A budget of atrazine and desethylatrazine loads was computed for a 116-kilometer reach of the Cedar River in Iowa to determine where these compounds enter the river during base-flow conditions. Loads were determined by measuring discharge and concentrations of the compounds for four main-stem sites along the Cedar River and for 27 tributaries on September 20-22, 1989. Tributaries contributed 17 percent of the increased atrazine load and 24 percent of the increased desethylatrazine load measured between the extreme upstream and downstream sites on the Cedar River. The remaining 76 to 83 percent of the increased loads were attributed to input along the groundwater's main stem. The ground-water samples were collected at depths from 1 to 2 meters beneath the river bottom where the ground water was determined to be moving toward the river. The sources of atrazine and desethylatrazine detected in the ground water may include bank storage of river water or ground-water recharge originating at some distance from the river.