How important is geology in evaluating stream habitat?

Abstract

Geology plays an important role in the evolution of a stream and the formation of habitat therein. As geology varies across the landscape, the differential response of various geological types to environmental and anthropogenic factors may cause differences in stream habitat. In this study, we assessed the importance of accounting for geology when evaluating sediment-related, stream habitat data. Stream habitat data were collected from 424 sites on federally managed lands in western Montana, USA. These sites represented a variety of ecoregions, stream types, management practices, and geologies. The importance of accounting for geology in data analysis was evaluated using five sediment-related habitat variables and three analyses that examined (1) differences across geology for the entire dataset and for sites in reference and managed watersheds; (2) differences between reference and managed sites within geologies; and (3) the relative strength of geology as a factor when accounting for the effects of management, stream type, and ecoregion. In some cases habitat variables differed across geology; however, this was typically due to differences within managed sites and the plutonic geologic class. Habitat variables collected from reference sites generally indicated better habitat quality than those collected from managed sites; this result was consistent regardless of underlying geology. Geologic class was only a significant factor for one of the habitat variables when management, stream type, and ecoregion were taken into account. Similarly, ecoregion was only significant for a single habitat variable. In contrast, stream type was a significant factor for all five habitat variables and management was significant for three. Underlying geology differentially affects sediment-related habitat variables. However, differences across geologic types are less common and pronounced when watersheds are non- or minimally impacted by human activities. Our results suggest that stratifying habitat data solely based on geology is likely to prove insufficient when attempting to account for variability between sites. However, despite this complexity, we believe that the results support the consideration of geology as one of several sediment-influencing factors when setting stream habitat targets and evaluating habitat conditions.