Dairy Creek, located in Washington County, Oregon, has a drainage area of approximately 230 square miles, and includes West Fork, East Fork, and McKay Creek drainages. Dairy Creek is a major tributary of the Tualatin River which experiences algal problems during the late summer when stream flows decrease and water temperatures increase. The upper reaches of Dairy Creek are forested land, while the lower reaches are devoted to intensively irrigated agricultural production. It has been proposed that excessive algae blooms are associated with elevated nutrient concentrations during summer low-flow conditions. The objective of this project was to evaluate nutrient loading to the Tualatin River from the Dairy Creek watershed. Water quality parameters were selected to examine relationships between precipitation, flow, solids and nutrient constituents. The following conclusions were reached: 1. Surface runoff and erosion processes did not appear to occur and hence had little impact on water quality during the months of July through October during 1990 and 1991. 2. Groundwater moving to the stream appears to drive flow and have significant influence on water quality during summer low-flow conditions. 3. Nutrient concentrations are greatest during the summer. This may be a consequence of increased solubility and greater contact time between soils and groundwater moving toward the creek. A second alternative is the winter time sediment deposition. 4. Most of the nutrient and solids loading occurs during the winter and early spring at times of highest stream flows.