Summer flooding in Delaware River Basin to increase in size and frequency, study finds

Summer flooding in the Delaware River Basin is expected to increase in both size and frequency as the climate continues warming, intensifying future floods along the Delaware River’s mainstem, a study found.

Matthew Cooper, a climate scientist who authored the April 2025 study while working as a postdoctoral research associate at Pacific Northwest National Laboratory, attributed the change to a process called “flood synchronization.” 

The flood-prone Delaware River has many branches, or tributaries, that meet and combine at the mainstem. As the climate warms, the entire basin will likely see more rain than in the past.

Floodwaters in the northern Delaware River Basin could create a sort of snowball effect of flooding — minus the snow — when they meet with other flooding tributaries and grow in intensity downstream. As the climate warms, Cooper said, these synchronized floods will likely happen more often.

In the summer, Cooper said, floods are “likely to both occur more often and be larger across the entire river basin.”

Cooper found the number of floods in the cold season, roughly November through April, could decrease thanks to a loss of snowpack and ice melt events, often called “nuisance flooding,” in the northern reaches of the basin — namely, the Lehigh, Mongaup-Brodhead, East Branch Delaware and Upper Delaware sub-basins.

In their place would be a warmer winter atmosphere that breeds more moisture for rain and larger floods; so overall, less frequent but more intense winter flooding.

“The snowpack sort of ‘spreads out’ the flooding over time,” Cooper said. “When you lose that snowpack, you get less of those smaller-intensity floods, and instead you get the more punctuated, intense rainfall-driven floods.” 

Those flooding events would likely sync up with mainstem flooding, too, meaning winter floods downstream would become more intense.

“When you warm things up, you get a big increase in the number of floods for the entire river basin to sort of synchronize in time,” he said.

Cooper said his study can’t predict exactly where and when flooding could hit the hardest as the climate warms. But areas that have historically dealt with flooding will see more in a warming world, he said, especially developed areas along the mainstem.

“The overall increase in rainfall intensity, together with the increased synchrony of upstream-downstream flood waves, produces larger floods along the mainstem Delaware and Schuylkill rivers in the densely populated portions of the Philadelphia Metropolitan Area,” Cooper said.

Year-round, Cooper said, the study showed an increase in flood synchronization and size. But while the winter months would see more intense but fewer floods, both the flood intensity and frequency would increase in warmer months.

“In future simulations, the flood risk is really at its highest in the warm season,” Cooper said.

Cooper’s study used a hydrologic model, or a digital simulator that took historic weather data then overlaid scientific predictions of weather pattern changes. He said this is “the latest and greatest way” of studying how warming temperatures could affect the watershed.

The model allowed Cooper to look at simulations of each subbasin independently to see how they flood in the future.

In both the winter and the summer, Cooper found that the more northern and mountainous tributaries are responsible for the heightened flood synchrony predicted in coming years, because of both the loss of snowpack in the winter and because higher elevation areas are predicted to see more intense rainfall in the summer.

While those uppermost tributaries are responsible for a large volume of floodwaters, “flood severity typically accumulates downstream.”

“The most severe impacts often occur in floodplains and low-lying coastal areas where storm surge, tidal, and riverine flooding converge with prevalent human development,” according to Cooper’s study.

Flooding history

Flooding events have rocked the Delaware River Basin in the past, creating change in more ways than one. The basin’s record floods brought on by Hurricanes Connie and Diane in 1955 were one of the reasons the Delaware River Basin Commission, an inter-state and federal commission that manages the basin’s water resources, was formed in 1961. The flooding also spurred talks of the controversial Tocks Island Dam project, which displaced around 600 families and ultimately didn’t come to fruition.

Tropical Storm Ivan hit the watershed in 2004, kicking off the first in a series of floods that breached dams, prompted multiple states of emergency, and that together caused a cumulative $745 million in damage across New York, New Jersey and Pennsylvania in just over two years.

Flooding conditions recently swept across the Northeast, with flash flood warnings issued for areas including Levittown and Philadelphia as well as flood-plagued Camden, N.J. In May, flooding conditions closed the Delaware River Water Gap, a somewhat rare occurrence.