Assessment of stream ecosystem function and sensitivity in the Bighorn National Forest, Wyoming

This paper presents a hierarchical analysis of stream ecosystem distribution and sensitivity to natural and anthropogenic disturbances for the Bighorn National Forest, Wyoming. We designated stream gradient, flow regime, and lithology as environmental parameters that would result in the most robust, readily applied, and parsimonious description of physical and chemical characteristics of individual stream segments. We used these parameters to map the spatial distribution and relative abundance of stream habitats in the study area. We then used the proportion of each sixth-level hydrologic unit boundary comprised by each category of stream gradient, flow regime, and lithology as input to an agglomerative cluster analysis, which identified six clusters for the 74 watersheds intersecting or within the national forest boundary. Five of the six clusters have predominantly high gradient streams and runoff dominated by snowmelt or mixed snowmelt and rainfall. Most watersheds on the Bighorn National Forest are sensitive to alterations in water supply because of the relatively small size of streams. Although watersheds are generally less sensitive to changed sediment supply, low-gradient stream segments create sensitive sites within individual watersheds. Field verification studies indicate that this approach reasonably characterizes physical channel properties and biological associations for the study area.