River flood seasonality in the Northeast United States: Characterization and trends

Abstract

The New England and Mid‐Atlantic regions of the Northeast United States have experienced climate‐induced increases in both the magnitude and frequency of floods. However, a detailed understanding of flood seasonality across these regions, 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 basin, and many aquatic and riparian organisms are adapted to flood seasonality, as are human uses of river channels and flood plains. Changes in flood seasonality may indicate changes in flood‐generating mechanisms, and their interactions, with important implications for habitats, flood plain 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 flood records that have, on average, 85 years of data and three peaks per year—thus providing more information about flood seasonality than annual maximums. The results show rich detail about annual flood timing across the region with each site having a unique pattern of monthly flood 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 the annual timing of flood‐rich seasons has generally not shifted over the period of record, but 65 sites with data from 1941 to 2013 revealed increased numbers of June–October floods—a trend driving previously documented increases in Northeast U.S. flood counts per year. These months have been historically flood‐poor at the sites examined, so warm‐season flood potential has increased with possible implications for aquatic and riparian organisms.

Trends in Global Flood and Streamflow Timing Based on Local Water Year

“…The methods presented here do not replace nonparametric methods of investigating changes in seasonality for complex situations where the distribution of timing is multimodal (Dhakal et al, 2015). Across the northeast of the United States, annual timing in the flood rich cool season has not shifted, but increased flooding has been observed in the warm seasons, shifting flood seasonality (Collins, 2019). In Norway, greater dominance or rainfall‐driven flooding will shift flood timing differently between catchments (Vormoor et al, 2015).…”

Spatially coherent regional changes in seasonal extreme streamflow events in the United States and Canada since 1950

“…In the western United States and Canada, various streamflow metrics have shifted earlier by 1 to 4 weeks (30,31), likely because of warmer alpine temperatures increasing winter-season melt and the percent of precipitation falling as rain (18). In the Northeast United States, Collins (20) found minimal evidence for shifts in the timing of annual peak flows, but Hodgkins and Dudley (32) showed that the centroid of winterspring streamflow has shifted at 32 to 64% of snowmelt-affected sites in the eastern North America. In the western United States, drought has and is projected to limit snowpack generation and groundwater recharge (33)(34)(35), a pattern reflected in increased frequency of extreme low flows in the Pacific Coast and Pacific Northwest hydro-regions during summer and fall ( fig.…”

Changing River Flood Timing in the Northeastern and Upper Midwest United States: Weakening of Seasonality over Time?

“…One major advantage of the use of circular statistics instead of linear statistics on the timing of flood events is its ability to provide an improved understanding of variables modeled as circular random processes (for example, the timing of an event within a cycle). Several studies have used a circular statistical approach for assessing the seasonality of flood events [5,[17][18][19][20][21][22][23][24]. The majority of these studies have used two summary statistics based on mean and variability for analysis of timing of flood events.…”