Abstract
Excessive suspended-sediment from predominantly agricultural landscapes remains a major environmental challenge in the Illinois River Basin, degrading water quality, altering channel morphology, accelerating backwater infilling, and increasing navigation and restoration costs. Understanding how sediment concentration and flux respond to hydrologic variability and watershed change is critical for effective river management. This study evaluates long-term suspended-sediment dynamics across six major Illinois River tributaries using the Weighted Regressions on Time, Discharge, and Season (WRTDS) and its autoregressive extension (WRTDS-K) applied to the Illinois Benchmark Sediment Monitoring Program records. Flow-normalized concentration and flux trends were analyzed at annual and seasonal scales and combined with flow-class sediment regime diagnostics. Results show that sediment responses are highly heterogeneous across tributaries and cannot be explained by streamflow change alone. Elevated discharge, particularly mid-high and high flows, dominates sediment export across all sites, while seasonal responses are strongest during spring and fall and, in some tributaries, winter. The Spoon River at London Mills and the downstream LaMoine River at Ripley showed sustained increases in both flow-normalized concentration and flux, indicating persistent sediment availability and strong sediment mobilization. In contrast, the Vermilion River, the upstream LaMoine River, and the Sangamon River showed declining or weakly responsive sediment trends under modest or decreasing long-term flow changes, suggesting sediment supply limitation, enhanced watershed retention, or other watershed-scale controls. These findings show that basin-scale sediment dynamics emerge from tributary-specific sediment regimes rather than uniform system-wide responses. The combined use of WRTDS and WRTDS-K provides a robust framework for distinguishing hydrologic amplification from sediment-supply controls and supports spatially targeted sediment management across the Illinois River Basin.