Spatial scales of diatom-based stream monitoring and assessment have expanded from traditionally local reaches or rivers to basins, and regions in the US. Our knowledge on large-spatial scale diatom distribution patterns in the western US is very limited. The main objectives of this study are to characterize broad-scale benthic diatom assemblages in the western US streams/rivers with least anthropogenic disturbance and then relate the distributional patterns with measured environmental variables in the west. A total of 691 taxa were identified. Median taxa richness is 39 with a range of 11-88. The top 5 most abundant taxa are Achnanthidium minutissimum (mean=24.6%), Cocconeis placentula (3.4%), Rhoicosphenia abbreviata (3.0%), C. placentula var. euglypta (2.9%), and Amphora pediculus (2.8%). Most of the taxa are rare with low abundance. Approximately 72.6% of the taxa occurred in ≤ 3 sites with ≤1% relative abundance. The NMDS ordination configuration based on diatom assemblages strongly correlates with water quality variables (e.g., conductivity, ANC) and watershed characteristics (e.g., mean watershed slope) but less strongly with reach-scale physical habitat conditions.
