Explore ISWS watershed maps and track vital streamflow data — including monthly statistics, flow magnitudes, and 10-year low flow frequencies — to inform your land and resource planning efforts.
Watersheds
Major Watersheds of Illinois Map
Our major watersheds of Illinois map shows the general boundaries of the state’s major watersheds, or the land area that drains directly to a common stream, river, or lake.
Illinois River Watershed Map
The Illinois River watershed is vital to Illinois. It covers 44 percent of the state and has a drainage area of 28,906 square miles, including approximately 3,058 square miles in Indiana and 1,070 square miles in Wisconsin.
View and download our Illinois River watershed map.
Streamflow
Illinois Streamflow Assessment Model
The Illinois Streamflow Assessment Model is an online GIS tool that estimates streamflow magnitude and frequency — especially average to low flows — to support water management and supply planning in Illinois.
Learn more about the Illinois Streamflow Assessment Model or see the interactive streamflow map.
Provisional Monthly Streamflow Network
ISWS’ Water and Atmospheric Resources Monitoring Program posts provisional monthly mean streamflows and related statistics for Illinois rivers and streams, based on data collected by 26 U.S. Geological Survey gaging stations.
Learn more about the Provisional Monthly Streamflow Network or view its streamflow data.
7-Day 10-Year Low Flow Maps
Eleven regional maps show the lowest average flow expected every 10 years for most Illinois rivers and streams. The maps also show associated effluent discharges, water withdrawals, and flow regulations.
Explore the 7-day 10-year low flow maps.
Peoria Lake Bathymetry Map
This 1996 Peoria Lake bathymetry map features bathymetric contours derived from a triangulated irregular network (TIN) built using Z-values from surveyed cross-section points.