Patterns of grain-size dependent sediment transport in low-ordered, ephemeral channels

Sediment was sampled during 9 flow seasons at the outlet of a 4.53 ha sub-watershed within the Walnut Gulch Experimental Watershed in southeastern Arizona to characterize transport. Measured concentrations averaged near 14,000 mg L-1 with maximum values routinely surpassing 30,000 mg L-1. Sediment transported in flow was found to systematically vary by grain-size. Fine material concentrations (< 0.25 mm) were found to peak early in each flow event, with some relation to seasonality or recovery period, and gradually wane throughout the event duration. The concentration of the coarser grain-size fraction (0.25 mm - 8.0 mm) was likely entrained from the channel bed and did not vary systematically with discharge. The coarse fraction made up an increasing percentage of the total sediment transport at larger total sediment concentration values. The different transport patterns of the two grain-size fractions are likely promoted by their transport mode. The coarse fraction was estimated to travel as suspended sediment or bed load while the finer fraction was estimated to travel as wash load, with little vertical stratification throughout the depth profile of the flow column. Study results show that the range of grain-sizes available for transport will likely play a large role in the rate and timing of the sediment delivery in flow. The observed behavior of the coarse and fine sediment fractions leads to the majority of the fine grained sediment yield occurring during the rising limb of the hydrograph while the majority of the coarse grained yield occurs during longer, waning limb along with the majority of the flow volume.