The modern-day lower Illinois River flows within a greatly oversized valley formed by an old course of the Mississippi River and glacial outwash events. As a result, the section of the Illinois from its mouth near Grafton to Henry, Illinois is characterized as a low-gradient, aggrading bed stream with large backwater areas. Sedimentation in the river threatens to convert the system from one of river/backwater areas to one of channel/marshes. The sediment budget of Peoria Lake, a major backwater lake along the river, includes input and output through the Illinois River and inputs from several small direct tributaries. Results after two-year monitoring of these small tributaries indicate that they are a significant factor in the sediment budget of Peoria Lake.