Low-flow characteristics of streams in the Kishwaukee River basin, Illinois

Seven-day, 2-year, and 7-day, 10-year natural low flows are estimated at five continuous-record gaging stations based on streamflow records and records of wastewater effluent furnished by treatment plants. Low flows are also estimated at 22 partial-record stations by relating natural low-flow estimates for long-term stations to discharge measurements at partial-record stations. Standard error of estimates for the 27 gaged sites averaged 33 percent for 2-year estimates and 51 percent for 10-year estimates. A technique for estimating flows at ungaged stream sites is based on drainage area and indexes of streamflow recession. Streamflow-recession indexes were used to account for effects of geology on low flows. Multiple-regression analyses relates the low flows to drainage area and streamflow recession rates. Average standard errors of estimates of 61 and 97 percent were indicated for the 2-year and 10-year estimating equations, respectively. Wastewater treatment plant location and effluent discharged during 7-day low-flow periods in 1981 are presented for use in adjusting natural low-flow estimates at gaged and ungaged sites to represent 1981 streamflow conditions. (USGS)