New Hampshire rivers change in wake of recent floods, warming climate
Scientists are studying the changes in New Hampshire's rivers and streams after periods of heavy rainfall.
The year 2023 will be remembered for relentless rains and frequent flooding.
"We saw about 100 flash flood events in the state of New Hampshire in 2023," said Sarah Jamison, a senior hydrologist with the National Weather Service. "Over the last preceding decade, we had about 150 total."
While that might have been a fluke year, 2023 could potentially be a sign of future changes to precipitation patterns. A warmer climate could lead to more frequent and intense rainfall in New England.
>> Download the free WMUR app to get updates on the go: Apple | Google Play <<
"There was this feedback loop where the wet pattern we were in just kept feeding itself over and over," Jamison said.
She said last year's intense rainfall and floods were the result of a weather pattern that resulted from a stronger summer El Niño.
"If we look at 2023 in isolation, a lot of the credit or blame goes to El Niño, but we can't ignore the influence of a warming climate," Jamison said.
The Pemigewasset River in Plymouth is one of many New England rivers that were hit hard by recent floods. Scientists are studying the changes in rivers across New Hampshire and how it could impact communities along them.
In order to keep track of ever-changing water conditions, Rick Kiah and his team from the New England Water Science Center take measurements from 60 gauges at rivers across New Hampshire.
The measurements include river height and stream flow, a measure of how much water is moving over a particular area.
"We relay that through the GOES satellite system in real-time, so our other federal partners, like the Weather Service, can capture that data and do forecasts," Kiah said.
That data is then studied by Rob Dudley, a supervisory hydrologist at the New England Water Science Center who has investigated climate-related trends in New England's watersheds for more than two decades.
"We've definitely seen trends in increasing precipitation," he said. "On the whole, we really have been seeing larger magnitudes in those more frequent floods."
According to Climate Central, the Northeast has seen, on average, about a 60% increase in heavy precipitation since the late 1950s. Along with rainfall, temperatures play a big role in changing river conditions.
"We have been seeing later ice formation and then earlier ice-off on the rivers," Dudley said.
Winter is the fastest-warming season in New Hampshire. Dudley said this will likely have an impact on how rivers flood in the future.
"With less ice, the expectation is that there's fewer ice jams that would be contributing to flooding," he said. "And yet, you have more rain-on-snow events."
While the increase in flood events is not expected to make flooding worse, it could increase the rate of erosion.
"If you've got a change in the flow regime and the water is transporting and depositing sediment, if the character of that is changing over time, it's certainly going to have some effects on the shape and function of the river," said Dudley.
That's a concern for the many homes and businesses along New Hampshire's waterways. Some Granite State rivers have records that date back more than a century. Having such a long record of information helps scientists determine the conditions needed for a future major flood event.
"With the historic record that we collect, we can fine-tune what that flood level is, and then that will translate to our flood insurance rate maps to help with sustainable building in the future," Dudley said.
The data being collected across New England should help forecasters better manage the increase in flooding events. The next step, experts said, is building resilience for vulnerable communities.