U.S. Geological Survey U.S. Geological Survey
Mark A. Uhrich and Dennis A. Wentz
The Willamette Basin National Water-Quality Assessment (NAWQA) includes two main tributaries to the Columbia River, the Willamette and Sandy Rivers in the State of Oregon, USA. 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. The initial study design was implemented by use of a Geographic Information System (GIS), a computer software and hardware system that captures, stores, manipulates and analyzes digital-spatial data. To relate water quality to land use, the basin was first subdivided by GIS into homogeneous areas of land type and cover, using ecoregions, hydrogeology, and land-use data layers. By aggregating data layers into a single digital-spatial data set, water-quality sampling sites were selected in representative homogeneous regions. The GIS and related spatial data were used to assess basin characteristics, including basin and subbasin boundaries, hydrography, precipitation, runoff, soils, and digital elevation models. The percentage ecoregion, hydrogeology, land use, and annual precipitation for each subbasin also was determined by GIS, and hydrologic model parameters, such as percent slope and aspect of a specified stream reach and subbasin, were evaluated. The GIS has proven an invaluable tool for assessing the spatial extent of watersheds and their relations to land-based and anthropogenic factors.