Increased Frequency of Low-Magnitude Floods in New England

Recent studies document increasing precipitation and streamflow in the northeastern United States throughout the 20th and early 21st Centuries. Annual peak discharges have increased over this period on many New England rivers with dominantly natural streamflow-especially for smaller, more frequent floods. To better investigate high-frequency floods (< 5-year recurrence interval), we analyze the partial duration flood series for 23 New England rivers selected for minimal human impact. The study rivers have continuous records through 2006 and an average period of record of 71 years. Twenty-two of the 23 rivers show increasing trends in peaks over threshold per water year (POT/WY) - a direct measure of flood frequency -using the MannKendall trend test. Ten of these trends had p < 0.1. Seventeen rivers show positive trends in flood magnitude, six of which had p < 0.1. We also investigate a potential hydroclimatic shift in the region around 1970. Twenty-two of the 23 rivers show increased POT/WY in the post-1970 period when comparing pre-and post1970 records using the Wilcoxon rank-sum test. More than half of these increases have p < 0.1, indicating a shift in flow regime toward more frequent flooding. Region wide, we found a median increase of one flood per year for the post-1970 period. Because frequent floods are important channel-forming flows, these results have implications for channel and floodplain morphology, aquatic habitat, and restoration.