In the twentieth century, probability became an important tool in the understanding of flood recurrences and magnitudes. This article focuses on the development of probabilistic flood understandings in the United States. Early efforts focused on projecting flood volumes, but maps of flood risk, brought about in large part by the National Flood Insurance Program, did much to cultivate this way of thinking in a broad audience. Engineers such as Weston Fuller and Allen Hazen, and geographer Gilbert White, play important roles in the trajectory developed in the article. The closely related ideas of the hundred-year flood and the hundred-year floodplain became standard terminology for communicating flood risk, but the knowledge behind them has been called into doubt by the realization of rapid, anthropogenic climate change.