Contribution of return flows to streamflow in selected stream reaches in Illinois, 1988-89

Water returns by sewage-treatment plants and various water users can be a significant part of streamflow. Knowledge of the effect of return flows on stream- flow is needed for purposes of assessing streamwater quality and water supply. The results of a study by the U.S. Geological Survey, in cooperation with the Illinois Environmental Protection Agency, are presented to describe the contribution of return flows to streamflow for five stream reaches in Illinois during 1988-89. The reaches studied were South Branch Kishwaukee River upstream from De Kaib, Mackinaw River upstream from Congerville, Addison Creek upstream from Bellwood, Flag Creek upstream from Willow Springs, and Thorn Creek upstream from Glenwood. The annual average contributions of return flows to streamflow in each reach ranged from 1 to 99 percent of the annual average streamflow. Return flows significantly affected streamflows in the Flag Creek and Thorn Creek reaches. Significant return flows probably sustained streamflows in the Addison Creek, Flag Creek, and Thorn Creek reaches. Return flows exceeded streamflow during at least 1 month during the 1988 drought at all of the study reaches, except the Thorn Creek reach. Of the amount of return flows that exceeded streamflow, all reaches lost an estimated 0.01 to 0.34 Mgal/d (million gallons per day) to evaporation and 0.36 to 1.33 Mgal/d to infiltration. An analysis-of-variance test indicated that return flows did not differ among seasons. A second analysis indicated that the proportion of return flows to streamflow did differ seasonally. Return flows during July through September 1988-89 constituted a large proportion (67 percent) of the annual streamflows.