Downstream channel changes on a contracting, anabranching river: The Lachlan, southeastern Australia

Downstream trends in flow, channel morphology, and sediments were investigated over 220 km of the middle and upper alluvial Lachlan River in southeastern Australia. The Lachlan is a perennial river that rises from temperate, humid highlands and flows inland into a semiarid low gradient plain lacking perennial tributaries. Long daily stage records were used to calculate flow parameters and flood frequencies in single and anabranching channel reaches of the river. Morphological trends were based on cross section and height data compiled from new and water authority data, and from particle size trends constructed from bed and bank sediment samples. Mean and peak flows, channel size, and bed particle size all diminish in a downstream direction. In the partially confined piedmont zone, channel and floodplain morphology feature large flood effects and large channel dimensions that reflect extreme flow variability. On the alluvial plains, the unconfined, single-channel contracts with distance downvalley; and anabranches develop at high flow stages. The development of a full anabranching pattern is associated with large flood volume losses. Diminishing flood volumes, mean flows, and channel dimensions is attributed to storage of flood waters in lakes, floodways, and lagoons and through transmission losses during overbank flows. The frequency of bankfull discharge consistently decreases downstream, and may reflect the long duration and lower frequency of floods in the lower reaches of the river. Particle size of bed sediment fines downstream at rates consistent with those reported from sand-bed rivers elsewhere. This is attributed to a combination of hydraulic sorting and low rates of sediment supply from the catchment.