River flooding can happen any time of year in the Northeast United States. But are there any seasonal patterns that can be identified? This question is explored in a new paper, River flood seasonality in the Northeast United States: Characterization and trends, published on December 2, 2018 in Hydrological Processes.

Understanding when floods happen can help scientists understand how floods are generated, a pressing question in regions with snow as climate changes. Knowing when floods are most likely to happen can help biologists understand how aquatic and floodplain ecosystems function.

This knowledge also helps communities plan for projects that could be affected by flooding, such as building and repairing roads and bridges. Local authorities also need to consider the effects of flooding in managing reservoirs for drinking water, navigation, flood protection, and public safety.

A new NOAA study that looked at 90 watersheds identifies in greater detail than previously known the times of year floods are more likely to happen in the Northeast U.S. Although each watershed has a unique pattern, three general patterns emerge, all of which include the late-winter/early spring (March-May). The most common pattern is for a watershed to have one flood season in the late-winter/early spring. Other watersheds have one flood season that spans the winter-spring (December-May). Coastal areas often have two flood seasons: late-winter/early spring and fall/fall-winter.

The study also provides new evidence challenging an old assumption: that spring floods in the Northeast U.S. are primarily snowmelt-related. Because March-May is a leading flood season in parts of the region without significant snowfall, it is likely that seasonally high soil moisture associated with months of plant dormancy (leaf-off) and low evaporation from short days is at least as important as snowmelt for generating floods at this time of year.

Researchers also investigated whether there were any trends in flood seasons. They found no evidence that flood seasons are shifting earlier or later in the year, but flood occurrence is increasing in the warm season (June-October) – a historically flood-poor time of year across the region. This may have implications for the region’s aquatic and floodplain organisms, for example changing the best time for fish to lay eggs.