Understanding changes in the timing and magnitude of streamflows available to maintain riverine biodiversity and ecological function, while also meeting the increasing societal demands on water resources, is a global priority. A rigorous quantification of seasonal changes in streamflow is needed to inform water management and policy under global change. We introduce a hierarchical Bayesian model that quantifies spatiotemporal variation in seasonal streamflow profiles and allows for inferences on temporal trends in model-based indices related to streamflow timing and magnitude shifts.