A spatially explicit method for determining the effects of watershed scale land use on stream conditions

The primary goal of this research was to identify the impacts that individual agricultural land uses have on fish communities in small streams located in the Willamette Valley of western Oregon. The diverse nature of the land use features of the valley provided a challenging but useful system for the differentiation of the impacts of various land uses. This manuscript first presents the methods used to select the watersheds for sampling. Selecting basins that allowed for the detection of various land use impacts while capturing conditions that were representative of valley-wide land use characteristics required an innovative sampling design. Histograms representing the land use characteristics of the small watersheds in the Willamette Valley were used for this purpose. The second manuscript in this document outlines the development and implementation of a spatially explicit method that allows for the isolation and quantification of the impacts of specific land uses. Developing a series of raster files (grids) using a Geographical Information System (GIS) allowed for the determination of the flow path lengths of each pixel (cell) in a watershed. The inverse values of the flow path lengths were used as measures of the potential impact that each cell in a watershed had on stream conditions. Sums of these potential impacts were calculated for each land use in a watershed and used as independent variables in modeling efforts. The selection techniques used to identify the sampled watersheds with those of the inversely weighted flow path method enabled the identification of the land use practices that have the greatest impact on stream fish assemblages. A model using the inverse values of the squared in-stream and out-of-stream flow path lengths was the most useful for describing the impacts of basin-scale land use on stream fish populations