The accumulation of organic matter and the influence of geomorphic variability along the Barwon-Darling River, Australia

Organic matter, in its various forms, is thought to be important for the ecological functioning of large lowland river systems. During periods of low flow, organic material accumulates on the floodplain and other in-channel surfaces and this material then provides a potential energy source for riverine food webs when it enters the river during flooding. Variations in the type and amount of this organic material may therefore have implications for aquatic food webs in Australian lowland river systems. In this paper we consider spatial variations in organic matter content present on in-channel floodplain surfaces between different geomorphic settings along the Barwon-Darling River. In general, floodplain surfaces in wider, unconfined geomorphic settings were found to contain greater amounts of surface leaf litter and sediment organic matter than surfaces in more confined geomorphic settings, at both the valley trough scale (leaf litter-239.7g/m² ~ 93% greater; sediment organics-8.7% ~ 50% greater) and the channel cross-section scale (leaf litter- 17.8% greater; sediment organics- 10.4% greater), Across both valley trough and river channel conditions, floodplain surfaces higher in the bankfull channel contained on average, twice the amount of surface leaf litter (350.1 g/m² ±64.2) and sediment organic matter (10.1% ±1.3), than surfaces at lower elevations (Leaf litter-61.5g/m² ±16.5; sediment organics-4.9% ±0.7). This study suggests that large scale geomorphological conditions can influence organic matter accumulation on in-channel floodplain surfaces. Overbank flows occur relatively infrequently in large lowland river systems hence smaller in-channel pulses and the maintenance of in-channel geomorphic structures could also prove important to the ecological integrity of these systems.