Hydrogeomorphic Classification of Western Streams for Ecological Assessment

Geophysical controls such as climate and geology interact with human activities to shape the fundamental drivers (e.g. flow regime, thermal regime, pollutant stressors, habitat structure) that make fluvial systems similar or different ecologically. These fundamental drivers are manifested differently across watershed, valley bottom, reach, and local spatial scales. Although geophysical setting is a fundamental determinant of variation in aquatic communities, physical habitat assessment techniques and popular, existing geomorphic classifications of streams do not necessarily provide an adequate multi-scale description for assessing ecological condition. In particular, the integration of hydrologic and geomorphic classifications at the reach, valley, or watershed scale does not exist, despite the fact that such integration may be a powerful way to stratify stream habitats, as has been shown for wetlands. We are developing a hierarchical classification framework for stream environments in the western US that integrates geologic context, a hydrologic regionalization approach, and analysis of drainage network structure with traditional reach-scale geomorphic classification. Extensive physical and biological data from over 300 first through fourth order streams across six ecoregions in Colorado, Oregon, and Washington and a comprehensive matrix of multi-scale hydrogeomorphic descriptors are being analyzed to link the classification with expectations of taxa distribution. For example, in the Cascades Ecoregion in Oregon, up to 72 percent of the variation in macroinvertebrate genus richness at a site can be explained using no more than four geophysical descriptors from both the reach and watershed scales. Given the current emphasis on ecoregional and local scales in biomonitoring, this approach is providing essential information on how geophysical factors measured at scales intermediate to ecoregions and stream reaches can be used to further understand and predict biotic condition at individual sites distributed across the landscape and the costs and benefits associated with different spatial resolutions of geophysical data in ecological assessment.