The relative contributions from potential sediment sources to the suspended load of the Wildcat Slough, a headwater tributary in central Illinois, are estimated as a proxy for understanding sediment and nutrient transport in low gradient, intensively managed watersheds. Sediment and nutrient fluxes from agricultural areas to rivers have led to nitrate (NO3) concentrations that exceed US Environmental Protection Agency water quality standards and phosphorus (P) concentrations that promote algal growth, decreasing water quality across the state of Illinois. This study assesses contributions to the sediment load as a function of land use and channel morphology in Wildcat Slough, a low gradient, intensively managed 61.3 km2 (23.67 mi2) watershed in the Upper Sangamon River Basin of central Illinois, United States. Land use is dominated by row crop agriculture. Potential sediment sources include row crop fields, forest, floodplains, river banks, pastures, and grasslands. The majority of the stream is constrained within a deep, channelized ditch fed by tile drains. However, in the lowermost reaches, Wildcat Slough freely meanders and has established fluvial features including point bars and cut banks.
A suite of tracers (delta carbon 13 [δ13C], phosphorus [P], magnesium [Mg], manganese [Mn], and carbon [C]) differ significantly between the potential sediment sources. An unmixing model uses the concentrations of the tracers in source samples and in suspended sediment collected during high discharge events to estimate the relative contribution from each source to the suspended load. The majority of suspended sediment was sourced from channel banks (40% to 65%) and forest (35% to 55%) with very little derived from the agricultural uplands (10% or less). The lack of sediment from uplands signifies a disconnect between the uplands and the channel, reflecting the low relief of the uplands and the dominance of tile drains in routing water to the channel. These findings suggest that efforts to limit the sediment-born nutrient load should focus on near-channel areas in meandering reaches. Best practices in similar landscapes may include installation of a buffer between meandering channel reaches and fertilized land, or limiting fertilizer application near channel banks, to diminish the inputs to the nutrient load via natural channel migration.
Neal, C.W.M. and Anders, A.M. (2015): Suspended sediment supply dominated by bank erosion in a low-gradient agricultural watershed, Wildcat Slough, Fisher, Illinois, United States. Journal of Soil and Water Conservation 70 (3): 145-155. DOI: 10.2489/jswc.70.3.145