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What causes spring floods? » Yale Climate Connections

Spring is flood season for much of the U.S. From the Pacific Northwest to New England, melting snow combines with spring rains to raise water levels in rivers, lakes, and streams. Sometimes, a perfect storm of factors creates a major flood.

Everything from soil to snowpack to overnight temperature plays a role in how significant a spring flood may be. Because so many different elements are involved, predicting flood risk in a given year can be difficult. As climate change influences many of those variables, prediction may get even more difficult.

“If you are missing a certain ingredient, you go from just having some nuisance flooding to maybe a catastrophe. And so that makes it really challenging to know in the future where the risks lie.” said Melissa Widhalm, the associate director at the Midwestern Regional Climate Center.

Rapid warmups cause snowmelt flooding

Rapid warmups and warm overnight temperatures can dramatically increase the rate of snowmelt and lead to flooding. Very warm temperatures are the primary drivers of extreme snowmelt flooding in the U.S. according to research by Xubin Zeng, a professor at the University of Arizona. Zeng and his coauthors mapped and studied extreme snowmelt events in the continental U.S. between 1988 and 2017.

Todd Shea, meteorologist in charge at the National Weather Service office in La Crosse, Wisconsin, said that you can have almost every other factor pointing to spring flooding, and flooding still might not occur if the temperature warms up slowly.

“I do remember one year where everything was looking really gloomy, like we were going to a pretty nasty spring melt, but we had the perfect melt, very slow and gradual,“ Shea said.

Shea also mentioned that rapid warmups often come with overnight temperatures above freezing, allowing snow to continue melting through the night. The ideal scenario, he said, is when daytime temperatures warm up enough during the day to gradually melt the snow, then go back to below freezing at night. The slower the snow melts, the less likely there is to be flooding.

Climate change is expected to make warm days, and warm nights, more likely in the early spring and late winter. “Unfortunately, when we’re talking about climate change, we’re talking about extreme temperature swings being a real threat,” Widhalm said. “We’re talking about having more of that erratic, warmup, really cool down, really warmup in sort of these fast snaps, and that’s when we start to see issues.”

Rainfall on top of snow makes the snow melt faster

According to Zeng’s research the second factor most likely to lead to rapid snowmelt is ain, expected to be a growing problem particularly in the Sierra Nevada Mountains of California and in the Pacific Northwest.

Eunsang Cho, a hydrologist working at NASA’s Goddard Space Flight Center, said that this kind of rain-induced rapid snowmelt happened in Northern California in 2017 during the Oroville Dam failure. A lot of snowpack accumulated in the Sierra Nevada mountains that year, and when an atmospheric river-driven rain event fell on that snow, it sped up the melting and rapidly filled Lake Oroville. The high rate of release needed to keep up with the precipitation and melt event damaged the dam’s main spillway, prompting evacuations for downstream communities.

This problem is not constrained to just the western U.S. Zeng’s research found the Northeast is also particularly at risk from rapid snowmelt flooding due to rain. Rain in late spring and winter is also becoming more common in the Midwest, according to Widhalm. And precipitation overall is increasing as well in those months.

“Here in Indiana, we’re talking about up to a 20% increase in precipitation in the winter,” She said. “We’re talking about a 13 to 16% increase in the spring. So this is a lot of water at a time when we’re already vulnerable to flooding.”

The soil’s ability to store water impacts flooding

Shea said that when the National Weather Service creates spring flood outlook reports, an important factor they consider is soil moisture content and how much water it will be able to store when the snow melts.

If soil is already saturated from precipitation in the summer and fall, then it will not be able to store as much runoff. The soil makeup – like whether it is predominantly clay or sand – also affects how much it will be able to soak up. Cho said that the Red River of the North Basin in Minnesota and North Dakota is particularly flood-prone because of the soil makeup.

“It’s a really flat area, and the soil is very impermeable, which means it’s very clay soil,” He said. “Even though some small amounts of the snow melted, it easily caused flooding.”

This kind of impermeability also happens when the soil is frozen as frozen soil is also very impermeable. This may become more common in the Midwest due to climate change where snowpack may be reduced and leave the ground uninsulated. This allows the soil to freeze deeper.

“If I get a big rainstorm on top of that, what’s going to happen to that water? Well, it’s going to hit the ground like a rock, and it’s going to run off to our streams and quickly increase our stream flows,” Widhalm said.

Cho also mentioned that in urban areas along the Atlantic Coast this issue may be compounded by sea level rise and impermeable paved surfaces. Sea level rise can raise groundwater levels, making the soil more saturated and less able to store water from snowmelt. And paved surfaces prevent runoff from soaking into the soil.

Water levels and ice coverage in rivers

Similar to soil, the capacity of watersheds to store more water matters during spring flood season. If rivers and other bodies of water are already full before the spring warm up arrives, there’s a higher chance of flooding. Additionally, if ice on rivers has frozen thickly that can cause problems. Shea said that when the ice is thick it often breaks up later in the spring which can cause ice jams and flooding.

With climate change increasing extreme precipitation everywhere and overall precipitation in many places east of the Rockies, the risk of there already being a lot of water in the watershed before the snow melts increases.

Snowmelt flooding is a compound event.

All of the experts interviewed for this story emphasized that no one thing causes a spring flood. Snowmelt flooding is a compound event which can make it unpredictable.

Widhalm cited the extreme flooding in Nebraska in spring 2019 as an example. “In 2019, when they had those terrible, terrible floods in March, well, let’s look at what happened,” She said. “In February, they had record snowfall, they had record cold. Well, then you had March and you had this intense warm up.” The combination of those three things, plus rainfall during that warm up led to flooding.

Because there are so many variables in a snowmelt flood, it’s hard for experts to predict whether or not climate change will make them worse. But some regions – Cho’s research flagged the Pacific Northwest, Alaska and Canada as particularly vulnerable – can expect a higher risk of extreme spring flooding in the future. And climate change may increase the frequency, if not the magnitude, of these kinds of events in other parts of the northern U.S. as well.

“We’re talking about weather superimposed on climate,” Widhalm said. “To get that perfect storm of events to line up to create a catastrophe, thankfully, is pretty hard to do. But if you are juicing up the system, if you are creating an environment that is more conducive to rapid storm development, or intensification, now we’re talking about increasing the potential to have larger snow dumps, we’re creating the potential for more rapid temperature swing environments.”

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