Trout Distribution and Habitat in Relation to Geology and Geomorphology in the North Fork Humboldt River Drainage, Northeastern Nevada

Abstract

We studied the existing distribution of native Lahontan cutthroat trout Oncorhynchus clarki henshawi and exotic brook trout Salvelinus fontinalis with respect to geologic and geomorphic land-classes in the upper North Fork Humboldt River drainage, Nevada. We evaluated habitat conditions in study sites to determine which measured components of habitat structure provided the best discriminators among study stream reaches in the different land-classes and among trout-supporting and unpopulated study reaches. At a finer level of resolution, we used the habitat attributes with the most discriminatory power to plot the distributions of study areas by land-class and by presence or absence of trout along coordinate axes reflecting environmental gradients defined by these attributes. Elevation, substrate embeddedness, and streamflow were the variables with the most discriminatory power among land-classes defined by parent geologic material (geologic district), but gravel abundance in the substrate was more useful than streamflow in further discriminating among land-classes at the lower-level classification defined by geomorphic character (landtype association). Plots ofstudy areas along environmental gradients defined by these variables visibly separated study areas by land-class. Trout distributions at specific sites were clearly related to geologic district and, to a lesser extent, to landtype association. Trout were almost exclusively restricted to sites in the sedimentary mountains defining the western boundary of the drainage, and occurred elsewhere only in study areas that were upstream from the fine-textured valley floor. Of the variables measured, embeddedness appeared to be the most likely cause of the segregation of trout by geologic district. Although unmeasured variables (e.g., temperature, winter conditions, and turbidity) cannot be eliminated as potential limiting factors, peripheral evidence suggests that they alone probably are not determinants of trout distribution in the region. Trout were common in the sedimentary geologic district, but did not occur in all study sites. Important discriminating attributes in this region were stream width, abundance of large substrate (rubble and boulder), and streamflow; trout were principally associated with sites characterized by wider, well-watered stream reaches containing high percentages of large stream-bottom particles. Study sites meeting these criteria were concentrated in high mountain areas influenced by Pleistocene glaciation, but were also present in the fluvial canyons. All sites containing brook trout were in drainages that had been glaciated. These sites would normally be considered “better” trout habitat; native cutthroat trout occupied the more degraded sites.