Case Study of Suspended Sediment Transport in Two Illinois Watersheds, Examining Effects of Area and Gradient

This work examines suspended sediment transport in two agricultural-dominated watersheds—Six Mile Creek (SMC) and Money Creek (MC)—in central Illinois, focusing on the influence of watershed area and gradient. Both watersheds, which drain into key municipal reservoirs (Evergreen Lake and Lake Bloomington), exhibit similar land-use and weather patterns but differ in watershed size and stream gradient. Field data collected from July 2012 to March 2013, consisting of both base flow and storm events, revealed that storm-driven flows account for most of the suspended sediment loads in both watersheds, transporting 99.8 percent and 97.0 percent of total loads for SMC and MC, respectively. Sediment loads were the lowest in the fall and peaked in the late winter/early spring, correlating to an increase in discharge. Despite MC’s larger area, SMC’s higher gradient resulted in greater normalized sediment export per unit area, emphasizing the dominant role of gradient over area in sediment mobilization. Soil erosion potential mapping, integrating stream power index (SPI), and soil erodibility (K-factor) identified erosion hotspots near stream channels in MC and the interior (upland) fields in SMC. The findings suggest that gradient, storm frequency, and storm intensity, rather than base flow or watershed size, are the principal drivers of sediment export in these agricultural watersheds.