Geospatial Techniques for Developing A Sampling Frame of Watersheds Across A Region

Robert E. Gresswell, George W. Lienkaemper, Aquatic Ecologist and Geographical Information Specialist, USGS Forest and Rangeland Ecosystem Science Center, , Corvallis, OR, and Douglas S. Bateman, Department of Forest Science, Oregon State University, Corvallis, OR

Current land-management decisions that affect persistence of native salmonids are often based on studies conducted at individual sites in a single watershed. Although this approach is useful for some purposes, extrapolating results to areas that were not sampled is usually not appropriate. Recent investigations of habitat relationships for coastal cutthroat trout (Oncorhynchus clarki clarki) located above natural barriers to anadromous salmonids required a methodology for extending the statistical scope of inference. Because a database with locations of isolated populations did not exist, initial steps involved identifying watersheds in the sampling frame. Mid-size watersheds west of the Cascade Mountain divide were delineated using 30-m digital elevation models. A database with the location of natural barriers to upstream fish movement was developed to identify the population of watersheds potentially containing cutthroat trout. Isolated watersheds with coastal cutthroat trout were subsequently stratified by ecoregion and erosion potential based on dominant bedrock lithology (i.e., sedimentary and igneous). A stratified random sample representing approximately 20% of the watersheds was selected using proportional allocation. The resulting sample is being used to investigate distributional patterns of isolated assemblages of coastal cutthroat trout at spatial scales ranging from local (channel units) to regional (western Oregon).