A growing body of evidence, both anecdotal and systematic, suggests that flooding has become more frequent and more severe on many rivers. River systems and flooding can change over time in response to: (1) climate shifts, (2) land-use change, or (3) river engineering. Rigorous documentation of hydrologic change is challenging because long-term trends are superimposed on short-term fluctuations, and because the three mechanisms above operate simultaneously and their effects are difficult to quantitatively separate. Systematic shifts in fluvial systems and flooding may be suggested by geomorphic analysis, hydrologic data, or numerical modeling of hydraulic and climatic behavior. Geomorphic evidence provides a unique perspective, extending over long periods of time and spanning multiple climatic cycles and often predating most human modification. Analyses of hydrologic data provide precise empirical and statistical measures of climatic and river conditions and change. Finally, both hydraulic modeling and climatic modeling allow forward simulation and assessment of the sensitivity of river systems to various perturbations. Recognition of rapid change in flood behavior on rivers has implications that are both practical and pressing. Current guidelines for, and implementation of, flood-frequency analysis in the U.S. utilize the assumption that flood data are homogeneous and stationary in time. For example, a current Federal reanalysis of Mississippi River flooding assumes no significant effect of land-use change, climate shifts, or downstream engineering structures. This study has preliminarily concluded that the 100-year flood level of the Mississippi River at St. Louis should be lowered by 0.5 ft. In contrast, new research incorporating hydrologic change suggests that the 100-year level at St. Louis should be raised * 4 ft, in spite of the addition of massive reservoir capacity upstream. Accurate and updated probabilities are required for a proposed new generation of U.S. floodplain maps, the cost of which is estimated at ~$1 billion nationwide. Assessing the impacts of systematic fluvial and climatic change requires new tools and techniques from a broad range of fields.