Many of the lakes and wetlands are disappearing because of sedimentation resulting from agriculture activities, levee building, and urbanization. Seventy-six percent of the lakes in the LIRB assessed by the IEPA are substantially or moderately affected by sediment input. Wetlands Many wetland or formerly wetland areas in the LIRB are glacial in origin. Blocks of ice, from retreating continental glaciers, were left buried in hills of gravel that the glaciers had pushed forward. Warmer temperatures melted the blocks of ice to form lakes.
Wetlands provide a sink for nutrients and sediment that filters water.
Presettlement wetlands were widespread throughout the Illinois River Valley and counties bordering the Illinois River. Lacustrine wetlands are found in small areas along the Illinois River from Peoria to Beardstown. The riverine wetlands are the most abundant wetland system in the LIRB, especially west of the lower Illinois River. The riverine wetlands are abundant but are much smaller individually than palustrine or lacustrine wetlands. The natural wetland distribution, by river basin, shows the Illinois River Basin to contain the most wetlands followed by the La Moine and Sangamon River Basins.