Abstract:
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.
