Climate and land use changes have both disrupted freshwater ecosystems, and these changes will continue and potentially worsen into the future. These changes will favor some taxa and impair others, resulting in changes in richness and composition. However, the amount of impact due to changes in climate relative to changes in land use are not clear. Understanding which changes are likely to result in the greatest changes in composition and losses of biodiversity is necessary for effectively prioritizing actions and resource commitments. Therefore, we built models predicting richness and community composition for both fish and macroinvertebrates in US rivers and then applied forecasts of future climate and land use to these models to predict how richness and composition will change in response to environmental changes. We used fish and macroinvertebrate occurrence data collected as part of the EPA National River and Stream Assessment as response data for random forest models of assemblage richness and multivariate random forest models of assemblage composition. For predictors in our models, we used a combination of static environmental factors, like soils and topography, and dynamic factors like climate and land use. We derived values for dynamic factors from climate (MACA, CMIP5) and land use (FORE-SEE) models, using current projections for richness and composition for model training and then using mid-21st century projections to predict future richness and composition. The fish richness model explained 65% of the variation and the macroinvertebrate model explained 38%. For both models, temperature and forested land use were the most important dynamic predictors and had approximately equal influence. But maps of the predicted changes indicated regional differences in richness change, with increasing fish and macroinvertebrate richness occurring in the western US and decreasing richness occurring in the Appalachians. Increased richness coincided with increased precipitation in the west, whereas decreased richness in the Appalachians was associated with increased temperatures. Improving our understanding of the relative impacts of climate and land use changes will help us make better decisions about priorities for management and mitigation of these threats.