First, for 119 stream gauges we calculated 8 flow metrics to describe high and low flow magnitude, frequency, duration, and timing. Second, we used cluster analysis to assign the 119 sites to 4 hydrologic classes. Third, we developed a GIS database to characterize 10 watershed variables at all Colorado Plateau stream reaches. Fourth, we built a statistical model to link hydrologic classes and GIS-derived watershed variables at the 119 gauged sites. Fifth, we used the statistical model to predict the hydrologic class at ungauged sites based on their watershed variables. Finally, we mapped aquatic invertebrate community data from 13 gauged and 89 ungauged sites onto the hydrologic classification to test the biological relevance of the flow typology.