Human impacts on suspended sediment and turbidity in the River Murray, South Eastern Australia: Multiple lines of evidence

Abstract

European settlement has led to increased loads of fine suspended sediment (SS) entering the River Murray, Australia's largest, and arguably, most important river. The River Murray's anthropogenic sediment history can be divided into four periods with varying source areas, sediment loads, and seasonal patterns. The Aboriginal period (before 1840) was characterized by clear water at summer low-flows in the River Murray and its southern tributaries, with more sediment coming from the northern catchment than the southern, and the Darling River being turbid at all flows. There is little evidence that Aboriginal burning resulted in any measurable increase in SS. SS loads peaked in the 1870s and 1880s (the gold and gully period, 1850-1930) as valley floors were incised by gullies (mostly in northern tributaries), and gold sluicing flushed huge amounts of sludge into southern tributaries. Sedimentation in wetlands and on floodplains increased by 2-10 times in this period, and the biota in wetlands switched from clear water to turbid water communities.In the hiatus period sediment supply from gullies and gold mining waned and low flow SS concentrations returned to low levels. Dam construction through the 1960s and 1970s (the regulation period, 1960 on) disconnected the River Murray from catchment derived sediment. Despite this, SS levels increased again: now largely derived from instream sources including bank erosion from long duration summer irrigation flows, the spread of bottom-feeding carp (Cyprinus carpio), and wave erosion from boats. Erosion switched from winter to summer dominated. Significant investment in securing water for the environment in the Murray-Darling Basin could be complemented by addressing inchannel sediment sources in the River Murray itself to reduce turbidity. Overall, European era SS concentrations remain relatively low with small sediment delivery to the ocean (0.1Mt per annum), despite high catchment erosion rates. This is due to poor sediment delivery efficiency through the low-gradient landscape.

Riverine landscapes, water resource development and management: A view from downunder

“…Humans have modified riverine landscapes through land‐use changes, urbanization, and the construction of dams and weirs that regulate downstream flow and sediment regimes (Vitousek, Mooney, Lubchenco, & Melillo, 1997). These pronounced and persistent modifications can result in regime shifts, a state change, of rivers as evident in the sediment regime shifts illustrated by Rutherfurd et al (2020). Many argue riverine landscapes have flipped into a new basin of attraction or state—the Anthropocene—through periods of intense degradation (sensu the hydrological changes described by Higgisson et al, 2020).…”

“…Excess sediment can have long lasting deleterious effects on river ecosystems. Rutherfurd et al (2020) show the legacy of human activities on suspended sediment loads in the River Murray. Four sediment periods are described—The Aboriginal period (pre 1840), Gold and gully period (1850–1930), Hiatus period (1930–1960), and Flow regulation period (post 1960).…”

“…Each sediment period has different sediment source areas, sediment loads, and seasonal patterns. Both Gibson and Hancock (2020) and Rutherfurd et al (2020) advocate that advancing and incorporating sediment knowledge into current river and water management strategies would benefit environmental conditions of river ecosystems.…”

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

“…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.…”

“…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(Davies et al, , 2020Rutherfurd et al, 2020). Recognising the extent of riparian disturbance caused by dredging provides further perspective on the anthropogenic transformation of river systems.…”