Junction of the Warrego and Darling rivers Selected Area 2021-22 - Appendix G In-channel Features

The inland rivers of the northern Murray-Darling Basin are well known for their geomorphic diversity, owing to their highly variable flow regimes, and the flat landscapes that they drain. This includes channel features such as secondary channels, anabranches and flood runners and various sedimentary features such as floodplains, benches, and point bars (Thoms et al. 2004). The geomorphic diversity of floodplain rivers has been shown to influence the distribution of resources throughout them (Southwell and Thoms 2011, Belletti et al. 2017). Knowledge of the resource distribution within the landscape and how this interacts with river flows is necessary to understand how these systems can be best managed to promote increased system productivity (Bunn et al. 2006, Thorp et al. 2006).

The channel and riparian areas of the Darling/Baaka River in north-west NSW is characterised by a low flow channel bordered by a series of sedimentary surfaces contained within a broader channel trough (Southwell 2008, Woodyer 1975). These sedimentary surfaces or in-channel benches have been formed by the contemporary flow regime of the river (Thoms and Olly 2004) and can be found at various heights or elevations within the bankfull channel of the river (Thoms and Sheldon 1997, Southwell 2008). Benches have been shown to store large quantities of organic material and nutrients which becomes available to the river during inundation. Lowes et al. (2002) showed that the amounts of carbon, nitrogen and phosphorus released from bench surfaces differed by river reach and bench elevation within the channel on bench surfaces in the Walgett to Bourke reach of the Barwon-Darling/Baaka River. Bench sediment texture was also shown to influence the release of nutrients into the water column. Benches also form in-stream habitat for juvenile native fish during flow events (NSW DPI 2015) and harbour invertebrate egg banks that can emerge upon wetting, providing an energy source to the river. As such they have become a target for environmental water to provide for the exchange of material at relatively low flow levels.