U.S. Geological Survey U.S. Geological Survey The Willamette Basin NAWQA includes two main tributaries to the Columbia River, the Willamette and Sandy Rivers. Objectives of the assessment are to evaluate status and trends of surface- and ground-water quality and to determine natural and human factors that affect water quality. The study focuses on agricultural and urban nonpoint sources of nutrients and pesticides. A GIS was used to select the best locations for water-quality sampling sites within the study basin, based on a composite of homogeneous areas of ecoregions, hydrogeology, and land use. This map depicts a three-dimensional perspective of these data layers using ARC/INFO GRID and the resulting aggregation to a single data layer showing site locations. Nutrient and pesticide data from a synoptic study of the Pudding River subbasin are also shown along with percent land use. A GIS proved an important tool in assessing land-based and anthropogenic factors in watersheds, and their relation to water quality.