The Occurrence of Large Floods in the United States in the Modern Hydroclimate Regime: Seasonality, Trends, and Large‐Scale Climate Associations.

Many studies investigate river floods by analyzing annual maximum series that record the largest flow of each year, including many within‐bank events inconsequential for human communities. Fewer focus on larger floods, especially at the continental scale. Using 473 streamgages across the conterminous United States with near‐natural flow from 1966 to 2015, we characterized the seasonality, occurrence, and climatic associations of the 10 largest and 2 largest floods at each site. These are often overbank events that have ecosystem functions and pose risks to humans. We grouped sites into 14 clusters corresponding to climatic and physiographic regions and characterized their flood seasonality at a monthly resolution using a probabilistic method. We then evaluated annual occurrence regionally and nationally, and seasonal occurrence regionally, by complementing a traditional approach to trend analyses with a novel method based on expected occurrence. Relationships between flood occurrence and climate indices were also investigated. Large floods have strong seasonality in some regions, but in areas with numerous flood‐generating mechanisms, seasonality is more complex. There is little evidence nationally that large riverine floods are more or less frequent than expected in recent years and only two regions show significant trends in annual counts; few show seasonal trends. We found some regional relationships between flood counts and climate indices, annually and seasonally; nationally the Pacific North American pattern is related to annual counts of the 2 largest floods. Large‐flood occurrence was generally stable across the United States in the last five decades; this may or may not continue with projected warming. Plain Language Summary: Flood studies usually focus on records of the largest streamflow of each year because they are widely available and provide relatively large samples. However, the largest flow of each year is not necessarily big enough to inundate the floodplain and pose risks to human communities. Our study investigated only the largest observed floods in the record, ones that occur roughly every five to fifty years. We used a set of 473 gages that measure near‐natural river flow across the United States. We studied many aspects of these consequential events at regional and national scales for the period 1966–2015, including seasonality, changes over time, and associations with known climate patterns. We found that the seasonality of large floods is strong in some regions, but weak or complex in others where floods have a variety of causes. There is little evidence that large floods have become more or less frequent with time, but we found some regional and national associations between large‐flood occurrence and known climate patterns such as the Pacific North American pattern. Large‐flood occurrence has generally been stable in the last five decades, but continued re‐evaluation of the historical record is important as new data are collected. Key Points: First broad characterization of large‐flood occurrence in the conterminous United States in the modern hydroclimate, including seasonalityBoth traditional and novel methods to assess temporal distribution of large floodsFew significant temporal trends, but significant relations to climate indices for some regions and seasons [ABSTRACT FROM AUTHOR]