The upper Big Sioux River Basin, in northeastern South Dakota, was studied to determine the origin and transport of nutrients in the Big Sioux River. The area was divided into three segments. Two of the segments, the Big Sioux River and Willow Creek Basins, were primarily agricultural drainage areas, and one segment was the city of Watertown. Sampling stations were selected to facilitate the determination of nutrient inputs to the river from each of these segments and to determine the nutrient load in the river leaving the study area. Nutrients studied included total orthophosphate and the particulate and dissolved fractions of organic carbon, organic nitrogen, and total phosphate. (Modified author abstract)