Historical dredge mining as a significant anthropomorphic agent in river systems: A case study from south-eastern Australia

Abstract

Bucket dredging to mine and extract gold and tin from rivers is a global industry that has had a range of negative effects on physical environments. These include the destruction of riparian soil profiles and structures, artificial channel straightening and loss of in-stream biodiversity. In this paper we evaluate the immediate effects and long-term consequences of bucket dredging on rivers in Victoria and New South Wales during the period 1900–1950. High quality historical sources on dredge mining are integrated with geospatial datasets, aerial imagery and geomorphological data to analyse the scale of the dredging industry, evidence for disturbance to river channels and floodplains and current land use in dredged areas. The study demonstrates that the environmental impact of dredging was altered but not reduced by anti-pollution regulations intended to control dredging. An assessment of river condition 70–100 years after dredge mining ceased indicates that floodplains and river channels continue to show the effects of dredging, including bank erosion, sediment slugs, compromised habitat and reduced agricultural productivity. These findings have significant implications for river and floodplain management.

Keywords

Australia dredging late-Holocene New South Wales rivers Victoria

Introduction

The enduring impact of historical mining on rivers globally is increasingly well-documented. Extensive research has identified the long-term effects of mining on river hydrology and associated contaminant loads in floodplain sediments (e.g. Clement et al., 2017; Dennis et al., 2003; Foulds et al., 2013, 2014; Gilbert, 1917; Hudson-Edwards et al., 1995, 2001, 2005; James, 1999; Lewin and Macklin, 1987; Macklin, 1996; Macklin et al., 2006; Macklin and Lewin, 2018; Rohe, 1983; Singer et al., 2013). A lesser-known dimension of historical mining activity is the use of bucket dredges to process metalliferous gravels in riverbeds and floodplains. Dredge mining disrupts rivers by excavating beds and banks to depths of up to 40 metres, discharging fine sediments as waterborne waste and redepositing gravels on-site as unsorted and consolidated masses that are vulnerable to erosion and devoid of organic matter.

Dredge mining has been used around the world in locations where metals, particularly gold and tin, occur in alluvial (placer) floodplain deposits, including Asia, North and South America, Africa and Australia and New Zealand. In Thailand, dredge mining has contributed to the degradation of an estimated 480 km² of land (Tanpibal and Sahunalu, 1989: 21) while in Malaysia, unremediated dredge tailings result in a ‘landscape scale patchwork of sand slimes and open water’ (Tompkins, 2003: 55). The remediation of dredge-mined land in the tropical conditions of Indonesia and Malaysia has also been the subject of studies by soil scientists and foresters (Agus et al., 2019; Ang, 1994).

The long-term effect of dredge mining on rivers has yet to be systematically studied. Impacts include the destruction of riparian soil profiles and structures, artificial channel straightening and loss of sinuosity, declines in in-stream biodiversity, changes in riverbank fabric and increased erosion from channel boundaries, all of which have lasting implications for river structure and functioning, ecosystem health and agricultural productivity. Dredging generally occurred in the upper portions of catchments, in regions often assumed to have experienced less anthropogenic disruption. Dredging also profoundly affected floodplains which had further consequences for the functioning and evolution of river systems.

In this paper we evaluate the immediate effects and long-term consequences of bucket dredging on rivers with a multidisciplinary case study drawn from rivers of mainland south-eastern Australia. The time elapsed since the peak of the industry in the early 20th century and the high quality of archival records facilitate a longitudinal study of river change and re-adjustment. The study integrates historic documentary sources, geospatial datasets and aerial imagery to analyse the scale and extent of the dredging industry, evidence for river and floodplain disturbance and current land use in dredged areas. This approach allows the historical causes and timing of river disturbance to be matched with assessments of river condition in the present day, 70 to 100 years after mining ceased.

Dredge mining is only one of numerous major interventions to which Australian rivers have been subjected since the arrival of Europeans in 1788 (e.g. Gell et al., 2007; Maheshwari et al., 1995; Portenga et al., 2016; Rutherfurd et al., 2020). Waterways have been dammed, diverted, channelized, de-snagged and deepened. Irrigation schemes have diverted significant volumes of water between catchments, while extensive erosion during the settlement period has generated and transported immense quantities of sediment. These changes have typically been ascribed to land clearance for agriculture, forestry, river navigation, urban and regional water supply and power generation. The effects of mining on Australian river systems, however, have received little attention despite the economic importance of the historical mining industry and the well-documented effects of modern mining on the physical environment. Recent research has indicated that the legacy effects of mining on Australian rivers has been both widespread and significant, including the discharge of hundreds of millions of tonnes of mining waste into Victorian waterways during the colony’s 19th-century gold-mining boom (Davies et al., 2018b; Mudd, 2013). Historical metals mining also affected rivers in New South Wales, Queensland and Tasmania (Kerr, 1989; Knighton, 1989; Wegner, 2009). Much of the associated sediment and contaminants remain in river systems today as floodplain deposits and sediment pulses or sand slugs.

While the environmental impacts of modern mining receive considerable public and scientific scrutiny, the enduring legacy of historical mining remains largely unrecognised and overlooked by management authorities. Historical mining played a significant role in anthropogenic river change in Australia, including the discharge to rivers of sediment liberated by surface alluvial and underground mining (Davies et al., 2018b, 2020; Rutherfurd et al., 2020). Recognising the extent of riparian disturbance caused by dredging provides further perspective on the anthropogenic transformation of river systems. Other historical modifications to waterways such as the installation of dams and weirs, de-snagging, channelization and wetland drainage are widely recognised as altering the functioning of rivers today (Macklin and Lewin, 2018; Wohl, 2004). Dredging had comparable effects, but its impact has been forgotten in Australia because the industry ceased operating many years ago. Many of the most intensively dredged rivers, including the Ovens and Loddon in Victoria, are part of the Murray-Darling Basin, Australia’s largest and most economically important river system. Understanding the legacy effects of dredging is highly relevant to river management today.

Historical context

Mining dredges were floating factories used to extract and process minerals from alluvial sediments. Two main kinds of technology were used: bucket dredges and hydraulic pump dredges. Both methods were efficient at working low grade ore deposits at minimal expense. In each case the engines, pumps and other machinery were mounted on the deck of a floating barge or pontoon (Murray, 1901; OMA, 1899). Bucket dredges, the focus of this paper, operated either within a river channel or in a pond adjacent to the river (Figure 1). A continuous chain of buckets attached to a movable frame extended from the front of the barge to excavate a working face. The buckets carried excavated material onto the barge for processing, where it was washed through a large, perforated rotating trommel or drum and then onto separating tables to concentrate the mineral. In the early years of the industry, unconsolidated spoil was dumped out the rear of the dredge to fill in the pit behind as the plant moved forward. Later operations, however, stripped and stacked topsoil and used tailings elevators to distribute finer dredged material on top of coarser tailings to remediate dredged ground. The extent to which mercury was used in processing for gold amalgamation is at present unclear although there is evidence that some operators reprocessed dredge concentrates using mercury barrels and copper plates (Watson, 1938). Bucket dredges gradually increased in size and capacity to eventually work rivers on a massive scale. They were most suited to working in partly confined valleys where minerals were concentrated in deep alluvial deposits of gravels, sands and silts.

Figure 1.

Tronoh bucket dredge on the Ovens River at Harrietville in north-east Victoria in 1948.

Bucket dredging originated on the Otago goldfields of New Zealand’s South Island in the 19th century. Miners in the region adapted techniques used to dredge silt from harbours and waterways to work the rich alluvial gravels in the beds of Otago’s large snow-melt rivers. The industry boomed with over 240 dredge companies operating by 1902, mainly in Otago and Southland (Burton, 1902; Hearn and Hargreaves, 1985). By the early 20th century bucket dredging had spread around the world including to the Philippines, Mexico, Brazil, Chile, Italy, Burma and Papua New Guinea (Aubury, 1910; Nelson, 1976; Waterhouse, 2010). Dredging for gold was also well established in the United States and Canada, with operations in California (Figure 2), Montana, Idaho, Oregon, Colorado, Alaska and the Yukon (Aubury, 1908; Morse, 2003; Spence, 1996, 2016). The potential to dredge for tin was also widely recognised and tin dredging was carried out in Australia, Nigeria, Thailand, Indonesia, Malaysia and elsewhere in southeast Asia from 1906 onward (Fell, 1939; Hillman, 2005; Kaur and Diehl, 1996; Ross, 2014: 465–468; Yip, 1969: 132–138).

Figure 2.

Dredged gravels along the Yuba River, Yuba City, California. The ridges are the result of spoil stacked by the elevator at the rear of the dredge (photo S. Lawrence).

The first dredges in New South Wales and Victoria began operating around 1899. Dredging technology was popular in eastern Australia where gold yields from more conventional methods were in decline by the end of the 19th century (Clift, 1975). Bucket dredges revived the gold mining industry, especially in Victoria, originally the powerhouse of Australian metals mining. Victoria had experienced one of the world’s richest alluvial gold rushes in the 1850s and subsequently produced about 2% of all the gold ever mined globally, mostly prior to 1914. Approximately 60% of Victorian gold production has been from secondary alluvial (placer) deposits, including deep leads, with the remaining 40% from primary quartz-vein sources (Phillips et al., 2003: 379–380).

Bucket dredging processed large areas of alluvial ground with few workers and low operating expenses. Operations typically covered their costs from yields as small as 15–20 Troy ounces (0.46–0.62 kg) of gold per week (Lloyd, 2006: 166). At the peak of activity in Victoria in 1910, the 58 operating dredges averaged just 0.15 grams of gold per cubic metre of material treated (Sellars, 1911). The consequence of this efficiency was enormous volumes of alluvium mobilised to recover payable gold. Each dredge worked on average 4–5 hectares of ground per year, generally to a depth of 3–6 metres. By this period the volume of material processed by each dredge generally ranged between 3,800 and 10,000 m³ per week (Sellars, 1914).

The dredging boom was short-lived, however, and the number of dredges declined dramatically during the First World War. The industry had virtually ceased by the mid-1920s but slowly revived during the 1930s in response to higher gold prices. There were fewer dredges, but operations were generally much larger and more efficient than before. By the late 1950s dredging had effectively ceased in Victoria and New South Wales although tin dredging continued in Queensland into the 1980s (Wegner, 2009). The earliest dredges were largely unregulated and public opposition to the environmental destruction they caused was immediate and intense. Dredging ultimately provided the catalyst for Australia’s first legislation to impose environmental controls on the mining industry (Lawrence and Davies, 2014, 2019). From 1904 Victoria introduced legislation to regulate dredging, with New South Wales following in 1907. Queensland and Tasmania did not implement measures to protect the environment from dredging until the 1940s (Wegner, 2009).

Scale of the industry

The scale and environmental impact of bucket dredging can be seen in the number and geographical distribution of dredges and the volume of sediment disturbed.

Geographical distribution of bucket dredging

The centre of the dredging industry in Australia was along waterways draining the Great Dividing Range north and south of the River Murray. At least 104 bucket dredges operated in Victoria between 1899 and 1955, while 68 operated in New South Wales over a similar period (Clift, 1975; Davies et al., 2018a). The industries in Queensland and Tasmania were much smaller with only a few dredges operating in each state. Several Victorian dredges operated in rivers that drained south into Bass Straight, including the Tambo, Mitchell and Thomson catchments in Gippsland, but most were in rivers flowing north into the Murray, with activity concentrated along the Loddon, Mitta Mitta, Kiewa and Ovens/Buckland catchments (Figure 3). In New South Wales bucket dredges were active in the Shoalhaven, Moruya and Clarence valleys draining east into the Tasman Sea and in the upper portions of rivers forming part of the Murray-Darling Basin, including the Murrumbidgee, Macquarie and Lachlan catchments (Figure 4). Bucket dredging for tin occurred in tropical north Queensland from 1939 until the 1980s, principally on the Herbert River which drains east into the Coral Sea and the Great Barrier Reef (Wegner, 2009). In Tasmania, suction dredges worked alluvial tin deposits at several locations from the turn of the century, including in the Ringarooma catchment in the north-east (Kerr, 1989).

Figure 3.

Map of Victoria with location of bucket dredge operations.

Figure 4.

Map of New South Wales with location of bucket dredge operations (after Clift, 1975).

Bucket dredging thus affected both coastal rivers and inland rivers of the Murray-Darling Basin. The industry operated in rivers with comparatively low rainfall and low flows as well as those in higher rainfall areas including tropical north Queensland and the New South Wales south coast. In several of these areas dredging was limited to only one or two operations. In other catchments, such as the Ovens in north-east Victoria, virtually contiguous leases covered extended reaches of the river, including 50 km from Harrietville downstream to Myrtleford (Figure 5). Most dredging occurred in streams that were 4th order using a Strahler classification and on average 37 km from the top of the catchment. Catchment area varied enormously, however, between 1.5 and 5,007 km², with an average of 426 km². Some dredged reaches were thus only 2.5 km from the top of the catchment while others were up to 220 km from the top. The total length of dredged reaches in Victoria is 118.2 km, which is 0.5% of the 30,000 km of major streams in the state. The real figure is likely higher because of our incomplete knowledge of the extent of dredging operations at each lease.

Figure 5.

Bucket dredging leases on the Ovens and Buckland Rivers in north-east Victoria.

Sediment volumes

A proxy for estimating the effect of bucket dredging on river valleys is the volume of alluvial material processed and returned to river systems as waste. The Department of Mines in Victoria recorded detailed statistics of material treated by individual dredging operations (measured in cubic yards) and published the results under the category ‘Dredge Mining and Hydraulic Sluicing’ in the series Annual Report of the Secretary for Mines (1900–1919) and Gold and Mineral Statistics (1920–1955). Aggregation of these annual volumes reveals that the total volume of alluvial sediment processed by bucket dredging in Victoria between 1900 and 1957 was at least 182 million m³. Bucket dredges in New South Wales processed at least 90 million m³ between 1899 and 1958 (Clift, 1975). The increased efficiency of dredges in the final phase from the 1930s to 1950s is apparent in the large volumes of sediment produced by a comparatively small number of operations (Figure 6).

Figure 6.

Annual bucket dredging operations and sediment volumes for Victoria 1900–1955 and for New South Wales 1900–1958.

Bucket dredging produced far greater quantities of sediment per unit of gold recovered than other methods of mining. Our previous analysis of historical records from the 19th century indicates that gold mining, prior to dredging, produced at least 650 million m³ of sediment between 1851 and 1900, all of which was dumped as tailings into nearby watercourses (Davies et al., 2018b). Hydraulic pump dredging was a further source of debris in rivers during this period, with available data from Victoria showing 55 million m³ of material discharged between 1900 and 1915. Bucket dredging thus mobilised around 20% (182 million m³ out of c.887 million m³) of the alluvial sediment produced by the gold mining industry in Victoria. At the same time, the 33 tonnes of gold obtained from bucket dredging represented only 1.3% of the total Victorian historical gold production of approximately 2500 tonnes (Phillips et al., 2003).

Figure 7 presents data relating to the volume of alluvium processed per ounce of gold recovered in Victoria: mining 1859–1891, bucket dredging 1901–1915 and bucket dredging 1949–1955 (underground mining and surface alluvial mining other than dredging were dramatically reduced by the First World War). Alluvial gold yields were at their highest at the start of the gold rush in the 1850s, exceeding more than 2 million ounces (62 tonnes) per annum, but declined rapidly thereafter (Serle, 1968). Bucket dredging in the years prior to the First World War generally yielded much smaller amounts, between 20,000 and 50,000 troy ounces (622–1555 kg) per annum. At the peak of the gold rush in the late 1850s miners needed to wash on average only 3 m³ of alluvium for each ounce of gold. This increased steadily through the following decades until around 50 m³ was washed per ounce. With the introduction of bucket dredging in 1900, however, volumes of alluvium increased dramatically to 160–200 m³ or more for each ounce of gold. In the final phase of dredging in the 1950s the industry processed 200–380 m³ per ounce. Bucket dredges thus processed much larger volumes of alluvial material to recover proportionately smaller amounts of gold than miners in the 19th century.

Figure 7.

Annual gold yield from mining in Victoria and volume of sediment mobilised per ounce, 1859–1891, 1900–1915 and 1949–1955.

Data on volumes of sediment mobilised by individual bucket dredges in Victoria are also available for the years 1903 to 1915 and for the largest dredges that operated after the Second World War. Aggregating these data with the location of dredges provides evidence for the volume of alluvial material processed by the industry within specific river catchments (Figure 8; Table 1).

Figure 8.

Recorded volumes of alluvium processed by bucket dredging in Victorian river catchments 1903–1915 and 1938–1957 and in NSW river catchments 1899–1958.

Table 1.

Bucket dredging operations and recorded volumes of processed alluvium for river catchments (shaded in grey columns) in Victoria and New South Wales.

Victoria	Bucket dredges	Alluvium processed m³	NSW	Bucket dredges	Alluvium processed m³
Avoca	2	5,066,000	Clarence	5	255,000
Goulburn	2	1,169,000	Macquarie	16	54,224,000
Kiewa	6	3,391,000	Moruya	17	17,197,000
Loddon	14	29,270,000	Murray	2	42,000
Mitchell	2	699,000	Murrumbidgee	4	11,442,000
Mitta Mitta	10	13,940,000	Namoi	2	750,000
Moorabool	1	178,000	Shoalhaven	15	6,069,000
Ovens/Buckland	59	123,747,000	Snowy	4	497,000
Snowy	1	54,000	Tuross	2	522,000
Tambo	1	420,000	Wagonga	1	n/a
Thomson	1	135,000

Sources: Clift (1975) [NSW]; Annual Report of the Secretary for Mines (1900–1919) and Gold and Mineral Statistics (1920–1955) [Victoria]; Canavan (1988), Lloyd (2006), McGeorge (1964); Table excludes four unknown dredging locations in Victoria; Volumes rounded to nearest 1000.

Bucket dredging in Victoria had its greatest impact on the Ovens River and its tributary, the Buckland River. Dredging operations between 1900 and 1955 processed at least 123 million m³ of alluvial deposits in this catchment, which represented 67.9% of the Victorian total. The Loddon River (29 million m³) accounted for 16% of the total, along with the Mitta Mitta (7.6%), Avoca (2.8%) and the Kiewa (1.9%). Rivers in Gippsland draining south into Bass Strait, including the Mitchell and Tambo, were each affected only moderately, with one bucket dredge recorded in each catchment. On the Snowy River, one dredge operated on the Victorian reach, along with four dredges upstream in New South Wales, although the overall impact was modest.

The uneven and limited coverage of available historical evidence from New South Wales means it is difficult to assess the full impact of bucket dredging on the state’s rivers (Clift, 1975). Nevertheless, based on available data, the Macquarie River was most affected, with 16 recorded bucket dredges processing 54 million m³ of alluvial material between 1899 and 1958. One dredging operation, Wellington Alluvials (1938–1958), contributed 37.6 million m³ (70%) of this total. Bucket dredges also intensively worked deposits along coastal rivers in the south-east of the state, including the Moruya (17 million m³) and the Shoalhaven (6 million m³).

Regulation of dredging

Environmental controls evolved slowly over the life of the industry, generally increasing as the economic and political influence of mining waned (Lawrence and Davies, 2019: 178–208). Regulations that constrained industry were matched by the development of more efficient technology that allowed dredges to excavate deeper into floodplain and channel gravels and work lower-grade ore deposits (Davies et al., 2018a). Regulation was greatest in Victoria and New South Wales and less in Tasmania and Queensland where laws to protect the environment were not passed until 1948 (Wegner, 2009: 208, 210).

The first phase of the industry spans the period between the introduction of dredging in 1899 and the start of regulations under Sludge Abatement Boards from 1905 in Victoria and 1907 in New South Wales. During this early period there were up to 26 bucket dredges operating in Victoria and 24 in New South Wales. Gold yields were high, with the Victorian Mines Department reporting that the bucket dredges working in 1905 produced 28,485 ounces (886 kg) of gold (Sellars, 1906: 87). At this stage the maximum depth of dredging was 8 metres, with most operations averaging depths of 4 to 6 metres. Dredges worked directly in river channels and as well as on adjacent banks and terraces. Waste was generally discharged directly into the rivers with no provision for settling dams. The heavier fraction of gravel and cobbles tended to remain in the dredge basin while finer silts and sands were washed downstream.

The second phase of the industry commenced with the introduction of environmental controls from 1906, which dictated where dredges were permitted to operate and led to changes in the technology used. Leases issued in Victoria from 1906 onwards prohibited bucket dredges from operating directly in river channels although plant working under earlier leases continued to do so. Under the direction of the Sludge Abatement Board, companies also began to experiment with methods for remediating dredged land. Topsoil was stripped and stacked prior to the commencement of dredging and then returned to the surface as works were completed. After a period for settling, the topsoil was mechanically levelled and reseeded with pasture grasses or orchard trees. These steps became routinely included in lease covenants from at least 1911 (Dredging Board, 1914: 6). The industry incorporated these modest restrictions and continued to expand, peaking between 1908 and 1913. More than 50 dredges operated each year in Victoria through this period with an average annual gold yield of 56,585 ounces (1760 kg). Each of these dredges was processing approximately 3800–10,000 m³ of alluvial material per week (Sellars, 1914: 78).

The third phase spans the period from the First World War until the end of commercial dredging in the southern states in the 1950s and in Queensland and Tasmania in the 1980s. During this phase dredging operations grew ever larger and more efficient (Figure 9). The new generation of bucket dredges routinely worked gravels up to 30 metres deep with the largest, the Tronoh at Harrietville in the Ovens valley, capable of reaching 40 metres. Heavier engineering meant that dredges could process larger boulders and more consolidated sediments (Lloyd, 2006: 199). Coupled with more efficient processing plants, this meant that new reaches of rivers with even lower-grade deposits could be dredged profitably. The Victoria Gold Dredging Company’s operation at Newstead on the Loddon in central Victoria, for example, was viable with an average yield of only 0.14 grams of gold per cubic metre. The operation moved nearly 15 million m³ of alluvial deposit over its 10 years of operation (McGeorge, 1964: 55). Dredges on the Herbert River in Queensland moved up to 3 million m³ a year in the 1940s and 1950s (Wegner, 2009: 204).

Figure 9.

Cocks Eldorado dredge on Reedy Creek (Ovens catchment) in north-east Victoria, 1936–37. The dredge is the large structure floating on the artificially impounded and inundated floodplain.

During this period environmental regulation was routine in Victoria and New South Wales. The apparent success of settling dams and remediation was such that it was used by the industry to justify its continued existence. The Mining and Geological Journal claimed in 1939 that dredged land had not only been restored to ‘a condition satisfactory for pastoral or agricultural purposes’, but in many cases it could be ‘materially improved’ (Dickinson, 1939: 13). Operations such as the Newstead dredge were industry leaders, their remediation efforts showcased in professional journals with photographs of lush paddocks and reports of high crop yields (Dickinson, 1939; McGeorge, 1964; Swift, 1950). Rising costs and falling mineral prices, however, brought an end to dredging in Victoria in the 1950s. Tin dredging continued in Queensland until the 1980s.

Documentary evidence of effect on rivers and floodplains

Contemporary sources provide numerous and diverse first-hand descriptions of direct onsite and indirect offsite impacts. Sources include annual reports from the Sludge Abatement Board and Annual Reports for the Secretary of Mines, articles in the Mining and Geological Journal, Parliamentary debates, newspaper accounts and witness testimony at inquiries including the Dredging and Sluicing Inquiry Board in 1913. The board described bucket dredging as ‘industrialised vandalism’, while the press called them ‘desolating dredges’ (Dredge Board, 1914: 18; The Age, 1909, 1912). These and many other descriptions provide evidence of the immediate effects of the industry and allow subsequent changes to be tracked through time. Observations describe activity in river channels where dredging was taking place, on adjacent floodplain terraces, and at downstream locations.

Dredging had a direct impact on stream beds and channels from the outset, including inversion of alluvial profiles in riverbeds and high instream sediment loads (Table 2). During the first phase of the industry many dredges worked directly in river channels, with fine silts and sands dumped at the base of dredge ponds or washed downstream while heavier gravels and cobbles were deposited at the surface. Logs and snags in channels were also removed to aid dredging operations, a process regarded at the time as beneficial to rivers as it reduced damage by floods. Discoloration of waters was also widely reported. With the beginning of regulation from 1905, earlier dredges continued working in streams, but new operations were confined to the terraces. Regulations specified that river channels worked by dredges had to remain open and free flowing, while riverbanks were to be maintained at their original height and condition (Sellars, 1907: 98–99). During this phase the primary effect on streams was the release of fine tailings sediment. Impact on streams intensified again in the 1940s and 1950s as larger operations were granted permission to divert rivers (Figures 10 and 11). New channels were constructed so that dredges could work the old stream beds, which were then backfilled and re-soiled. This approach limited the release of fine tailings into waterways but dramatically transformed river morphologies, replacing the original complex structure and habitat with simplified, channelized reaches, comprising banks of unconsolidated dredge gravels (Figure 12).

Table 2.

Historical evidence of dredging effects on stream channels.

Phase I, 1899–1905	Reference
. . .mining operations are carried on to a very large extent in the streams. . . the beds of some of the streams are being turned upside down in the course of dredging operations.	Hansard [Thomas Ashworth], 1903 vol. 105: 970
Bucket-dredging merely lifted stuff from the river, and put it back again. It cleared the stream, and deepened the stream, and removed all logs from it.	Hansard [Edward Miller], 1903 vol. 107: 2081
Phase II, 1906–1919
. . .the water of the Ovens, though still discoloured, has not been in such a condition as to cause damage or affect stock.	SAB 1911: 68, report for 1910
The removal of thousands of logs from the river and creek beds by these dredges in the course of their operations, is a much-needed improvement.	SAB 1911: 72, report for 1910
Phase III, 1920–1955
. . .the dredging of the [Ovens] river will be an eventual blessing, they say. At present it is filled with snags and bars of silt, and barriers of gravel. In its present condition it is easily flooded.	The Age (1934a: 9d)
To enable work to proceed a diversion of the Ovens River was affected in compliance with conditions laid down by the State Rivers and Water Supply Commission.	Swift (1950: 16)
This channel [diversion of the Loddon River], which was approximately one mile long, was constructed wholly through dredged land, and special precautions, such as the grassing of the banks, were necessary to guard against subsequent scouring.	McGeorge (1964: 53–54)

Source: Quotes in Tables 2 –4 are quoted as in the original sources: Hansard: parliamentary debates. Legislative Council and Legislative Assembly; MAM: mount Alexander mail; OMA: ovens and murray advertiser; SAB: sludge abatement board.

Figure 10.

Construction of channelized reach of Loddon River through the Victoria Gold Dredging Company lease at Newstead, 1945.

Figure 11.

LiDAR image of channelized reach of Loddon River through the Victoria Gold Dredging Company lease at Newstead, Victoria. Dredge tracks are visible on either side of the river.

Figure 12.

Unconsolidated gravels in dredge tailings exposed in channelized Loddon River diversion, Newstead, central Victoria. Note that this is a very unnatural deposit, with matrix supported gravels that extend close to the surface of the floodplain (photo I. Rutherfurd).

The proximal effect on floodplains traversed by dredging was severe (Table 3, Figure 12). By 1913 terraces were excavated to depths of 12 metres, and this increased to 30 metres or more by the 1940s (Lloyd, 2006: 204). In the first phase of operations, the soil stratigraphy was inverted with layers of fine silts often topped by coarse sediment, the result described as ‘a shingled beach’ and useless for any kind of agriculture (Dredging Board, 1914: 9). Additionally, the unconsolidated gravels that now formed the riverbanks were highly vulnerable to erosion during floods. As regulations developed there were attempts at reconstructing the original soil horizons and returning topsoil to the surface. Advance stripping, soil stockpiling and re-soiling with silt distributors was attempted by several dredge companies but improvements were limited. The resulting soil was described as ‘disintegrated’, and only around one-third of the original topsoil could be saved (Sellars, 1910: 112). Dredged areas also remained elevated above surrounding land due to the poor sorting and unconsolidated nature of the ground. In some locations dredging filled in old shafts and levelled spoil heaps, which encouraged proponents to argue that dredging was improving the land (Dickinson, 1939: 13; SAB 1909: 69). By the 1940s re-soiling was more sophisticated, with coarse gravels returned to the base of the profile followed by coarse sands and then topsoil. The ground remained very porous, said to be good for silviculture, and there was limited success in some places with re-planting grasses and clover for grazing (Dickinson, 1939: 17).

Table 3.

Historical evidence of dredging effects on adjacent floodplains.

Phase I, 1899–1905	Reference
The real damage through dredging [on the Mitta Mitta] is not caused so much by the sludge that is sent down, as by tearing up the country and making it loose. When the snow melts and the heavy floods occur in October, November and December, you have a stream, perhaps a mile wide and several feet deep, carrying everything away and ruining thousands of acres of land.	Hansard [Albert Craven], 1903 vol.105: 1023
The deposits had so filled up the [Loddon] watercourse that it quickly flowed over the adjacent low-lying lands on which he found sludge, clay and silt covering the grass.	MAM (1905b: 2f)
Phase II, 1906–1919
[The re-soiler at Sandy Creek on the Ovens delivers] top soil in a practically dry and satisfactory condition on to the gravel, is fairly effectively restoring the surface ground. . .	SAB (1910: 72), report for 1909
. . .the top soil of to-day [near Eurobin on the Ovens] is composed largely of rather saline silt brought down by flood waters since the inception of mining and other settlement, and that the richest earth from an agricultural point of view is to be found 2 or 3 feet below the present surface, so that the mingling of the top 3 or 4 feet would be beneficial rather than injurious. . .	SAB (1909: 71), report for 1908
Probably about one-third of the alluvium [topsoil, on the Ovens] was conveyed by the belt and dumped in a dry and useful condition, one-third went by way of the well hole into the operating pond, and one-third was scooped up with the auriferous gravel by the buckets, and after passing through the sluice-box was discharged in the form of sludgy water. . .	SAB (1911: 74), report for 1910
. . .the soil becomes completely disintegrated; the position of the constituents are altered in relation to one another, the bottom becomes the top; and it is idle, in the opinion of the Board, to contend that land so treated is not seriously injured permanently. . .	Dredging Board (1914: 16)
Phase III, 1920–1955
It is a saying in the Ovens Valley that wherever pines and wattles are growing, there will be found dredged land.	The Age (1934b: 9e)
Dredged and resoiled some 30 years ago, this area [at Campbell’s Creek] now produces better fruit than any other part of the property, and has also grown prolific lucerne crops. Mr Robinson attributes its added fertility to the fact that dredging has broken up the hard clay sub-soil, so allowing a freer circulation of water and permitting the tree roots to spread to greater depths.	Dickinson (1939: 19)
The terrain thus formed [by dredging at Eldorado] cannot be said to be ideal for any particular purpose, except perhaps for pine or wattle plantations which have been very successfully grown under less favourable conditions.	Dickinson (1939: 17)
As there was no soil worthy of the name in the first place [due to previous mining at Eldorado], the only possibility is for top-dressing and planting with trees such as black wattles. . .Already parts of the settled areas are under vegetation and on some old dried-out settling dams the growth of couch grass is prolific. One area of ten acres was planted with black wattles about six years ago and some of these trees are now 20 feet high.	Swift (1950: 14)

The offsite, downstream effects of dredging were similar to impacts from other forms of gold mining (Table 4). Water-borne tailings increased turbidity for considerable distances, with the Ovens River described in 1906 as ‘a seething mass of muck’ (OMA, 1906). Samples taken by the Sludge Abatement Board between 1906 and 1914 provide an important and detailed historical record of water quality (Rutherfurd et al., 2020). Mine tailings filled waterholes in streambeds, covered banks with silt and eventually choked the channels. Floods carried sands and gravels further downstream and overflowed across floodplains. Following regulations in 1907, settling ponds were used but during floods these could be breached or overflow and remained as point sources of sediment. Improved settling dams were a feature of the final phase of dredge mining and companies took pride in the clarity of the water returned to streams (e.g. McGeorge, 1964: 73).

Table 4.

Historical evidence of downstream effects of dredging.

Phase I, 1899–1905	Reference
. . .the deposit of silt and sludge in the bed of the river [Loddon] had greatly increased. Instead of the bed being formed of sand and shingle, as previously it was full of mud and slime. . .	MAM (1905c: 2f)
On each side of the channel there are mudbanks. . ..Fryers Creek was practically filled up with sludge, sand and stones. Each flood took large quantities of this material down the river. . .	MAM (1905a: 2f)
The [Loddon] riverbed was then [prior to 1901] a bed of sand, gravel and natural black soil. There is a deposit of mud nearly 2 feet deep over portions of the bed. . .	MAM (1905): 2f
The natural channel of Campbell’s Creek had become entirely filled up and the flow was spread out and shallow.	MAM (1905b: 2f)
Phase II, 1906–1919
The material dealt with . . . being principally well-washed gritty sand and gravel, does not, when disturbed, travel far down the stream under normal conditions.	SAB (1911: 72), report for 1910
. . .under heavy flood influences the disintegration of the country by dredging may facilitate the escape of large bodies of worked material to lower levels. . .	Dredging Board (1914: 11)
Phase III, 1920–1955
A substantial levee bank along the [Loddon] river boundary serves as protection against possible flooding and all danger of stream pollution is virtually eliminated. . .	Dickinson (1939: 15)
. . .the [Cocks Eldorado] company has constructed six large slum dams, covering more than 70 acres, for settlement purposes. . .	Dickinson (1939: 17)
The majority of the discharge water from the dredges seeps back into the working paddocks. During its sub-surface journey the water is more or less completely filtered and enters the river carrying little or no sediment.	Dickinson (1939: 19)
. . .the dredge pond water contains up to 2,500 grains of solids per gallon [35 g/L] [but] its cleansing is affected quite simply. Large settling areas were built and . . . some of the sludge water was diverted through the tailings area of the Adelong Company. . . These tailings, about a mile in length by a quarter of a mile in width, acted as a perfect filter. The water passed out at the lower end without a trace of colour. . .	Swift (1950: 16)
. . .the Ovens River below Freeburgh. . .is crystal clear and there is absolutely no indication that two dredges are using its water a short distance upstream.	Swift (1950: 17)

Assessing the environmental legacy of gold dredging

The contemporary condition of Victorian rivers provides a measure of the environmental legacy of dredging 100 years after the industry began and 60 years after the last big dredges in Victoria ceased working. Most offsite impacts, which are indistinguishable from the effects of other forms of Victorian metals mining, are addressed elsewhere (Grove et al., 2019). It is important to note, however, the specific downstream impacts caused by unconsolidated floodplain sediments. Where rivers run through land that has been dredged, or in a reach that has been channelized because of dredging, the unconsolidated ground is more liable to erosion. This appears to be the case along the Loddon River where erosion from the channel within the former lease has caused a slug of sand and gravel to fill the riverbed downstream (Figure 13). Another large sand slug on Reedy Creek, a tributary of the Ovens River, is likely to be a combination of sediment from earlier sluicing operations and dredging at Eldorado early in the 20th century.

Figure 13.

Slug of sand and gravel in the Loddon River downstream from Victoria Gold Dredging Company lease at Newstead (photo S. Lawrence).

Analysis here focuses on the impacts of dredging on floodplains. Spatial data on dredging operations derive from two Geoscience Victoria databases, ‘VicProd’ and its derivative ‘VicMine’, which has more precise classification of mining types (Department of Primary Industries, 2002). Both databases have point data accuracy of up to +/− 25 m. A complementary set of spatial data used is Mineral Tenements, ‘MINTEN’, which shows mining lease areas (Department of Economic Development, Jobs, Transport and Resources, 2013). Of the 104 dredging operations identified from historical reports, 68 bucket dredging leaseholds can be spatially located. Dredge leaseholds were examined using LiDAR and aerial data from 2009–2010 (Figure 14). Not every leasehold was dredged, and in many leases only part of the area was mined by the dredge. The presence and impact of dredging on these sites was assessed from topographical changes shown by LiDAR, from vegetation changes, and from limited field inspections. Evidence of continuing physical evidence of dredging in rivers and floodplains includes dredge pools, depressions, linear dredge tracks, the absence of floodplain features such as palaeo-channels, the presence of channelized streams and differences in vegetation.

Figure 14.

Dredged agricultural land on the Ovens River near Freeburgh in north-east Victoria: (a) 2010 aerial photography. (b) 2010 LiDAR with a two times hillshade elevation exaggeration. The river is flowing north-west between the dredge leases. The LiDAR reveals linear dredge tracts in the bottom right section (Box 1) and a segment of palaeochannel in Box 2. (c) Shows linear dredge tracts in more detail. (d) Shows the palaeochannel on undredged floodplain in Box 2 in more detail.

Characteristics of dredged locations indicate the conditions favoured by dredge operators. Dredging exploited floodplain pockets where palaeo-gravels had accumulated. These are places where palaeochannels had sufficient energy to transport minerals while coarse floodplain sediments were confined by bedrock. Minerals carried downstream in the channels thus became concentrated in the sediments rather than being dispersed over an unconfined floodplain. At these locations, valley confinement and valley bottom flatness averaged 92%, providing room for dredges to manoeuvre and stack tailings (Gallant and Dowling, 2003; Stein et al., 2002).

LiDAR analysis of surface elevation differences identified evidence of dredging at 52 of the 68 sites where bucket dredging leaseholds have been spatially located. This may reflect the recovery of some dredged sites to the point where there is no longer any evidence visible using these methods. It may also reflect the fact that not all the lease areas were dredged. Of the 52 sites identified as being dredged, 14 had evidence of a dredge hole and 30 had multiple stripes left by the dredge tracks. The remainder had homogenised floodplains compared to proximal sites with features such as palaeochannels. In most cases re-soiling and remediation has been only partially successful (Figures 15 and 16). Three quarters of the sites are treed, 60% as woodland or scrub that shows patchy vegetation and bare soil, despite these being floodplain locations that should be prime agricultural land. Evidence for the continuing degradation of these sites is that only one third of the sites were producing economic returns in 2010 through plantation forestry, grazing or cropping and only 13% are being used for cropping, the highest-value use. Further evidence of the degraded state of these soils is anecdotal information from farmers. Along the Loddon River, for example, landholders report that dredged lands are inferior to their other land and susceptible to the formation of sinkholes.

Figure 15.

Present land use on sites (n = 52) where bucket dredging occurred in Victoria, identified using aerial imagery from 2010 and current satellite imagery.

Figure 16.

Dredged agricultural land on the Ovens River near Freeburgh, Victoria (note the gravels at the surface (photo P. Davies).

The depth of dredging, combined with the disturbed stratification of the sediment, means that bucket dredging appears to have significantly altered the productivity of the land and will continue to do so for decades if not centuries. This has consequences for riparian rehabilitation programs in addition to agricultural viability. Many revegetation programs rely on the germination of seed already present in the local soils yet there is little prospect of intact soils with seed banks surviving on dredged land.

Discussion and conclusion

Spatial analysis of dredging lease records indicates that river dredging in south-eastern Australia is far more widespread than previously realised, with rivers on both sides of the Great Dividing Range affected. Some catchments were severely affected, notably the Ovens and Loddon in Victoria and the Macquarie and Moruya in New South Wales. The impact of dredging changed over time as technology and regulation evolved and the effect on rivers will vary according to when the dredging took place. The effects, however, will be substantial regardless of when dredging occurred.

Mechanical dredging represents a substantial and little appreciated historical impact on rivers and floodplains in eastern Australia. This technology arrived late on the goldfields at the turn of the 20th century, at a time when yields from ground sluicing were in long term decline. Dredging was most substantial in Victoria where dredges produced over 20% of the sediment delivered to rivers but recovered just 1.3% of the total gold yield. This can be considered the ‘industrial’ phase of alluvial gold mining in which sophisticated technology and substantial capital allowed a few individuals to benefit from exploiting low grade mineral reserves, at the expense of substantial environmental degradation. However, dredging was also seen to be a useful land use for areas that had already been damaged by mining, and it appears that dredging did improve land in some cases. In addition, dredging experienced the most substantial regulation of any alluvial gold mining practice.

Three distinct phases of dredging have been identified, each of which affected rivers in specific ways (Table 5). Knowing when dredging operations were carried out thus provides an early indicator of the kind of disturbance likely. During the earlier phases, rudimentary technology meant that dredging was shallower and the depth to which soil structure was disturbed is less. Due to the lack of regulation during this period, however, dredging took place directly in river channels and there was neither retention of tailings nor any attempt at remediation. The overall impact on channels, floodplains and downstream reaches during this phase was considerable. During the later phases more efficient technology and tighter regulation changed where and how dredging occurred but did not alter the overall environmental result. Engineering became more robust which allowed dredges to reach greater depths than has generally been appreciated. Later dredges were thus able to excavate as much as 40 m below the surface, churning the entire soil profile even when preliminary stripping of topsoil was undertaken. The inevitable destruction of the soil structure has implications for contemporary revegetation work that depends on an intact surface seed bank and for the capacity of floodplains to support high-value agriculture.

Table 5.

Phases of bucket dredging and its effect on rivers.

Bucket dredging phase	Position and maximum depth of dredging	Distributor and stripping operations	Legacy
Pre-regulation (1899–1905)	Riverbed and Floodplain (3–4 m)	Coarse sediment left on floodplains	Removal of bedforms, bed stratigraphy and release of large quantities of fines downstream. Floodplains unproductive
Regulation – WW1 (1906–1914)	Floodplain (10–12 m)	Re-soiling experiments begin	Loss of floodplain stratigraphy and topographic diversity by removing paleochannels. Poor floodplain productivity
Post WW1 (1915–1950)	Floodplain and within channels of diverted rivers (30–40 m)	Full pre-stripping and distributors that create a better mix of particle sizes in the stratigraphy	Loss of floodplain stratigraphy and topographic diversity by removing paleochannels

Increased regulation moved dredges out of river channels and led to remediation measures that made some attempt to recreate the original soil structure, replace topsoil and revegetate. This was a welcome reprieve for river channels and contrasts dramatically with California (see Figure 2), where there was no remediation. Our research indicates, however, that the effect of remediation was largely cosmetic. Even where state-of-the art methods were used, such as the Victoria Gold Dredging Co. lease at Newstead along the Loddon River, an industry showpiece in the 1930s and 1940s, the site continues to show evidence of disturbance. Poorly sorted and unconsolidated soils have resulted in the formation of sinkholes, the erosion of exposed riverbanks and the development of a large sand slug below the former dredge lease.

Dredging was concentrated in the upper reaches of river systems and this has several implications. One is the potential for changes in the dredged reaches to influence river function downstream. Dredging operations released tailings that led to increased sediment loads and aggraded river channels. This added to sediment volumes mobilised by surface alluvial mining in the 19th century and resulted in channels spilling onto their floodplain more frequently. These processes changed the stratigraphy of channel margins and adjacent floodplains, often depositing layers of fine sediment topped by coarser material (Cargill 2005; Grove et al., 2018, 2019). Floodplains also aggraded, homogenising their surfaces and reducing vegetation complexity.

The erosion of unconsolidated floodplains would likewise contribute to high sediment loads downstream both during and after dredging. Further implications follow from the location of dredges, which were generally above modern water storages. These are reaches considered to have had less human intervention than parts of the catchments below water storages. The lack of major storages on the Ovens River has led to the perception that it has high natural values as an undisturbed catchment and has been listed as a Heritage River by the Victorian Government. Our research has demonstrated, however, that the Ovens is one of the most heavily dredged rivers in Australia and its upper sections have been severely impacted by industrial activity. River reaches above water storages are often used for baseline scientific studies of biodiversity and river health, yet it is now clear that the baseline being recorded in many instances is itself already in a highly disturbed condition. A further implication in Victoria is that some of these areas are coming under increasing development pressure as urban dwellers seek to move to smaller, more scenic rural communities. This is true of both the Ovens and Loddon catchments. Improved understanding of land-use history is critical to the development approval process as dredged lands may present risks of land subsidence and chemical contamination, and as riverbanks may be more prone to erosion.

The impacts of dredge mining both onsite and offsite are considerable and ongoing. They include channelizing and reduced river sinuosity, increased erosion, damage to soil structure and reduced capacity of floodplains to support vegetation. All of these suggest avenues for further research to determine if the unconsolidated soils of dredged floodplains are more susceptible to erosion, to quantify anecdotal evidence of sinkholes, and to investigate the chemical signatures and possible presence of contaminants in dredged soils. Answers to these questions have implications for living with and managing rivers. Unconsolidated riverbanks may trigger the installation of armouring to prevent erosion, revegetation programs may need strategies that are not reliant on seed banks, agricultural producers may have to take action to remedy sinkholes and improve soil structure and quality, and planning agencies may need to consider potential threats to buildings and people from unstable ground and potential chemical residues.

The multidisciplinary approach used here has provided insights with implications for environmental scientists, land managers, historians and local communities. The integration of historical, geospatial and geomorphological data repositions historical mining as a major industrial activity with significant environmental consequences. The synthesis of historical evidence at a regional level makes large-scale patterns visible and provides an invaluable source of evidence for environmental scientists and land managers. By using the documentary record the analysis can extend beyond living memory, complementing baseline studies of river condition but also highlighting their shortcomings. Profound and widespread human intervention in natural systems is not just a feature of the post-war period now being described as the ‘Anthropocene’. It has a much longer history and one that continues to shape current circumstances.

Footnotes

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by funding from the Australian Research Council (DP160100799).

ORCID iD

Peter Davies

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