Diverse Physical Processes Drive Upper‐Tail Flood Quantiles in the US Mountain West

Abstract

Abstract Floods in the mountainous western U.S. typically arise from rainfall, snowmelt, or rain‐on‐snow (ROS). This implies that streamflow records comprise “mixtures” of flood regimes with differing physical characteristics. This runs counter to the assumption, made in flood hazard practice, that flood observations are independent and identically distributed. In this study, we examine 308 watersheds and 281 (91%) of which exhibited substantial roles of at least two flood regimes, with rainfall and ROS accounting for the very largest floods. We demonstrate that flood mixtures can have dramatic impacts on upper‐tail flood quantiles—rarer than the 0.01 annual exceedance probability. While the geographic distribution of mixed flood regimes is explained by climate and elevation, such regimes are subject to future change due to climate warming. The complexities brought about by diverse flood mixtures require process‐based approaches for understanding and modeling future flood frequency distributions.