FLOOD SEASONALITY IN THE NORTHEAST UNITED STATES: CHARACTERIZATION AND TRENDS

The New England and Mid‐Atlantic regions of the United States have experienced hydroclimatic increases in both the magnitude and frequency of river floods. However, a detailed understanding of flood seasonality across the Northeast U.S., and how flood seasonality may have changed over the instrumental record, has not been established. The annual timing of river floods reflects the flood‐generating mechanisms operating in a watershed and many aquatic and riparian organisms are adapted to flood seasonality, as are human uses of river channels and floodplains. Changes in flood seasonality may indicate changes in flood‐generating mechanisms, and their interactions, with important implications for habitats, floodplain infrastructure, and human communities. I applied a probabilistic method for identifying flood seasons at a monthly resolution for 90 Northeast U.S. watersheds with natural, or near‐natural, flood‐generating conditions. Historical trends in flood seasonality were also investigated. Analyses were based on peaks‐over‐threshold (POT) flood records that have, on average, 85 years of data and 3 peaks per year—thus providing more information about flood seasonality than annual maximums. The results show rich detail about flood timing during the year across the region with each site having a unique pattern of monthly occurrence. However, a much smaller number of dominant seasonal patterns emerged when contiguous flood‐rich months were classified into commonly recognized seasons (e.g., Mar‐May, spring). The dominant seasonal patterns identified by manual classification were corroborated by unsupervised classification methods (i.e., cluster analyses). Trend analyses indicated that flood‐rich seasons have generally not shifted earlier or later in the year over the period of record, but 65 sites with data from 1941‐2013 revealed increased numbers of June‐October floods—a trend driving previously documented increases in regional flood counts per year. These months have been historically flood‐poor at the sites examined, so warm‐season flood potential has increased with possible ecological and human-use implications.