Murray Beaches

Beaches occur all along the Murray, however they are most spectacular in the middle section, especially around Tocumwal and Cobram and then, once again, downstream of Wentworth, where they may be up to half a kilometre long and a hundred metres deep. Most are isolated, treasured family camping spots which families return to over generations, interwoven with stories. 

Beaches build in times of high flow, when the river has enough current to carry sand, and are gradually eroded in between these times where there are high levels of boat traffic, like around Echuca. Sometimes high rivers are few and far between: the 2010 - mid 2012 high river followed two decades of predominantly low rainfall. It was a pleasure to paddle the river following this event and to see beaches reborn, their shape pristine, sculptured by the river's currents.

There is no one form of beach, each reflects the environment in exists in and the nature of the river in that place. Some are long and flat, reaching far out into the river, catching unwary boaters unawares, whilst others are steep and high. Some are symmetrical, having the same gentles curves to their outlines on both the upstream and downstream sides, others are truncated, dropping suddenly on their downstream side, worn away by whirl-pool like eddies as if they had been sliced by a cake knife. It is these truncated beaches where many non-swimmers have drowned after unexpectedly finding themselves in deep water. Locals call these places 'holes'. Holes are usually on the downstream side of beaches, but can occur anywhere along them, as the water is influenced by clay, or rock reefs and snags. 

Beaches make great places to camp, as they are often beyond the reach of the notorious falling branches of the otherwise beautiful river red gums. For the canoeist, they also provide a place to pull up where their feet, free from mud, can stay clean.

But how exactly do they form?

The expansive beaches of Tocumwal and Cobram owe their generosity to two sand sources. In this area of the river, the banks are sandy and light. Paddling down the river you can see where the river has cut into them, causing trees to fall and making the fastest flowing part of the river both confusing and potentially dangerous. I have the feeling that the river is able to change its course here more easily than in other sections due to the soft banks. As the banks fall in the sand is washed downstream by the fast current that eroded it. Current always flows faster on the outside of bends, so this is where the cutting occurs. On the inside it flows slower, or even backwards as an eddy. With less speed, the water can no longer carry the coarse sand grains and drops them. The beach will reach out to where the current is fast enough to carry sand rather than drop it. So beaches are formed from the coarse particles of the banks that the river has washed away upstream of them. 

Coolgeography.co.uk

Formation of meanders and oxbow lakes

The second source of sand is from the granitic hills of the upper river valleys. When heavy rains cause strong flows, then sand is washed down these valleys in what is known as a 'sand slug'. Picture an area of sand being pushed further downstream with every flood, or high river. Sand slugs can be small (only tens of metres), or very large and run for almost 100 kilometres. The removal of trees by farmers seeking uninterrupted grazing spaces (remnant of fertile British fields and removed from the uncivilised Australian bush) and the removal and disturbance of top soil by miners in waves of gold rushes, led to unnaturally large amounts of sand moving down the granite mountain streams of North eastern Victoria and the Snowy Mountains of New South Wales. In the Yackandandah Gold Rush of 1852 saw the movement of ten kilometres of river bed, four hundred metres wide and ten metres deep down the river in their search for the precious mineral. This sand and stone slug, partially caught by the Yarrawonga Weir wall, is still moving and is now around Cobram. The squarish stones on the shallow river bed in this area come from those miners back in the 1850's. It is moving around a kilometre a year.

Sand slug in a mountain stream in the USA.

Sand choked river, Tasmania: ABC Science: Australia - a land of sand rivers

Caring for our country project. North East CMA

Once the river enters the river red gum forests of Barmah, Millewa, Gunbower and Koondrook the beaches are more modest and only appear post flood, or high river.

NSW DPI: Sand Slug Bannockburn Creek

Downstream of the weirs of Mildura and Wentworth where the Darling River joins the Murray, the beaches are very large once again. Following the weirs, the turbulence caused by the release of water from the weirs could be eroding sand, but I do not know this. The sand could be simply coming from bank erosion, now visible after the many weirs in this region. However, it could be due to erosion from farmlands along the Darling River. This is an area that I need to investigate further.

Sand slugs and the large beaches that they form can be threats to the original fauna and flora of an area, as this study into the habitat requirements of freshwater mussels in NSW found.

Scientific readings and further information sources.

geological

• Davis, J. A. and Finlayson, B. 2000
  
  The fate of the sediment depends to a certain extent on its particle size distribution. Fine sediments (clays and fine silts) remain entrained in flow for sufficient time to be transported significant distances downstream from the site of erosion or out onto floodplain units where they are stored for long periods of time (Meade 1988). To transport the sand and gravel fraction, on the other hand, requires much more energy. As a result, this fraction remains in the stream channel and is transported slowly downstream in an episodic manner by large flood events. If the material eroded from a catchment is predominantly fine the major form of degradation is a deepening and widening of the channel, as well as sedimentation downstream on the floodplain. If the eroded material is predominantly coarse, degradation takes the form of changes in channel dimensions and stream sedimentation, or sand slug development...
  
  A sand slug is a discrete body of sand deposited in a stream channel... (it) is a discrete volume of sand and/or gravel material that is released into a stream channel and only very slowly transported out of the stream network by the stream flow. The slug can fill the width of the channel to depths of the order of metres, and extend over distances of hundreds to thousands of metres. The front of the slug is referred to as its ‘snout’, and this can be a well-defined face or front, downstream of which negligible deposition is apparent...
  
  In many instances a sand slug will transform a stream channel, producing a shallow flat bed. This alters the channel roughness and reduces the channel capacity, altering the stream hydrology and hydraulics. The stream will break out of the channel more frequently, low flows may occur beneath the sand and pools will no longer persist during dry spells. Large woody debris is submerged by the sand and the channel boundary material is often altered. Such changes have an impact on in-stream habitat and thus the stream ecology. Alexander & Hansen (1986) found that the introduction of sand into a stream in the upper midwest of the USA resulted in the channel becoming shallower and wider. As a consequence the static water volume decreased, channel diversity was reduced, fish cover was reduced and velocities increased, all of which contributed to a more stressful environment for fish...
  
  In south-eastern Australia sand slugs derived predominantly from stream erosion have tended to be associated mostly with granite catchments (Rutherfurd 1996). Granite catchments produce sediment that is dominated by sand-sized particles and so it is no surprise that when stream erosion and gullying occur in these catchments, sand slugs usually result. This report focusses on the tributaries to the Goulburn, the effects of sand slugs on ecology, the lessons to be learnt for land managers and strategies for rehabilitation. Davis, J. A. and Finlayson, B. 2000, p1-2.
• Interviews (with landholders about the creeks that ran through their property) were not formal in the sense that set questions were given; instead the interview was based on the general question, ‘How has the creek changed over the years and what do you think has contributed to that change?’ Davis, J. A. and Finlayson, B. 2000, p12.

ecological effects of sand slugs

• Sand slugs often lead to shallower river beds, raised water temperatures and a reduction in the biodiversity of benthic life. Davis, J. A. and Finlayson, B. 2000, p1.
• Increased sediment inputs have two broad effects, specifically changes to water quality (see next  section) and geomorphology. Increased sediment inputs led to some river reaches becoming dominated  by sheets of sand rather than a mix of material such as silt, clay, gravel or cobble (Moran et al. 2005). The effects of this change appear variable (Bond & Downes 2003; Downes et al. 2006) but may represent a decrease in habitat heterogeneity. Increased sediment inputs also led to the infilling of deep water holes in river channels. Water holes represent a critical refuge habitat and their persistence is heavily dependent on their depth. Reductions in water hole depth reduces the availability of refuges making systems less resilient to drought (Bond & Lake 2005). Gawne et al 2011 p10
• Observed sections of river that had shallow pools and runs, structural woody habitats smothered by sand, and sand bars that limited fish passage during low flow. Boys et al, 2005.

gold mining

• Historically, goldmining has been noted as highly detrimental to drainage line stability because of some of the practices employed. Gullies were dug up and puddling machines were used to wash sediment dug out of streams and hill sides, before flushing it downstream (Powell 1976). Later came the introduction of hydraulic sluicing in which miners used jets of water to displace alluvial deposits (Shakespear et al. 1887). These alluvial mining practices resulted in severe environmental degradation, usually in the form of downstream siltation (Powell 1976).

sand quaries

• very extensive and deep water worn gravel and sand deposits across the valley floor. Situated below several metres of overburden, the deposits extend deeper than the thirty-metre level to which they are usually worked. Below that level problems associated with the watertable and buried ancient timbers are two factors affecting the economics of extraction. p40 Trueman 2007.

anecdotal evidence

• "The sand was white like snow and the rocks and stones were nicely coloured in the river... before the green slime got into the river from Dartmouth, well the old man said it was buggered..." Recollections of Les ‘Brickie’ Franks of Tallangatta, interviewed in 2006 aged 67. p73. Trueman 2007.
• "that was before the mid 1950’s, before the new weir went in. The Delatitie now is nothing like it used to be back then. The river was deeper and had much greater flow, maybe the flow was 25 times what it is now. It’s changed all right, it used to be pretty much gravel and sand. A lot of it is now mud and big boulders. You look at it and its not the same river any more." Recollections of Frank Moore of Max Cove, interviewed in 2006 at age 82. p93. Trueman 2007.
• ‘The cod weren’t big up there, dad said, and different to those in the Goulburn here. They put it down to the clearing of the land and the rabbits, and the creek the holes filled up with sand, they were up to 20 feet deep. I think it was back in the 1920’s, I can just remember the change in the creek, we used to have picnics out there, further down the creek, just this side of Jeffries.’ Recollections of Ron Bain of Seymour, interviewed in 2006 aged 88. p99. Trueman 2007.

References:

W. T. TRUEMAN, (2007). SOME RECOLLECTIONS OF NATIVE FISH IN THE MURRAY-DARLING SYSTEM WITH SPECIAL REFERENCE TO THE TROUT COD Maccullochella macquariensis. Summary & Source Material from the Draft Publication 'Bluenose: The Lost World of the Trout Cod'. Electronic Version, June 2007. Prepared as an Interim Report for Native Fish Australia (Victoria) Incorporated. http://www.nativefish.asn.au/files/Recollections_compressed.pdf

Davis, J. A. and Finlayson, B. 2000. Sand slugs and stream degradation : the case of the Granite Creeks, north-east Victoria. Cooperative Research Centre for Freshwater Ecology Technical Report 7/2000. December 2000.  http://freshwater.canberra.edu.au/Publications.nsf/0/b48c7eb100e9072bca256f0f0014aede/$FILE/Granite%20Creeks%20.pdf

Downes BJ, Lake PS, Glaister A, and Bond NR (2006) Effects of sand sedimentation on the
macroinvertebrate fauna of lowland streams: are the effects consistent? Freshwater Biology, 51(1):
144-160

Gawne et al 2011 A Review of River Ecosystem Condition in the Murray-Darling Basin The Murray-Darling Freshwater Research Centre http://www.mdba.gov.au/kid/files/175-review-ecosystem-condition-mdfrc.pdf

Boys CA, Esslemont G and Thoms M (2005) Fish habitat assessment and protection in the Barwon-Darling and Paroo Rivers. NSW Department of Primary Industries.

Rutherfurd, I. 1996. Sand-slugs in south east Australian streams: origins, distribution and management. In: Rutherfurd, I. & Walker, M. (eds.), First National Conference on Stream Management in Australia,
Merrijig, February, 1996, pp: 25–34.