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Illinois StreamStats

StreamStats for Illinois was developed in cooperation with the Illinois Department of Transportation; the Illinois Department of Natural Resources, Office of Water Resources; the Illinois Center for Transportation, University of Illinois at Urbana-Champaign; and the Federal Highway Administration.

Updated regression equations for estimating the magnitude and frequency of floods in Illinois were published on September 20, 2023, and were made available in StreamStats version 4 for Illinois on December 1, 2023. The regression equations, which were developed using streamgages within Illinois and bordering areas of Wisconsin and Indiana, can be used to estimate the 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probability statistics at ungaged basins in the seven hydrologic regions of Illinois (Over and others, 2023). The following report documents the equations used to estimate the statistics, describes the methods used to develop the equations and to measure the basin characteristics used in the equations, and describes the uncertainty associated with the estimates. Users should familiarize themselves with the report before using StreamStats to obtain estimates of streamflow statistics for ungaged sites.

  • Over, T.M., Marti, M.K., O'Shea, P.S., Sharpe, J.B., 2023, Estimating peak-flow quantiles for selected annual exceedance probabilities in Illinois (Report No. FHWA-ICT-23-014). Illinois Center for Transportation.

Click on this link to obtain more detailed information on the Illinois application, as well as specific sources and computation methods for basin characteristics.

The regression equations for estimating the magnitude and frequency of floods in Illinois (Over and others, 2023) are appropriate for locations in Illinois where the effects of regulation are minor. Methods for adjusting peak-flow estimates and corresponding confidence intervals for ungaged locations outside of northeastern Illinois (hydrologic region 2) were developed (Over and others, 2023) and implemented into StreamStats to account for urbanization effects. The regression equations for peak-flow quantiles are not applicable for large rivers including the Big Muddy, Fox, Illinois, Kaskaskia, Mississippi, Ohio, Rock, Sangamon, and Wabash Rivers.

The regression equations used in StreamStats to estimate streamflow statistics were developed using streamflow statistics and basin characteristics computed for USGS-operated streamgages. The equations are applicable with known accuracy when they are applied at locations with basin characteristics that are within the ranges of those used to develop the equations. Estimates for user-selected sites with basin characteristics that are outside of those ranges are extrapolated. These extrapolated estimates should be used with caution, as their associated errors are unknown and may be large. StreamStats outputs provide the ranges of applicability for each basin characteristic that is used as an explanatory variable in the regression equations and warnings when those ranges are exceeded.