conspicuous landscape element in this tropical mountain range (Kaser and Osmaston, 2002; Lliboutry et al., 1977a;Vilímek et al., 2005). At a global scale, since the 1950s the number of proglacial lakes has increased significantly, most noticeably in the Himalayas (ICIMOD, 2011; Richardson and Reynolds, 2000) and the Andes (Dussaillant et al., 2010), but also in the European Alps (Frey et al., 2010), the New Zealand Alps (Kirkbride and Warren, 1999) and the Rocky Mountains (Clague and Evans, 2000). Among them, the Cordillera Blanca represents one of the key locations of newly emerging lakes. Settlements in the Rio Santa Basin, and to a lesser extent in the Rio Marañon catchment, have been severely affected by failures of glacial lakes (Figure 1). These devastating events in recent decades have caused the lakes to be the focus of intensive natural hazard studies (Ames and Francou, 1995; Carey et al., 2012; Emmer and Vilímek, 2013; Hegglin and Huggel, 2008; Kaser and Osmaston, 2002; Lliboutry, 1977; Lliboutry et al., 1977b; Reynolds, 1992, 2003; Vilímek et al., 2005; Zapata, 2002).
Overview of the recent lakes and major glacier lake outbursts in the Cordillera Blanca (for references regarding outburst events, see Carey, 2010; Carey et al., 2012; Zapata, 2002).
In addition to the proglacial lakes, geomorphological evidence of paleolakes can be found throughout the middle and lower valley sections of the Cordillera Blanca. Some of the paleolakes from the southern part of the mountain range have been used as archives for reconstructing the glaciation stages of the Last Glacial Maximum (LGM) and the Holocene, and to identify the changing climatic drivers for the glaciations (Rodbell, 1993; Seltzer and Rodbell, 2005; Stansell et al., 2013). Geomorphological aspects of the lake settings have been previously discussed in the context of Quaternary glacial extensions and lake outburst hazards by Clapperton (1972, 1983, 1993), Kinzl (1935, 1942, 1949, 1950), Lliboutry (1977), and Sievers (1914). Since these pioneering works, little attention has been paid to the glacial geomorphological features and the altitudinal distribution of modern, Holocene and Pleistocene lakes as a whole. Moreover, the literature on hazard studies focuses mainly on the hazardous glacial lakes, which are located upstream of the urban centers of the Cajón de Huaylas, namely Huaraz (3090 m), Carhuaz (2650 m), and Caraz (2250 m), and which may pose a significant threat to the hydropower station at Huallanca (1820 m asl). Thus, the overarching motivation for this paper is to provide an overview of the lake types formed since the Pleistocene glaciation from a topographical-geomorphological perspective, in order to contribute to a better understanding of the lake formation in a broader spatial and chronological context, which might be useful for future hazard planning strategies.
II Rationale and methodical approach
The aims of the study are: (1) to explain the formation of the lakes, considering their topographical and geomorphological setting in the Cordillera Blanca, and to set up a genetic regional-specific lake classification; (2) to analyze the spatial dispersal pattern of modern and paleolakes in the altitudinal and central-peripheral arrangement; special emphasis lies on the interpretation of their occurrence in the context of the distinct Holocene and Pleistocene glaciation stages; (3) to demonstrate from these findings implications for the hazard potential of the lakes; and (4) to assess the identified lake types on a wider geographical scale looking at lakes in other high-mountain areas of the Peruvian Andes, the Andes Mountain Range as a whole, and the Himalaya-Karakoram Range.
The paper is methodically based on the approach of the glaciated valley land systems (Evans, 2003; Eyles, 1983) in high-relief mountain environments, applied here for tropical mountain regions. It analyzes and classifies recurrent, genetically linked land units associated with lake formations in terms of their origin and relative age in the framework of the glaciation history (cf. Iturrizaga, 2008). The underlying idea of the research approach is to consider the lakes more in their spatial arrangement, instead of focusing on the individual lake.
The regional focus of the study lies at the western part of the mountain ranges including primarily glaciers draining to the Rio Santa basin. Empirical data was collected on fieldwork during the years 2008–2013 in austral summer (October–January) and winter seasons (June–September). The investigations were carried out at the following locations and catchment areas: Pastoruri Glacier, Langanuco Valley, Rajucolta, Quilcayhuanca, Cojup, Llaca and Parón Valleys, Ulta, Llanganuco and Cullicocha Valleys, Santa Cruz, Honcopampa, Upper Quitarasca and Cedros Valleys, Ishinca Valley, Laguna 513, Champará Mountains, Honda and Alhuina Valleys, and Akilpo and Rio Negro Valleys. The investigations presented in this paper are partial results of a larger project dealing with the glacial geomorphology of the Cordillera Blanca.
Multi-temporal comparisons of historical and modern photographs from reports and publications as well as aerial images and satellite images (Servicio Aerofotografico Nacional Perú, 1948/1950, Google Earth) were analyzed in regard to the distinct stages of lake development. The chronological classification of the lakes is based on existing studies on the extent of Quaternary glaciation in the Cordillera Blanca provided, among others, by: Clapperton (1972, 1983), Farber et al. (2005), Kinzl (1935), Rodbell (1993), and Smith et al. (2005a) for the LGM; Rodbell et al. (2009) for the Lateglacial and Holocene; Glasser et al. (2009), Jomelli et al. (2009), and Seltzer and Rodbell (2005) for the Younger Dryas/Holocene; Röthlisberger (1986) and Solomina et al. (2007) for the Little Ice Age (LIA); and Rabatel et al. (2013) for the historical/modern glacier fluctuations.
III Environmental setting of the lakes
1 Topographical setting
The Cordillera Blanca shows the highest concentrations of peaks higher than 6000 m asl in Peru. The 180 km long mountain range is divided into 12 glaciated mountain groups (Champará, Pilanco, Santa Cruz-Pucahirca, Huandoy, Huascarán, Contrahierbas, Hualcán-Copa, Chinchey-Perilla, Huantsán, Yanamarey, Jatunllacsa, Caullaraju) and incised by up to 25 km long, NE–SW trending transversal valleys aligned almost parallel to each other (Figure 1). The central part of the Cordillera Blanca shows the highest relief amplitude culminating in the Huascarán Group (6768 m), with a relief amplitude of more than 4000 m asl towards the Cajón de Huaylas (2600 m). The relief from the highest peaks to the transversal valleys is generally only about 2500 m. Southwards towards the upper catchment area of the Rio Santa Valley, the topographical situation converts into more moderate relief conditions with elevational amplitudes of less than 1500 m.
The majority of the valleys at the western side of the Cordillera Blanca show a similar geomorphogenetic evolution with comparable topographical and morphological valley settings, especially in regard to similar longitudinal and cross-valley profiles. This homogeneity of the valley features provides a basic precondition to set up a systematic concept for the lake distribution, as a remarkably uniform recurrent pattern of the lake types occurs in different valleys. It is argued here that the topographical and climatic settings exercise a key control for their spatial arrangement.
The continental water divide between the Pacific and the Atlantic runs through the Cordillera Blanca. The inventory of glacial lakes of the ‘Unidad de Glaciología y Recursos Hidricos’ of the Autoridad Nacional del Agua in Huaraz (ANA, 2011) indicates 834 lakes with a total size of 58 km2, considering lakes with a surface area of more than 5000 m2. In contrast, only 223 lakes were registered in 1953 (Carey, 2005). Two thirds of the lakes are draining westward into the Rio Santa catchment area. The remaining lakes are associated with the Rio Marañon to the east and to a minor part with the Patavilca drainage basin to the south of the Cordillera Blanca (ANA, 2011).
2 Climatic setting
The Cordillera Blanca is located in the Outer Tropics and characterized by a pronounced seasonality (Kaser and Osmaston, 2002), which might considerably control the water level of the glacial lakes. During the austral summer months between October and May, when the major concentration of rainfall occurs (Kaser, 2001), the lakes reach their maximum water level. Thereby, liquid precipitation contributes to increased ablation on glaciers (Kaser and Osmaston, 2002). During the wet season, lakes are most prone to potential outburst events. In contrast, during the austral winter months from June to September subtropical dry conditions prevail, leading to the dominance of sublimation processes in the glacial mass balance (Vuille et al., 2008), and comparatively lower amounts of melt water discharge into the proglacial lakes. Snow does not play a vital role in providing meltwater as in other mountain areas. Temperatures stay rather constant throughout the year with deviations in the range of 1–2°C (Rabatel et al., 2013).
3 Glacial setting
The Cordillera Blanca shows the largest glacier cover (527 km2) among the mountain ranges in Peru. Substantial glacier shrinkage has started after the Little Ice Age Maximum (Racoviteanu et al., 2008). In the 20th century, the glacier surface area was reduced by a third (Georges, 2004; Rabatel et al., 2013). The reduction in glacier area is strikingly visible in the bare, greyish, and freshly polished exposed bedrock beneath thin slope glaciers, but also in the compelling formation of lakes.
The most common glacier types are small, thin, and steeply sloping glaciers. The average glacier size is only 0.7 km2 (Portocarrero et al., 2010). Valley glaciers account for 6% (ANA, 2011), and 3% are classified as debris-covered glaciers (Racoviteanu et al., 2008) with mostly non-composite glacial basins. They are no more than 5 km in length. The tongues of the mountain glaciers terminate at an altitude of 4500–5000 m asl. The lowest elevations are attained by debris-covered glacier tongues at an altitude of 4250 m asl (Jatunraju Glacier, Huandoy N-Side). The Equilibrium Altitude Line (ELA) runs between elevations of 5000 and 5200 m asl (Kaser and Georges, 1997; Smith et al., 2005b).
IV Lake types in the Cordillera Blanca
Lakes in mountain environments may be formed, among other ways, by means of tectonic, volcanic, landslide, glacial, fluviatile, and eolian activities and organic material (Costa and Schuster, 1988). Among them, glacial lakes are supposed to be the most widespread lake types worldwide and are one of the most diverse groups of lakes. In the Cordillera Blanca, the majority of lakes are of glacial origin (ANA, 2011; Klimeš, 2012). The term ‘glacial lake’ refers to lakes whose basins have mainly been formed by present and/or former glacial processes. They are generally classified into glacier-dammed lakes, which include a variety of ice-dammed (Costa and Schuster, 1988; Iturrizaga, 2011a; Tweed and Russell, 1999) and moraine-dammed lakes (Richardson and Reynolds, 2000), lakes eroded in bedrock (glacial-scoured lakes in piedmont areas and cirque lakes in mountain settings), and intra-glacial lakes at present glaciers such as supraglacial, englacial, and subglacial lakes. Glacially conditioned lakes, which are dammed by paraglacial landforms (Ballantyne, 2002), are also considered in this paper. According to the origin of the lake water, these might be distinguished into glacier-fed lakes and distal glacier-fed lakes, which are separated by rock or sediment from the parent glacier (Smith and Ashley, 1985). Glacial lakes also include lakes in currently deglaciated mountain terrain with a non-glacial water supply, such as in the Cordillera Negra, located to the west of the Cordillera Blanca. These lakes are termed ‘decoupled glacial lakes’.
A basic classification of glacial lakes in the Cordillera Blanca was established by Concha (1957) in the context of hazard assessments. He differentiates three lake types in regard to the nature of the lake barrier (morainic, debris, and rock; cf. Korup and Tweed, 2007) and in regard to whether the glacier tongue has contact with the lake. The latter criterion is relevant for the estimation of the hazard potential of glacial lake outbursts triggered by calving of the glacier tongue (Zapata, 2002). In this paper, a proglacial lake without ice-contact is termed a ‘decoupled proglacial lake’.
Based on the author’s field surveys carried out for the present study, the principal lake types have been determined in the Cordillera Blanca as follows (Table 1, Figure 2): (1) supraglacial lake; (2) moraine-dammed lake; (3) moraine-dammed pond; (4) piedmont lake; (5) glacier-dammed lake; (6) debris-dammed lake; (7) bedrock-dammed lake (glacial erosional lake); and (8) combined lakes. Other lake types, such as tectonic, englacial, and subglacial, may occur to a minor extent, but are not addressed in this paper.
Principal lake types in the Cordillera Blanca.
Topographic setting of lake types at the western side of the Cordillera Blanca.
1 Supraglacial lakes
Supraglacial lakes develop in depressions of impermeable ice on the glacier surface. As the majority of the glaciers are steep slope glaciers, supraglacial lakes are not so abundant in the Cordillera Blanca. However, some clean-ice glaciers show a considerably high density of supraglacial lakes. Almost the entire glacier tongue may be dotted over a length of about 1 km with supraglacial lakes (e.g. Kogan Glacier, Alpamayo N-Side; Pisco Norte Glacier, Pisco N-Side; Figure 3). The lake diameter generally does not exceed about 50 m. These lake clusters are located closely beneath the ELA at some of the highest low-inclined, temperate glacier tongues in this mountain range. They are presumably perched lakes, without connection to the englacial drainage network (Benn and Evans, 2010). Their morphological arrangement might be linked to internal glaciological features in combination with climatic factors.
Supraglacial lake clusters at (A) the clean ice-type Kogan glacier at 4850–4950 m asl and (B) the slightly debris-covered Pisco Norte–Glacier at 4950–5000 m asl.
Supraglacial lakes can coalesce at low-inclined glacier tongues and transform over time into a single proglacial lake, forming moraine-dammed lakes up to several kilometers in length (Reynolds, 2000). This transformation has been studied in detail at debris-covered Himalayan glaciers (Benn et al., 2012), where this process extents over several decades. In contrast, in the Cordillera Blanca it may last under 10 years (Reynolds, 2000). Large, long-term supraglacial lakes are rather uncommon because lakes may become relatively quickly decoupled from the parent glacier tongue. On some of the debris-covered glaciers, the supraglacial debris supply is so high in relation to the ice thickness that these glaciers have not developed large lakes (Figure 4). On the largest debris-covered glaciers, such as the Kinzl Glacier (also known as Llanganuco or Glacier 510, Huascaran North, N-Side) and Schneider Glaciers (also known as Mátara Glacier or Glacier 512, Huascaran South, N-Side), only small supraglacial ponds occur. They already existed in 1948/1950 (Servicio Aerofotografico Nacional Perú). Outbreach fans at their terminal moraines indicate small lake outbursts prior to this date. However, lake formation may still take place if the upper part of the glacier becomes decoupled from the glacier tongue. In the course of down wasting of debris-covered glaciers, the supraglacial lakes may also change into smaller thermokarst lakes (Huandoy East Glacier) or into lakes nested inside the emerging hummocky or ablation moraine in the dead ice zone (e.g. Laguna Llaca). These lakes are rather short-lived and infilled by sediment or drowned by a larger lake.
Debris-covered glaciers. (A) At the Kinzl Glacier (Huascaran North, N-Side) and the (B) Schneider Glacier (Huascaran South, E-Side) no large supraglacial lakes have developed at this time. (C) At the Huandoy E-Glacier, a 250 m long thermokarst lake (4620 m asl) has formed at the heavily debris-covered glacier tongue (all photos by L. Iturrizaga, June 2011).
The multi-temporal comparison of aerial images from the years 1948/1950 (Servicio Aerofotografico Nacional Perú) of the Pucajirca Glacier in the upper Quitarasca Valley demonstrates that Laguna Safuna Alta has emerged from a supraglacial lake of a clean-ice glacier. Only the glacier tongue had been covered by debris over a length of 500 m in the 1950s.
2 Moraine-dammed lakes
The most widespread and largest lakes are represented by proglacial lakes as bedrock-dammed (glacial erosional lakes) and moraine-dammed lakes (Figures 2, 5, and 14; ANA, 2011). The latter have been the main source of the destructive glacial lake outburst floods in the past. The moraine-dammed lakes may be found in the majority of the large transversal valleys at their upper end, often orientated in a SW–NE direction. They are predominantly situated in a strongly topographically confined topographic setting with steep mountain walls rising over 6000 m asl at a well-defined altitudinal range of 4300–4700 m asl (Table 1). The lakes form in the depression between the retreating glacier tongue and the downstream moraine arc. The largest lakes are impounded by a striking terminal moraine, the Great End Moraine (GEM), which was named ‘Hauptmoräne’ by Kinzl (1935). It is located at a maximum distance of 3–5 km from the highest peaks in the catchment area (Figure 5). The GEM is a prominent landform measuring about 70–150 m in height with a semicircular plan view, and forms mostly a lateral-end-moraine complex (Figures 5 and 11). It may be located either on sediment or on bedrock. The maximum lake size is controlled by the topography and ranges in the order of 1–2 km in length and a depth of 30–70 m. The valley topography dictates the oval shape of the lakes with a width of generally less than 500 m. Some of the largest lakes are Laguna Parón (40×106 m3), Laguna Arhueycocha (19×106 m3), Laguna Rajucolta (17×106 m3), Laguna Palcacocha (17×106 m3), Laguna Safuna Alta (15×106 m3), Laguna Tullparaju (12×106 m3), and Laguna Artesa (12×106 m3) (ANA, 2011). The remaining lakes generally have a volume of less than 10×106 m3. Compared with the Himalayan moraine-dammed lakes with volumes of >70×106 m3 (ICIMOD, 2011), the lakes here are rather small.
The cross-profile of the GEM shows an asymmetrical geometry with a steeply inclined inner slope (>60°) and a more gently sloping outer slope (30–35°). A major part of the moraines is composed of granites and granite-dioritic rock types from the Miocene Cordillera Batholith. The sediment matrix of the moraines generally consists of a heterogeneous mix of large angular to subangular rocks, gravel, and sands, with varying proportions of clay and silt (Figure 6). Lliboutry et al. (1977a) mentions 10–15% of fine sand, silt, and clay with a diameter of <0.2 mm and 30–50% of pebbles and boulders with a diameter of >2 cm in the moraine of Laguna Safuna Alta (Figure 6B).
At the base of the Huantsán (6395 m asl), Laguna Rajucolta (4250 m asl) is dammed by the Great End Moraine (GEM) (↓) measuring about 100 m in height. The lake is artificially drained and also used for hydropower. In the foreground, past remnants of an older lake basin (^) can be seen (photo by L. Iturrizaga, 24 July 2010).
(A) View downstream onto the inner side of the GEM dam of Laguna Llaca (4460 m asl) (cf. Figures 12 and 15). The moraine is composed of granite-diorite boulders up to several meters in diameter embedded in a sandy fine matrix. In the center part, an artificial dam for secure measures is visible (↓) (photo by L. Iturrizaga, 23 July 2010). (B) The 150 m high GEM dam of Laguna Safuna Alta (4350 m asl), located at the eastern side of the Cordillera Blanca (cf. Figure 13A), consists mainly of metamorphic and sedimentary rocks and shows a lesser content of large boulders in the terminal moraine, whereas they are abundant in the lateral moraines (photo by L. Iturrizaga, 4 September 2011). The matrix consists of gravel-sized and sandy debris. Note person for scale (←).
Genetically, the GEM was referred to as push moraine by Lliboutry et al. (1977a). However, the nature and heterogeneity of moraine outcrops in combination with their great height (Figure 6A) indicate that dominant processes may vary over time. Their origin might be referred to as polygenetic, involving dumping, push, thrusting, and ablation processes. In the case of the debris-covered Jatunraju Glacier, which was supplied by abundant rockfall events, dumping processes have considerably contributed to the moraine formation (Iturrizaga, 2013). Apart from the high-level transport pathway, englacial processes have to be considered for the formation (cf. Spedding and Evans, 2002). At the low level, transport-level thrusting processes may play a role in the moraine development. Displacement waves may modify the distal and proximal slopes of the moraines (e.g. Laguna Safuna Alta-Moraine; Figure 13A; Hubbard et al., 2005).
The widespread occurrence of the moraine-dammed lakes in the Cordillera Blanca raises the question about the controlling factors for the lake development as not all glaciated mountain ranges in the Andes have developed lakes during the modern glacier retreat to such an extent (cf. section VII). Throughout the Andes, immense end-moraine complexes that may impound lakes are found in extra-montane and intra-montane settings. The largest moraines occur in the piedmont area of the Patagonian Ice sheet on the Argentinian side (Clapperton, 1993). However, here the focus is on lakes in a confined high-mountain setting. The following preconditions are considered to be decisive for the formation of the GEM lakes: The critical peak elevation for the formation of moraine-dammed lakes (>1 km length) is situated at about 5600 m; respectively, the catchment areas should rise at least 500 m above the modern ELA. Below that elevation, the glaciation cover and the debris supply are too limited for the formation of larger moraines. The sudden change of the valley gradient from steeply inclined head walls to flat valley bottoms promotes the overdeepening process, forming the lake basin and the debris deposition in the form of large moraines acting as a lake barrier (cf. Hallet et al., 1996; Hooke, 1991). Flat potential proglacial areas are essential for the formation of larger lakes. In terms of the glacier geometry, a low-inclined glacier tongue (<2°threshold; cf. Reynolds, 2000) is necessary for the formation of supraglacial lakes as the initial stage of a moraine-dammed lake. The formation of large moraines requires a stable, low fluctuating glacier margin over a long time period. Low glacier velocities or even stagnant glacier termini are prone to the development of glacial lakes (Reynolds, 2000). The moraine dam has to be sufficiently stable and in parts impermeable to impound large water reservoirs. The large GEM is located below the permafrost zone and commonly not significantly ice-cored, so that a certain stability of the moraine dam is provided. The high-amplitude moraines, forming the dams, are characteristic for decoupled landsystems with inefficient glaciofluvial transport between the glacier and the proglacial area (Benn et al., 2003). The glacial meltwater discharge needs to be rather moderate or low, so that the end-moraine is not incised or destroyed. Discharge of the rivers in the Cordillera Blanca is commonly less than 10 m3/s (Mark and Seltzer, 2003). Crucial for the persistence of the lake is that the debris supply or the sediment discharge of the glacial meltwater is not too high, so that the lake can persist. In principle, the tropical glaciers here are characterized by a glacial environment of low sediment yield.
In general, a certain relationship between glacier geometry, meltwater discharge, sediment supply (cf. Rodbell et al., 2008; Zemp et al., 2005, 2008), and glacier dynamics must be present to form a moraine-dammed lake.
At least since the end of the 19th century, modern glacier retreat has led to a widespread distribution of moraine-dammed lakes. The exact onset of this formation is not known. A. Raimondi (cited in Kinzl, 1950) already mentioned in the 1870s that small lakes formed at the glacier tongues. It was suggested that the GEM has been formed over a range of some several hundred (Kinzl, 1950) to thousand years during the Holocene (cf. Rodbell, 1992; Röthlisberger, 1986; Stansell et al., 2013). Solomina et al. (2007) have attributed the moraine to the Little Ice Age based on extensive lichenometric studies. This age classification may indicate a minimum age. The onset of these multi-phase and polygenetic landforms, built up by moraine superposition, accretion, and dumping processes might have started earlier (Iturrizaga, 2013). It cannot be excluded that glacial lakes have formed behind the GEM dams at times of a lesser glacial extent in pre-Little Ice Age times. Smaller lakes are also dammed by low-amplitude moraines only several meters in height, such as the pre-GEM, which has formed before the GEM (see subsection 8,‘Combined lakes’, below).
3 Moraine-dammed ponds
Moraine-dammed ponds are distinguished from the above-mentioned larger moraine-dammed lakes (Figure 7). Whereas moraine-dammed lakes develop mostly at valley glacier tongues, moraine-dammed ponds are restricted to the proglacial areas of retreating clean-ice slope glaciers. The catchment areas remain generally below an altitude of 6000 m asl. The ponds are predominantly located at the trough shoulders of the Last Glacial Maximum and distributed in a well-defined altitudinal belt between 4800 and 5000 m asl. Due to the topographic and glacial constraints, the lake length does not exceed some 10–100 m.
(A) Moraine-dammed ponds (4820–4950 m asl) (↓) along the Santa Cruz Valley looking towards W (Google Earth image 2013). Some of the small lakes have already dried up (→). On the valley floor sediment fan-dammed lakes are visible (Jatuncocha and Ichicocha, 3850 m asl), which have been affected by a recent lake outburst (↑) of Laguna Artizón Baja in February 2012. (B) Sketch of the topographic setting of moraine-dammed ponds.
The lakes are dammed by perched morainic ramparts and aprons (Figure 7B). The moraines are deposited on an inclined bedrock base, preferentially in the transition zone from the flat part of the trough shoulder to the steep trough valley flanks. The inner slopes of the moraines measure only several meters in height. The lake basins might be partly formed by glacial erosion of the bedrock. The moraine dams are situated close to the ELA and thus are more likely to be ice-cored.
The ponds may occur in clusters at neighboring glaciers and be aligned along the festoon-shaped morainic aprons. Similar to talus screes, the coalesced aprons may extend over 2–3 km along the lower part of the proglacial zone of the slope glaciers. The pond formation is associated with the glacier retreat in the Little Ice Age, but somewhat different from the moraine-dammed lakes. Once the small lakes have formed, they rather quickly lose contact with the glacier and become decoupled proglacial lakes. The supraglacial lake phase at these steeply inclined glaciers is generally more short-lived in comparison to the moraine-dammed lakes.
Bi-temporal comparison of the present-day situation with aerial images from the years 1948/50 (Servicio Aerofotografico Nacional Perú) reveals that some of the moraine-dammed ponds already existed at that time. Since then, some ponds have not changed considerably in size. Several lakes have disappeared. The corresponding glaciers diminished so much that not enough meltwater was provided for lake feeding. The ponds may then have been entirely rain-fed. Lacustrine sediments of moraine-dammed ponds are abundant. Thus, the lifespan is rather short.
The moraine-dammed ponds drain by seepage or by small incised fluvial channels through the moraine. They may drain abruptly but, in contrast to the outbursts of larger moraine-dammed lakes, no large breach fans are produced due to the limited lake volume and the steep relief conditions. Despite their small size, the ponds can become hazardous when draining at once (cf. section VI).
4 Piedmont lakes
A further type, dammed by glacial sediments, is the piedmont lake, which mainly occurs as tongue basin lakes (Table 1). The lake barrier is not formed by a prominent morainic arc, but by amorphous sediments of predominantly glacial origin. The lake basins may have formed by glacial overdeepening processes in the Pleistocene till and outwash. The highest contemporary lake is Laguna Aguashcocha (4270 m) in the southern part of the Cordillera Blanca. The lakes in the piedmont area are mostly paleolakes (Figures 8 and 17), such as in the Rurec, Negra, Quilcay, and Ulta Valleys (cf. section V). Piedmont lakes are much more widespread in the central and southern part of the Peruvian mountain ranges.
(A) View towards the valley exit of the Chucchún Valley, entering the Rio Santa Valley. An ancient lake basin is located in the Pampa de Shonquil (3600 m asl) (^). It is embraced by a set of piedmont moraines, dating probably back to the Younger Dryas/Lateglacial/LGM (↓) (photo by L. Iturrizaga, 26 December 2011). (B) View from the Portachuelo de Honda Pass (4750 m asl) towards the upper Quebrada Honda. A lake (^) has been dammed in Holocene times by an end-moraine complex and partly by a rock barrier (4150 m asl). The location of the lake sediments at the valley floor is locally known as Viñopampa (photo by L. Iturrizaga, 31October 2012).
5 Glacier-dammed lakes
Glacier-dammed lakes are backed up by tributary glaciers, sealing off the main river. The cross-valley barrier might be composed of glacier ice and/or the lateral and latero-frontal moraine. The only present lake of this type is Laguna Parón (Figure 9; cf. section V). The glacier-dammed lakes might have been more frequent during the LGM and Lateglacial, when glaciers attained greater length.
The 3 km long Laguna Parón (4200 m asl) is blocked by the 200 m high pedestal moraine of the Jatunraju glacier tongue (↓) and a sediment cone originating from the opposite valley flank (→). The Jatunraju Glacier is flowing down from the Huandoy Norte (6395 m asl) (←) (photo by L. Iturrizaga, 26 July 2010).
6 Debris-dammed lakes
Sediment fans and debris cones originating from the tributary valley and adjacent slopes impound lakes in the relief-constricted valley sections of the transversal valleys (Figure 10), mainly at elevations of 3800–4000 m asl. During the Holocene, debris-dammed lakes were more widespread (cf. section V). The debris of the sediment fans frequently derives from the recycling of older slope moraines, such as debris flows (Iturrizaga, 2008). Thus, they can be classified as glacially conditioned, i.e. paraglacial landforms as part of the vertical debris cascade system (Ballantyne, 2002; cf. Iturrizaga, 2011c, for transglacial landforms in high-mountain settings). Equally, rock slides forming cross-valley barriers may be a result of debuttressing processes after deglaciation. Small rock slides can be found throughout the transversal valleys, especially towards the valley exits. Large cross-valley rock slides, which block the main valley, are not common in the resistant granitic rocks. On the whole, only a minor portion of the present lakes (6.5%) in the Cordillera Blanca account for this lake type (Concha, 1974, cited in Klimeš, 2012).
(A) Lagunas Orgoncocha (O) and Chinancocha (C) (3850 m asl) in the Llanganuco Valley are dammed by debris cones coming from the Huascaran-North Side (6655 m) (HN) and the Huandoy South Side (6160 m asl) (HS). At the trough shoulders, moraines (↑) may impound small ponds. Mass movements may recycle the moraines and contribute to the formation of the debris cones below (↓). The impounded lakes in the valley bottom may have developed during Holocene times when the Lateglacial glacier stream disappeared (photo by L. Iturrizaga, 25 June 2011). (B) Laguna Conococha (4025 m asl) (photo by L. Iturrizaga, 24 August 2011).
7 Bedrock-dammed lakes (glacial erosion lakes)
Besides the moraine-dammed lakes, glacial erosional lakes represent the major lake type in the Cordillera Blanca. They are dammed by bedrock and produced by glacial scouring (Figure 11). The formation of the lake basins is commonly a cyclic process, lasting over multiple glaciation phases. The onset of some of the basins may have started since the Last Glacial Maximum at different elevational levels. Nevertheless, they rarely appear in the middle sections of the main valleys as the valley bottoms are mostly filled with sediments. At the western slope of the Cordillera Blanca batholith, the lakes often occur as paternoster lakes in a staircase-type alignment, forming multi-step lakes with significant differences in elevations. Moraines may be located on top of the glacially scoured rock threshold. One of the biggest and deepest rock-dammed lake is Laguna Auquiscocha (Figure 1), located in the drainage basin of the Ulta Valley, with a volume of 50×106 m3, and a depth of 93 m (ANA, 2011). One of the most hazardous glacial erosional lakes in recent times has been Laguna 513 (Carey et al., 2012).
Glacial erosional lakes. (A) Laguna Taullicocha (4450 m asl), Santa Cruz Valley (photo by L. Iturrizaga, 28 August 2011). Combined moraine-bedrock-dammed lakes: (B) Laguna Rajucocha is impounded by a rock threshold and part of a paternoster sequence (cf. Figure 13) with (C) Laguna Azulcocha and (D) Laguna Yanacocha (3990 m asl) downstream, producing paternoster lakes (photos by L. Iturrizaga, 1 September 2011). (E) Areal-scouring lake clusters (4970 m asl) in bedrock in the paleo-proglacial environment of the Pastoruri Glacier, Caullaraju Massif (photo by L. Iturrizaga, 20 December 2008).
Glacial erosional lakes in the form of clusters of areal scouring lakes occur in the proglacial areas of the clean-ice glaciers, such as in the Caullaraju Massif (Figures 1 and 11). Their formation is enhanced by less resistant sedimentary rocks of the Jurassic-Cretaceous Periods. In the non-glaciated Cordillera Negra (8°41’–10°07’S), located on the western side of the Cajón de Huaylas, decoupled rock-dammed lakes are the dominant lake type. They occur at a narrow elevational level of 4400–4450 m asl mainly as cirque lakes.
8 Combined lakes
In terms of the spatial arrangement of the lake types, combined lakes are classified as a composite lake type. The lakes occur at nearly the same elevation or at successively lower elevations, and form characteristic lake sequences of two or more lakes in the Cordillera Blanca (Figures 13, 14, and 18). They occur as multi-moraine-dammed lakes or mixed combined lakes such as moraine-rock-dammed lakes or multi-debris-dammed lakes. They play a crucial role in the assessment of the hazard potential of lake outbursts (cf. section VI).
Laguna Llaca (4460 m asl), with the Ranrapalca (6162 m asl) as the highest peak in the catchment area, is impounded by the GEM. It has been ultimately built up during the Little Ice Age. The moraine shows an artificial dam at its upper crest as part of security measures for a potential lake failure. Further downstream, the Holocene pre-GEM-sequences in the form of several moraine ridges are visible (photo by L. Iturrizaga, 23 September 2010).
(A) Laguna Safuna Baja, dammed by the pre-GEM, and (B) Laguna Safuna Alta, dammed by the GEM, form combined lakes at the eastern side of the Cordillera Blanca. Laguna Baja receives its water through internal moraine drainage of the upper lake, which formed during the 1950s. Neither lake shows any open-air outlet channel. At the base of Laguna Safuna Baja, several springs emerge (photos by L. Iturrizaga, 5 September 2011).
Development of paired moraine-dammed lakes dammed by two successive terminal moraines (pre-GEM and GEM). (1) Initial formation of the pre-Great End Moraine complex (pre-GEM) during the Holocene (pre-LIA). (2) Final formation of the pre-GEM, glacier retreat, and subsequent long-term stable glacier tongue position with the formation of the Great End Moraine (GEM), development of supraglacial lakes, seepaging through the GEM (→), and formation of the pro-GEM lake. (3) Coalescing of supraglacial lakes to one proglacial lake, accelerated glacier retreat due to lake water contact, appearance of rock windows at the rock-step in the ice fall-zone, moraine dam failure, and glacier lake outburst. (4) Disintegration of the valley glacier into a slope glacier with a hanging glacier tongue and a regenerated or residual glacier cone at the base of the rock step; maximum lake length is reached. (5) Decoupling of glacier and lake, rock failures caused by debuttressing after deglaciation, and landslides at inner slopes of lateral moraines. (6) Development of new small lakes above the confluence step; lake level lowering or drying up of the GEM- and pro-GEM lake.
Characteristic stages of modern lake development in the Cordillera Blanca: transition of valley glaciers to slope glaciers without lake contact. (A) Initial phase: supraglacial lakes (↓), which may coalesce to a moraine-dammed lake. The glacial lake may drain through the GEM and form a pro-GEM lake (←). Santa Cruz W-Glacier (4760 m asl) (Google Earth image 2013). (B)Proglacial lake formation in progress, Laguna Llaca (4450 m asl) (photo by L. Iturrizaga, 24 July 2010). A hummocky dead ice landscape emerges near to the glacier tongue. (C) Disappearance of the valley glacier tongue and formation of a slope glacier with a hanging glacier tongue, which is still in lake contact. Arhueycocha Lake (4450 m asl) (photo by L. Iturrizaga, 27 August 2011). (D) Disintegration phase of the parent glacier into a slope glacier and a basal regenerated or residual glacier cone (↑). Laguna Palcacocha (4580 m asl) (photo by L. Iturrizaga, 29 July 2010). (E) Occurrence of rock windows at the confluence step (→). Laguna Rajucolta (4270 m asl). The residual ice cones may be subject to subglacial collapsing and trigger displacements causing glacier lake outburst. (F) The glacier tongue has retreated almost entirely on top of the confluence step. Laguna Quilcay (4350m asl) (photo by L. Iturrizaga, 31 July 2010). (G) Decoupling of the lake from the entire glacier at Laguna Shallap (4280 m asl) (photo by L. Iturrizaga, 25 August 2011). Maximum lake length is attained. (H) The GEM of Laguna Jancarurish (4300 m asl) was destroyed by a glacier lake outburst (→) in 1950 (photo by L. Iturrizaga, 6 September 2011).
Characteristic moraine sequences are found in the upper parts of numerous valleys (Kinzl, 1942; Röthlisberger, 1986). They are composed of the GEM and the pre-GEM, which was formed before the GEM (Figure 12). Both moraines are located in rather close vicinity to each other at a distance of 1–3 km. In contrast to the prominent sharp-crested GEM, the pre-GEM is a low-amplitude end-moraine complex, which usually does not exceed a few meters to tens of meters in height. The pre-GEM is often composed of several inserted moraine ridges or an irregular hummocky moraine landscape, which is eroded in its central part by the river.
The pre-GEM might dam a lake directly downstream of the GEM lake, termed here a ‘pro-GEM lake’ in regard to its spatial context, e.g. downstream in front of the GEM. The GEM lake and pro-GEM lake are classified as combined lakes (paired moraine-dammed lakes), which are aligned behind each other (e.g. Lagunas Qoyllurcochas, in the Quebrada Oja Malca, Champará, Lagunas Safuna Alta and Baja; Figure 13A). The pro-GEM lakes are usually situated at an altitude range of 4200–4600 m asl. The lower pro-GEM lake is dominantly fed by the water from the upper larger lake by internal seepage through the GEM-dam, and not necessarily by an open-air outlet. Thus it may be referred to as a distal proglacial lake, as it is separated from the glacier by the moraine. At the base of the GEM and pre-GEM multiple springs might be located (Lliboutry et al., 1977a). The distal lake may be situated in pre-existing basins of glacial or non-glacial origin. The upper lake often acts as a sediment trap, so that mostly very fine grained sediments are found in the lower lake (Stansell et al., 2013). Seepage can already take place before the existence of the upper GEM lake. Thus, the distal lake may pre-date the upper lake (e.g. Santa Cruz SW-Glacier, Cordillera Blanca, and Yerupia South Glacier, Cordillera Huayhuash). This combined lake type may have evolved from Holocene to modern times. Figure 14 shows an exemplary model for the development of paired moraine-dammed lakes.
The lower water bodies of combined lakes can be also dammed by one or several rock barriers in the form of paternoster lakes (Figure 11, B–D). The formation of the rock basins are longer-term processes as the initial carving might have already started during the Last Glacial Maximum. The rock basins then became ice-free during the Holocene.
9 Current transition phases in lake development
The majority of the valley glacier tongues have been subject to lake formations in the Cordillera Blanca. The timespan for the development of larger lakes ranges between under 10 years to some tens of years (cf. Kinzl, 1949; Lliboutry et al., 1977a). Due to the limited thickness and length of the Cordillera Blanca glaciers and the topographical limitations, the formation of lakes is a comparatively rapid process until their maximum expansion. As aerial analysis shows, several lakes had already reached their full size in the 1950s. Laguna Rajucolta was almost completely developed at that time. Other lakes were decreasing in depth, partly due to artificial draining (ANA, 2011; Reynolds, 2003).
Numerous glaciers have undergone or are in a crucial transition phase, converting from short debris-covered valley glaciers to slope glaciers. A noticeable change in the geometrical shape of the glacier occurs as it is no longer a form-constant glacier retreat. Most characteristic is the decoupling of the glacier tongue from the parent glacier due to the steep rock threshold at the upper head of the valley (decoupled proglacial lakes). Figure 15 outlines the major stages of the deglaciation process in association with the proglacial lake formation. The ice-cliff phase, in which the retreating valley glacier may form a calving front, is in general rather short-lived in comparison to those of the Himalayan glaciers. The shallow tongues of the valley glaciers become disconnected from the parent glaciers, which are located in the precipitous relief in the form of slope and hanging glaciers. The residual glacier tongue is little nourished by the parent hanging glacier and the glacier tongues transform rapidly into lakes. Some of the debris-covered glaciers convert into rock-glaciers or a hummocky debris landscape (Huandoy East Glacier, Schneider Glacier). A threshold for lake formation in terms of glacial water supply may be reached when glaciers shrink to a critical minimum size (cf. Mark et al., 2010).
The comparison of aerial images reveals that the supraglacial lakes at the Santa CruzWest-Glacier were remarkably stable over the last 65 years (cf. Figure 15A). The pro-GEM lake, Laguna Raucoltacocha, at the base of the GEM of the Santa Cruz West-Glacier, did not change considerably in size either. The pro-GEM lake Laguna Safuna Baja remained the same size during the evolution of Laguna Alta from 1948 to 2013.
Assuming a further glacier retreat, the elevational range of future lakes will be located mainly above the present GEM lakes (Figure 2) and mainly bedrock-dammed lakes due to the limited moraine formation. The potential for the formation of an extensive distribution of larger proglacial lakes is generally small in the higher altitudes due to the steep relief conditions and small glacier size. However, a pronounced flattening occurs in many longitudinal profiles in the elevational range of 4800–5200 m. These level areas may host small cirque lakes, but in some cases longer lakes could also develop, such as at the glaciers in the upper Quebrada Honda valley (Pucaranra-Pucaraju-Massif). Above Laguna Artesonraju (4300 m), which is dammed by the GEM, a new shallow proglacial lake (4750 m asl) has been formed above the confluence step. It has been estimated that the new Artesonraju Lake will attain a length of about 1500 m, a volume of at least 20×106 m3 with a depth of more than 50 m (McKinney et al., 2012). These new potential lakes play a vital role in regard to the future hazard scenario (cf. section VI).
V Paleolakes during the Pleistocene and Holocene
Glacial lakes have been a major landscape feature throughout the Pleistocene and Holocene in the Cordillera Blanca. Sievers (1914) has already stressed the role of the Pleistocene Glaciation in lake formation, referring to the lakes as ‘index fossils’ of the Paleo-glaciation. In this paper, it is argued that every major glaciation period has been accompanied by distinct assemblages of lake types in the Cordillera Blanca as shown in Figure 16. The lower limit of glacial lakes is commonly controlled by the LGM extent. Previous studies have demonstrated that the valley glaciers reached the Rio Santa Valley with their piedmont lobes and attained lengths in the order of 10–20 km and altitudes of 3400–3200 m asl (Clapperton, 1983; Farber et al., 2005; Kinzl, 1950). Wilson et al. (1967) proposed that the piedmont glaciers partly coalesced with the lowest ice margins at 2700 m asl (e.g. Quebrada Honda, at Marcará). Oppenheim (1945, 1946) and Clapperton (1972, 1983) assume a four-fold glaciation based on the classical alpine glacial chronology. Rodbell (1993) has classified four moraine groups: (1) Cojup moraines 3400 m asl, 440–76 ka; (2) Rurec moraines 3400–3800 m asl, 30–20 ka; (3) Laguna Baja moraines 3800–4000 m asl, 18–16 ka; and (4) Manachaque moraines above 4000 m asl, 13.2–10.4 ka. Glasser et al. (2009) reconstructed minor glacier advances for the Younger Dryas/Holocene (12.5 and 7.6 ka) by cosmogenic dating. Estimates on ELA lowering during the LGM vary greatly from 1500–2000 m (Clapperton, 1983) to only 400–900 m (Smith et al., 2005b).
Idealized longitudinal profile of a transversal valley of the western side of the Cordillera Blanca with a generalized pattern of the elevational distribution of glacial lakes from the Last Glacial Maximum to modern times.
1 Glacial lakes during the Last Glacial Maximum (LGM)
Remarkable U-shaped valleys were formed during the Pleistocene glacial period (Clapperton, 1983) in the granite and granite diorite complexes of the Miocene Batholith (Siame et al., 2006). The 500–800 m high trough valley flanks indicate a Pleistocene ice thickness in the range of up to 1000 m. The gentle longitudinal profiles and the trough shoulders offer one of the decisive topographical settings for the formation and spatial arrangement of the intra-montane lakes.
In the piedmont area of the western side of the Cordillera Blanca, large lateral moraine complexes, several kilometers in length, provide striking evidence of the Pleistocene glaciation extent (Figure 2). At some places, the moraines are cross-cut by the active Cordillera Blanca Normal Fault with slip rates in the order of 60 m (Figure 1; Schwartz, 1988; Siame et al., 2006). Thus, tectonic events might have played a role in lake formation, as well as indirectly in the context of triggering mass movements, but are not the subject of this paper.
After the retreat of the piedmont glacier lobes, lakes remained in some of the tub-shaped tongue basins (Figure 8). Their elevational distribution ranges from 3200–3600 m asl in the lower Santa Valley up to an altitude of 4300 m asl in the upper valley section. The majority of the LGM piedmont lakes were drained or filled by sediment. Ancient tongue basin lakes occur among others at the valley exits of the Quebrada Honda, at Pampa Jonca (3400 m), and at Pampa de Shonquil (3600 m) in the Rio Chucchún catchment, above Carhuaz (Figure 8A). Laguna Aguashcocha (4270 m asl; Figure 1) represents a recent, still existing example of this lake type. It has developed at the former terminus of the 13 km long Tuco Glacier. These tongue basin lakes are generally non-hazardous, unless an upstream flood triggers their outburst. In other mountain ranges of Peru, with shorter valleys in the range of 5 km, modern piedmont lakes are rather common landscape features and may even be longer than the parent valley (see section VII).
During the LGM, glacier-dammed lakes formed where trunk glaciers advanced from the Cordillera Blanca into the ice-free main valley of the Cajón de Huaylas. One of the lowest lakes of this type was supposed to have been located in the Rio Santa Valley at an altitude of 1850 m asl near to Molinopampa upstream of the Cañon de Pato (Kinzl, 1950), but this location is controversial.
2 Glacial and glacially conditioned lakes during the Late Glacial Period and Holocene
During the Late Glacial Period and Holocene, glaciers receded into the upper valley heads and the main valley became gradually ice-free (cf. Farber et al., 2005; Jomelli et al., 2009; Rodbell, 1992; Rodbell et al., 2009; Stansell et al., 2013). A large variety of lake types developed, such as moraine-dammed lakes, bedrock-dammed lakes, and to a lesser extent glacier-dammed lakes in a valley-confined high-mountain setting. The elevational range of glacial lakes formed during the Late Glacial/Holocene period ranges between 3600 and 4600 m asl.
The level valley floors of the main valleys are scattered with paleolake sediments which provide plenty of evidence of a former widespread lake distribution downstream from the present lakes (Figures 8B and 17). The lake beds are known locally as ‘vegas’, such as in the Quebrada Honda (Figure 17B). They are mainly dammed by low-amplitude end-moraine assemblages, which can be classified as early Holocene according to the existing glacial chronologies (cf. Röthlisberger, 1986). They are generally situated no more than 5 km downstream of the GEM, at an altitude range of 4200–4000 m asl.
As a consequence of deglaciation of the transversal valleys, debris cones from the tributary valleys began to extend into the valley floors of the main valleys during the Postglacial period (Figure 10A). The low valley gradients, the narrow valley cross-profiles, and the low discharge levels of the main rivers are advantageous settings for the formation of debris-dammed lakes. The catchment areas of debris cones may exceed 6000 m asl, such as at the South Side of the Huascaran-North. The debris supply area often consists of till or morainic ramparts mantling the trough valley flanks (Figure 7A), which are recycled by mass-wasting processes and debris flows through steep ravine-like gorges, cut into the valley flanks of the glacial troughs and shoulders in the course of the paraglacial landscape transformation. Many debris cones are still active today, such as in the case of Lagunas Chinancocha and Orgoncocha (Figure 10A). The debris flows coming from the Huandoy N-Side repetitively cause the destruction of the road down valley of Laguna Orgoncocha. During the 1970 earthquake, a rock slide occurred at the Huascaran N-Side and sealed the outlet of Laguna Orgoncocha. Currently, the debris-cone-dammed lakes only account for a relatively small portion of the lakes in the Cordillera Blanca.
Laguna Conococha (4025 m), which is located in the wide piedmont area in the source area of the Rio Santa, is dammed by the Rio Tuco sediment fan. Due to the low sediment supply to this river, the lake has survived until present times (cf. Kinzl, 1950). The lake level is currently artificially lowered by mining companies.
When the main valleys became ice-free, tributary glaciers may have dammed the rivers, such as the Jatunraju Glacier dam impounding Laguna Parón (Figure 9; Iturrizaga, 2013). It is primarily dammed by the enormous pedestal moraine of the Jatunraju Glacier and a debris cone, entering the Parón Valley from the North (Iturrizaga, 2013; Lliboutry, 1977). Laguna Parón is supposed to have existed during the entire Holocene (Figure 17A; Seltzer and Rodbell, 2005). The damming glacier itself may also host a proglacial lake as proposed by Lliboutry (1977) for the Jatunraju Glacier during Holocene times. Laguna Parón is the only recent lake which falls into the category of glacier-dammed lakes.
Lakes may have also developed at minor rock steps or overdeepened valley sections in combination with till from the LGM glaciation moraine.
VI Glacial lake outbursts in the topographical context: outburst cascades
Over a dozen catastrophic glacial lake outbursts have occurred during the last century killing more than 5000 people (Figure 1; Ames and Francou, 1995; Carey, 2010, and references therein; Vilímek et al., 2005; Zapata, 2002). From all the Peruvian mountain ranges, the Cordillera Blanca shows the longest records of glacial lake outbursts. From the other mountain ranges only a few are mentioned (such as in 1938 Laguna Mistral, Cordillera Conchucos, in 1941 Laguna Suerococha, Cordillera Huayhuash, in 1989 Laguna Chuspicocha, Cordillera Huaytapallana, in 1996/97 Laguna Salacantay, Cordillera Vilcabamba; Reynolds, 2003). The first glacial lake outbursts affecting settlement areas were reported from Monterey in 1869 and Laguna Rajucolta in 1885 (Ames and Francou, 1995; Zapata, 2002). Major settlements of the Cajon de Huaylas with high population densities are located on flood-prone zones of sediment cones ata distance of only 15–20 km from the hazardous lakes. Early lake formations in the beginning of the 20th century have been documented best by the development of 10 lakes in the valleys E and NE of the city Huaraz in the 1930s–1950s (Kinzl, 1935, 1950; Lliboutry, 1977). The lakes are situated over a N–S-transect of 65 km upstream of one of the most densely populated settlement zones in the Cajón de Huaylas.
Most of the glacial lake outbursts occurred at moraine-dammed lakes. The most devastating event was the failure of Laguna Palcacocha in December 1941, destroying a third of the city of Huaraz, causing several thousand fatalities (Kinzl, 1950). Glacial lake outburst floods occur as multi-process-type floods, including debris and mud flows, hyper-concentrated flows, and mud floods, which carry up to house-sized boulders, with impact zones of tens of kilometers downstream. Remedial measures, in the form of lake control by tunnels through the moraine or bedrock, open channels, and artificial dams work, have been undertaken at 35 lakes by Peruvian engineers, especially under the guidance of Cesar Portocarerro and Marco Zapata and involving the efforts of local people (ANA, 2011; Carey, 2010; Reynolds, 2003). Some of the glacial lakes, such as Laguna Parón, Laguna Rajucolta, and Laguna Aguashcocha, are used for hydropower generation by foreign companies. These activities have led to substantial conflicts in the context of local water management and natural hazard management (Carey, 2010). More recently, high-tech early-warning systems, provided by transnational research initiatives, are tested at Laguna 513 (Carey et al., 2012).
Many end-moraines possess a partial erosional incision and breach sediment fans indicating past glacial lake outbursts (Figures 5 and 15H). However, moraine-dammed lakes may drain abruptly not just once. After the catastrophic drainage of the lakes, a certain water volume may remain in the lake basins. They may refill due to continuing glacier retreat and become hazardous again. Laguna Palcacocha, with a volume of 9–11×106 m3, drained incompletely when the lake was still in evolution (Vilímek et al., 2005). Since then, it has reformed continually in the course of the further shrinking of the glacier tongue, posing a new hazard potential (Emmer and Vilímek, 2013).
Figure 18 summarizes the potential triggers for glacial lake outbursts. In general, most of the glacial lake outbursts occurred during the rainy season, when lake levels were high. Moderate or heavy rainfall, combined with other critical factors such as the degradation of the lake barrier over time, may cause a dam failure. Glacial lake outbursts have repetitively been generated by ice avalanches, originating on the steep mountain flanks (e.g. Laguna 513 in 2010) and leading to destructive displacement waves (Reynolds, 2003). They may originate from failures of small hanging glaciers located in the mountain walls, which are differentiated according to their starting zone in the break-type with wedge failure or the rampart-type with slap failure (Reynolds, 2003). In general, these ice avalanches from high-level zones might occur as multi-process events, in which rock slides initiate the ice avalanche, or vice versa. The starting zone might also be located at close vertical proximity to the lake surface inoverhanging or instable glacier tongues (low-level ice break-offs). Displacement waves may be produced by a variety of other processes, such as destabilized inner slopes of lateral moraines (e.g. at Laguna Palcacocha in 2003; Vilímek et al., 2005), landslides from adjacent slopes, sudden glacial meltwater input, or ice-cliff calving. For the outburst of the Jancarurish Lake of the Kogan Glacier in 1950, it was assumed that subaqueous calving events from a residual glacier cone generated a dam failure (Lliboutry et al., 1977a). Engineering works for an artificial drainage system have been also considered for the failure of the moraine dam (Carey, 2010). The devastating Ancash-earthquake in 1970 severely affected the recently constructed drainage tunnel in Laguna Safuna Baja (Zapata, 2002).
(A) Delta sediments in Laguna Parón, 4200 m asl (photo by L. Iturrizaga, 26 July 2010). (B) Lacustrine sediments with multiple organic layers in the Santa Cruz Valley, 4150 m asl (photo by L. Iturrizaga, 27 August 2011).
Triggers for glacier lake outbursts in the Cordillera Blanca.
Special attention shall be given here to the hazard potential of the lakes in the context of their topographic settings and the spatial arrangement of the lakes to each other. In Figure 18, combined lakes, consisting of two moraine-dammed lakes behind each other, are shown in order to address the phenomenon of lake outburst cascades. Outburst cascades in high-mountain settings and their hazard potential have been described from glacier-dammed lakes in the Hindukush-Karakoram Mountains (Iturrizaga, 2005). The drainage of an upper lake may trigger the failure of a lower lake. The additional water volume may significantly aggravate the hazard potential. Outburst cascades may even involve more than two lakes of similar or different genetic origin. The outburst hazard of pro-GEM lakes is generally comparatively low. However, an outburst of the upper GEM lake can increase the total flood volume.
When Laguna Palcacocha drained, the flood volume was significantly increased by taking up the water of Laguna Jircacocha (4.8×106 m3) in the middle part of the Cojup Valley (Vilímek et al., 2005) and causing major destruction in Huaraz. On the other hand, the lower lakes can serve as a retention basin and possibly prevent a destructive flood downstream. During the Artizón Bajo Glacier flood in February 2012 in the Santa Cruz Valley, the flood water drained into the debris-cone- dammed lakes Jatuncocha and Ichicocha (Figure 1) with only minor destruction in the valley downstream. The flood itself was triggered by a landslide into the upper lake, Artizón Alto (4750 m asl), which then drained into the lower lake, Artizón Bajo (4500 m asl) (cf. Emmer et al., 2014). This scenario might be emblematic for future lake-related hazards if glacier retreat continues. The upper new lakes at altitudes of 4700–5200 m may initiate the outburst of the lower lakes, mostly located directly downstream and separated by a steep confluence step.
1 Topographic setting of the lakes with reference to their hazard potential
The recurrent spatial arrangement of individual lake types allows the distinction of four principal distribution areas of modern lakes in the upper catchment areas in the context of the hazard potential: M-, T-, S-, and P-type settings (Figures 2 and 16). The differentiation is based on their topographic setting, the longitudinal valley profile downstream of the lakes, and the spatial relation of the lakes to the settlement areas, which are key factors for the hazard potential. All the permanent settlements are located in the Rio Santa valley, and not in the transversal valleys.
M-type setting
The lakes are situated in the upper heads of the main valleys. Among them are in particular the GEM lakes, which have been proved to be most hazardous in the past. The longitudinal valley profile is characterized by an extreme steepening in the headwaters (>50°) with maximum peak elevations of >6000 m asl. A drastic change in slope starts about 500 m below the recent Equilibrium Line Altitude. The longitudinal profile is then characterized by a remarkable continuous topographical levelling. The valleys show a vertical relief amplitude of only 300–400 m over a horizontal distance of 10 km. Minor rock steps in the longitudinal profile arise frequently and may occur in combination with lakes. The gently inclined valley bottoms, stretching over a horizontal distance of 15–20 km, offer favorable conditions for lake development (low-amplitude moraine-dammed lakes, sediment-cone dammed lakes). In the case of an outburst of the upper lake, they may play a crucial role in the development of the flood in regard to mitigating or even aggravating the effects of the initial lake outburst.
In the southern part of the Cordillera Blanca, the transversal valleys enter the Rio Santa Valley without a pronounced break in the longitudinal profile (e.g. Rajucolta Valley). In some cases, the valleys located North of Huaraz have a marked stepped longitudinal profile (e.g. Santa Cruz and Parón Valleys) or even gorge-like sections (e.g. Cedros Valley) in their lower part. In the case of a lake outburst, these steeper valley sections may accelerate the flood water.
T-type setting
The lakes are located at the Pleistocene trough shoulders and are chiefly small moraine-dammed ponds (Figure 7). They drain along the steep trough valley flanks (>45°) over a distance of only 1.5–2.5 km, such as at the northern valley flanks of Quilcayhuanca and Cojup Valleys, and the southern side of the Santa Cruz Valley (Figure 1). The vertical amplitude between the lakes and the valley bottom of the main valleys ranges mostly between 800 and 1000 m. The locations in the main valleys beneath these lakes are generally not permanently inhabited. The major groups of people affected by an outburst are temporary local dwellers and tourists in locations such as campsites. The lakes themselves are not that hazardous, but when draining into a lake in the main valley they might trigger a larger lake outburst flood. This could be the case for the moraine-dammed pond at the South Side of Aguas Nevados (Parón Valley) perched at the valley flanks above Laguna Parón.
S-type setting
The lakes are positioned at the head of the short and steeply inclined valleys, which are incised into the western slope of the Cordillera Blanca. They run in a NW–SE direction parallel to the main valleys. They are generally less than 10 km long and possess a steep and often markedly stepped longitudinal valley profile due to glacial overdeepening processes. They may show a vertical relief amplitude of 2000 m over a horizontal distance of only 10 km. The lakes are located in the upper valley head at an altitude of 4600–4200 m asl with catchment areas rising above 6000 m. Lakes occur here especially as combined lakes (bedrock- and moraine-dammed lakes). The moraine-rock-dammed lakes Cullicocha-Rajucocha drain from an elevation of 4630 m to the Rio Santa Valley at 1850 m asl over a distance of only 9.5 km (Figure 13B). Other examples of lakes in the S-type setting are Laguna 513 in the Chucchún Valley, Laguna Auquiscocha in the Catay Valley, and Laguna Churup, located directly East of Huaraz (Figure 1). These types of lakes are particularly hazardous as the settlement zones are in close proximity to the lakes. The Huascaran-ice avalanche of 1970 originated from such a valley type at the W-face of Huascaran Norte (6652 m), causing a death toll of about 6000 people in Yungay (Evans et al., 2009).
P-type setting
These lakes are situated in the piedmont area in the Cajón de Huaylas. The current piedmont lakes are mainly found in the upper part of the Rio Santa Valley, at an altitude of 4300–4400 m asl. They generally have a very low hazard risk, unless an upper lake outburst initiates an outburst cascade.
VII Implications for the distribution of glacial lakes throughout the Andes
At a global scale, two of the major glacial lake types at high-mountain settings, moraine-dammed lakes and glacier-dammed lakes, occur in specific mountain ranges. The key distribution areas of proglacial moraine-dammed lakes are in the Central and Eastern Himalayas (ICIMOD, 2011; Richardson and Reynolds, 2000), in British Columbia (Clague and Evans, 2000), and in the outertropical Andes. In other high-mountain regions, moraine-dammed lakes are almost absent, such as in the heavily glaciated Karakoram-Hindukush Mountains (Iturrizaga, 2011a, 2011b). In this mountain range, glacier-dammed lakes, impounded by cross-valley tributary glacier tongues, are the main temporary glacial lake type (Hewitt, 1982). During the Little Ice Age, glacier-dammed lakes were much more widespread when tributary glacier tongues sealed off the main rivers. When looking at the Andes along a N–S-profile, a distinct spatial distribution pattern of moraine-dammed and glacier-dammed lakes can also be determined (Figure 19). Even though glacier recession is dominant throughout the Andes (Le Quesne et al., 2009; Rabatel et al., 2013), some glaciers are prone to form proglacial moraine-dammed lakes.
Distribution of main glacial lake types across the Andes.
Glacial lakes are mainly absent in the Inner Tropics (2°N–8°S) due to the high ELA in combination with unfavorable topographical conditions for lake formation on the steep-sided volcanoes. In the Outer Tropics, a major concentration of high-altitude moraine-dammed lakes occurs in the Peruvian and Bolivian Andes (8–18°S), mainly above elevations of 4000 m asl. The satellite analysis shows that glacial lakes in general occur at an altitude of 3500–5000 m asl, with a core zone of 4300–4600 m asl in the Peruvian Andes. Proglacial lakes, similar to those of the Cordillera Blanca (intra-montane type with high-amplitude moraines in a confined valley setting) emerge only to a larger extent in the Cordillera Huayhuash (10°11–16’S) at 4050–4750 m asl, Cordillera Huaytapallana (11°55’S) at 4600–4700 m asl, Cordillera Vilcabamba (13°10–27’S) at 4400 m asl, and Cordillera Vilcanota (13°39’–14°29’S) at 4350–5000 m asl. The Cordillera Apolobamba (14°35–45’S) in the southern part of Peru and the Cordilleras Real (15°48’–16°20’S) in Bolivia, apart from small glacial erosional and moraine-dammed lakes in the higher catchments, show mainly piedmont lakes down to an elevation of about 4200 m. They may reach a length of up to 16 km (e.g. Laguna Sibinacocha in the Cordillera Vilcanota). They often occur as impressive combined lakes, composed of two or more lakes. In the mountain ranges of Peru, which rise only slightly above the Equilibrium Altitude Line with elevations in the range of 5500 m asl, glacial erosional lakes, sometimes with superimposed moraines, prevail – e.g. Cordillera Conchucos or Rosko (8°15–30°S), 4100–4600 m asl, and Cordillera Carabaya (14°00–22’S, Nevado Allincapac 5780 m asl), 3700–4800 m asl. Large parts of the mountain ranges are currently deglaciated and the lakes in terms of their water supply are decoupled glacial lakes.
In the Desert Andes (20–30°S), glaciers and thus glacial lakes are mainly absent due to the aridity and topographical constraints, leading to the highest Equilibrium Line Altitude worldwide, such as at the volcanoes Llullaillaco (6739 m) and Nevado Ojos del Salado (6893 m). In the Dry Central Andes (32–35°S), debris-covered glaciers reach a length of more than 12 km, but nevertheless moraine-dammed glacial lakes scarcely occur. Their absence might be partly explained by frequent glacier oscillations in length, which are unfavorable for the formation of end moraines. The dynamic glacier behavior is reflected in extreme cases by glacier surges of several kilometers in length (cf. Espizúa, 1986). Most characteristic for the Dry Central Andes are temporary glacier-dammed lakes at an elevation of 2650–3250 m asl. They are caused by the advance of tributary glaciers blocking the main river. The last lake impoundment was produced by the Grande del Nevado del Plomo in 1985 (Lleiva, 1999). The largest contemporary glacier-dammed lakes are developed in low-altitude, extra-montane unconfined mountain foreland settings below 1000 m asl at the outlet glaciers of the Patagonian Ice Sheets with the occurrence of recent lake outbursts (cf. Dussaillant et al., 2010). Modern moraine-dammed lakes (Worni et al., 2012) occur at retreating glacier tongues as well as at the Pleistocene piedmont moraines (38–52°S). In the Andes, the largest glacial lakes are formed by piedmont lakes, such as the Lago Argentino (1490 km2).
This overview shows that the distribution pattern of the lakes in relief-confined high-mountain settings is mainly a function of the glacier type, including the climatic-controlled nourishment conditions of the glacier, the glacier size, the topographical conditions, and the debris transfer system. Thus, despite the global trend of glacier retreat, a diverse pattern in the type of glacial lake formation occurs.
Most of the investigations on the formation of moraine-dammed lakes originate from the Himalayas (Benn et al., 2012; Watanabe et al., 2009). In this context, it is important to point out that the lakes of the Cordillera Blanca differ from those of the Himalayas. Several thousand lakes with lengths of up to 6 km formed in the Himalayas, especially in Nepal, Bhutan, and China (ICIMOD, 2011) at elevation levels between 4500 and 5500 m asl. The glaciers stretch upstream of the moraine-dammed lakes (e.g. Tsho Ropa Lake at the Trakarding Glacier, Rolwaling Himalaya, Nepal) for over 10 km and show ice thicknesses of several hundred meters in their ablation zones. The Khumbu glacier is still 400 m thick below the icefall (Gades et al., 2000). Therefore a large potential for further lake expansion exists. Some of the Himalayan glacial lakes are still in development (e.g. Imja Glacier Lake, Mount Everest, Nepal), half a century after their onset of formation. In contrast, the Cordillera Blanca glaciers are only several kilometers in length, and possess an average thickness of 31 m according to Ames et al. (1989). The glacial lakes have a limited development potential and in some cases almost reach their maximum length after just one decade of development. Due to small glacier size, the glacial hydrological system is not that complex (cf. Benn et al., 2012). Sudden sub- and englacial meltwater drainage does not play a vital role in outburst risk. Moreover, the Himalayan moraine dams are commonly built up by prominent end-moraine arcs, but they may also be composed of an extensive hummocky moraine landscape which contains ice cores/permafrost, resulting in different lake outburst risks. Although various glacial lake outbursts have occurred, they have not been as devastating as in the Peruvian Andes as they have affected less-populated mountain areas.
VIII Conclusions and outlook
This paper presents a synopsis of the principal types of lakes with their characteristic features in the context of their spatiotemporal distribution in the Cordillera Blanca. The uniform topographical setting with similar glacier types and comparable former glaciation extensions leads to a recurrent pattern of contemporary lakes in the altitudinal zonation. From the Pleistocene onwards, geomorphological evidence shows that lakes have always been a determining landscape element in the Cordillera Blanca. The ample distribution of paleolakes provides great potential for further studies in order to refine time chronologies of the Quaternary glacial sequences. During the Holocene, the main valleys might have even been partly more difficult to access due to the occurrence of lakes. Holocene glacial lake outbursts may have contributed to the formation of the mass-wasting dominated sediment fans in the Rio Santa Valley, on which major settlement concentrations are located today (cf. Bonnot et al., 1988). The distribution the lake types can be linked to some extent to distinctive altitudinal levels. The present hazardous moraine- and bedrock-dammed lakes, which began to evolve at least since the mid-19th century, generally occur within 5 km from the highest catchments with a core distribution zone between 4300 and 4700 m asl. Some of the larger hazardous lakes are in the critical transformation phase of valley glaciers to decoupled proglacial lakes with perched glacier tongues above the lake and unstable residual glacier cones at the base. Moraine-dammed ponds and supraglacial lakes at clean ice glaciers are found at elevations between 4800 and 5000 m asl. Debris-dammed lakes (3800–4000 m asl) are the most common lake type in the middle part of the valleys. Future studies may put a more detailed focus on the specific characteristics of the formation of glacial lakes in tropical mountain environments, such as the glaciological constraints for the formation of the supraglacial lake clusters on clean ice glaciers and their potential to form larger lakes, or the presence of ice cores in the moraines close to the ELA.
Glacial lake-related hazards are a key challenge for planning strategies in the Cordillera Blanca and highly complex in terms of the institutional, political, and social structures as well as in the context of risk perception from different actors (Carey et al., 2012; Hegglin and Huggel, 2008). For an appropriate hazard assessment of the glacial lakes a holistic approach, which considers the lakes in their topographic setting and spatial arrangement to each other, is necessary alongsideinvestigating the individual lake in detail. The hazard potential of more inconspicuous small and shallow lakes has been highlighted, such as the moraine-dammed ponds in the S-type setting. The latter are generally of low hazard risk, but may become hazardous if they are triggering or involved in an outburst cascade. Potential outburst cascades may become more complex as new lakes might emerge at higher altitudes, such as the recent 2012 Laguna Artízon Baja/Alto event.
Major settlement zones are located in highly hazardous flood-prone areas directly in the impact zone of glacial lake outbursts. The maintenance and improvement of technical measures, such as remedial works at the lakes and early-warning systems, are an urgent task. However, one has to keep in mind that they commonly only lower the risk of a devastating lake outburst. Considering the long life span of proglacial lakes and the complexity of outburst triggers in a seismically active region, the efficient use of the limited available safe settlement area, such as the reorganization of the internal spatial arrangement of housing areas and possible resettlement to safer locations, combined with educational schemes on the hazard situation, seems to be a key priority for sound hazard management in the longterm.
Footnotes
Acknowledgements
My thanks go to Reynaldo Charrier for his support during my research stay at the Universidad de Chile and Universidad Andrés Bello in Santiago de Chile. I am most indebted to Marco Zapata and the staff of the Unidad de Glaciología in Huaraz (Ancash), especially to Alejo Cochachin Rapre, for sharing their knowledge about the present situation on glacial lakes in the Cordillera Blanca and for access to the library. I wish to thank Roger Salazar (Pira, Ancash) and Jesús Falcón (Vaqueria, Ancash) for their assistance in the field and the staff from Albergue Churup (Huaraz) for the organization of the fieldwork. I would like to thank Adam Emmer, Wilfried Haeberli, and two anonymous reviewers for their highly constructive and helpful comments on the manuscript. I am thankful to Heike Baesecke (Winthrop, USA) for final corrections.
Funding
Research work was partly financed by the Alexander von Humboldt Foundation in the framework of a Feodor Lynen Research Fellowship.