Illinois
The report below documents the regression equations that are available in the Illinois StreamStats application for estimating flood-peak discharges of selected recurrence intervals for rural streams in Illinois. The report also describes the methods used to measure the basin characteristics and to develop the equations, and the errors associated with the estimated discharges. Users of the StreamStats application should familiarize themselves with this report before using StreamStats to obtain estimates of streamflow statistics for ungaged sites.
Interactive Map
Improved estimates for an ungaged site can be obtained by use of the drainage-area ratio method when the drainage area for the ungaged site is within ± 50 percent of the drainage area for an upstream or downstream streamgaging station. Methods described on page 24 of the above report can be applied to obtain these estimates. Weighted estimates for streamgaging stations in Illinois can be found in table 1, beginning on page 93 of the above report, or in the StreamStats reports for data-collection stations. The Data-Collection Stations link to the left explains how to obtain the reports. The National Streamflow Statistics program (NSS) also can be used to compute weighted flood-peak discharges for streamgaging stations and improved estimates for ungaged sites. The NSS can be obtained from http://water.usgs.gov/software/NSS/.
Data refinements and automated determination of basin characteristics have resulted in differences between basin characteristics and peak-flood discharges determined in the 2004 report, and those reported by the Illinois StreamStats application. Drainage areas, water percentages, and basin lengths computed by StreamStats were not significantly different from those published in the Soong and others (2004) report. Main channel slopes computed by StreamStats were slightly different from the published values on a statewide basis, even after application of a correction for bias, because of the difference in the definition, and therefore, computation of this characteristic. (The definitions for main channel slope are found in the 2004 report, page 73, and, for StreamStats, in the Basin Characteristics link to the left under Stream_Slope_10-85_Method.) On a regionally sampled basis, the slopes were not statistically significantly different, although some individual locations did have large differences. Where the differences exceeded 60 percent, the StreamStats slopes were higher, resulting in flood-quantile estimates that are larger than the published regional equation estimates. A very small mean difference in average permeability (about 0.25%) was found on a statewide basis, but the sensitivity of the equations to this basin characteristic is also very small, so no correction was applied. Both the adjusted and unadjusted values for main channel slope and basin length can be displayed by selecting the BasinChars tool after delineating a watershed; however, only the adjusted value is displayed on the Streamflow Statistics report obtained by using the FlowStats tool.
The overall differences in flood-peak discharges resulting from the differences in basin characteristics were statistically insignificant on a sampled region-by-region basis, except for Region 1, which had a mean differences ranging from 3.8% for the 2-year peak discharge estimate to 4.3% for the 500-year peak discharge estimate, where differences are computed as 100*(2004 estimate-StreamStats estimate)/2004 estimate. These differences are well within the standard error of estimate for the regression equations. However, the percent differences can be large for small watersheds of less than 0.5 square miles. The automatic delineation of watersheds should be reviewed and, if necessary, corrected using the EditBasin tool.
The USGS has developed StreamStats for Illinois to provide up-to-date flood-frequency information for the State, in cooperation with the Illinois Department of Transportation, the Illinois Department of Natural Resources, Offices of Water Resources, and the Illinois Center for Transportation, University of Illinois at Urbana-Champaign.
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