Shifting climate regimes are projected to increase the area of arid regions and result in more pronounced intermittency across river networks. Given these projected changes, understanding the factors contributing to species persistence under increasing aridity will inform long-term conservation of regional fish biodiversity. To investigate fish assemblage responses over large spatial and temporal scales, we compiled data on 194 fish species found in 1,379 arid land stream sites in the United States and Australia to explore how changing flow regimes are related to changes in fish richness and assemblage composition over fifty years. We compiled trait data for fish species in this dataset, including reproductive and life history strategies, habitat preferences, and diet to identify characteristics common to species in these systems and those eliciting the strongest responses to environmental change. Our data show declines in overall precipitation in concert with increasing temperatures over the last several decades. Climatic shifts were accompanied by declines in discharge, increased zero-flow days, and longer durations of no-flow periods. In these same systems, an overall linear decline in fish species richness was observed. A majority of xeric fish species are small-bodied, short-lived invertivores that are highly fecund and predominantly occupy habitats with sand and gravel bottoms. A higher proportion of these arid land taxa are endangered relative to all ray-finned fishes in the IUCN database. General declines in richness were observed in both the United States and Australia, though the longest datasets available were all from the United States, underscoring the need for accessible, long-term datasets on occurrence in other xeric regions of the world. This work highlights a critical conservation need for arid land fishes and identifies taxa that are especially vulnerable in aquatic communities that are likely to experience more severe drying events in the future.