Large floods in the United States: where they happen and why

The spatial distribution of large gaged floods throughout the United States shows that the locations of most of the largest flows are related to specific combinations of regional climatology, topography, and basin size. Key factors include the general northward trend of decreasing atmospheric moisture, proximity to oceanic moisture sources such as the Pacific Ocean and the Gulf of Mexico, and orientation of topographic features relative to directions of moisture flow, with the largest flows being at locations where topographically high areas are oriented perpendicular to directions of moisture flow. Additionally, the largest flows in large river basins are primarily caused by persistent climatologic conditions such as seasonal snowfall. In contrast, the largest flows in smaller basins are most commonly the result of intense precipitation due to convective storms.