Ecological - Second Order
Field studies that investigate sediment transport between debris‐flow producing headwaters and rivers are uncommon, particularly in forested settings, where debris flows are infrequent and opportunities for collecting data are limited. This study quantifies volume and composition of sediment deposited in the arterial channel network of a 14 km2 catchment (Washington Creek) that connects small, burned and debris‐flow producing headwaters (<1 km2) with the Ovens River in SE Australia. We construct a sediment budget by combining new data on deposition with a sediment delivery model for post‐fire debris flows. Data on deposits were plotted alongside the slope‐area curve to examine links between processes, catchment morphometry, and geomorphic process domains. Results show that large deposits are concentrated in the proximity of three major channel junctions, which correspond with breaks in channel slope. Hyperconcentrated flows are more prominent towards the catchment outlet where the slope‐area curve indicates a transition from debris flow to fluvial domains. This shift corresponds to a change in efficiency of the flow, determined from the ratio of median grain size to channel slope. Our sediment budget suggests a total sediment efflux from Washington Creek catchment of 61 x 103 m3. There are similar contributions from hillslopes (43±14 x 103 m3), 1-3rd stream order channel (35±12 x 103 m3) and the arterial 4‐5th stream order channel (31±17 x 103 m3) to the total volume of erosion. Deposition (39±17 x 103 m3) within the arterial channel was higher than erosion (31±17 x 103 m3), which means a net sediment gain of about 8 x 103 m3 in the arterial channel. The ratio of total deposition to total erosion was 0.56. For fines < 63 μm, this ratio was much larger (0.89), which means that fines are preferentially exported. This has important implications for suspended sediment and water quality in downstream rivers.