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

Abstract

The 7-day, 2-year and 7-day, 10-year (Qj^ 2 and °-7N 10) natural low flows are estimated at five long-term continuous-record gaging stations in the Kishwaukee River basin using a probability distribution based on daily discharge records and records of wastewater effluent furnished by treatment plants. The QyN 2 and °-7N 10 are a^so estimated at 22 partial-record gaging stations based on the relation of concurrent natural low flows at the longterm stations and the partial-record stations. The natural low flows were defined by low-flow discharge measurements minus the daily wastewater effluent from treatment plants. The standard error of estimates for the 27 gaged site,s averaged 33 percent for the 0,7^,2 estimates and 51 percent for the Q7N r io estimates. Estimates of the Qy^ 2 an(^ °-7N 10 at ungaged stream sites may be made based on drainage area and indexes of streamflow recession. Streamflowrecession indexes were used to account for the effects of geology on low flows. Relations of low-flow estimates to drainage area and streamflowrecession indexes were defined by multiple-regression analyses. The locations of wastewater-treatment plants and the amount of effluent discharged from each plant during 7-day low-flow periods in 1981 are given for adjusting natural low-flow estimates at gaged and ungaged sites to represent 1981 streamflow conditions.