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Morning Edition
Cathy Wurzer and Gracie Stockton

What is a flood stage? And why does measuring rivers matter? An expert explains

A flooded street with yellow caution signs
A flooded street in Windom, Minn., not far from the Des Moines river on Tuesday.
Dan Gunderson | MPR News

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Storms are getting stronger and more frequent with climate change, often meaning rivers rising and flooding more quickly, threatening landscapes, infrastructure and lives.

It falls to the United States Geological Survey field crews to measure floodwaters and collect water-quality samples in the wake of big bouts of rainstorms. Over the last week, more than a dozen scientists spread out statewide, from the Rainy River Basin to the Blue Earth River.

James Fallon, data chief for the Minnesota portion of the Upper Midwest Water Science Center of the USGS, told MPR News Wednesday that staff have been ensuring stream gauges are working properly, then taking discharge measurements.

“We know not only how high the river is, what the water level is, but how much flow is passing through each of the gauges that we operate,” he said.

The data is fed through satellite telemetry to the Weather Service in the Army Corps of Engineers, which in turn forecasts river flooding. USGS scientists also process and share the data publicly.

The National Weather Services’ corresponding flood levels are action, minor, moderate and major. Determining how high water must be to qualify for each level is a cooperation between the USGS, the NWS and individual communities.  

“So they want to know if it’s getting high enough to go over the levee or flood their wellfield or something like that. So they’re typically not usually like when the river leaves its banks. It’s more ‘what structures or infrastructure do we need to protect?’” Fallon said.

Rivers statewide are still swollen and cresting, some having reached major flood stage. The Minnesota River at Henderson broke its record high by a foot-and-a-half. The Rapidan Dam near Mankato was in danger of failing and a rushing Blue Earth River eroded its banks enough to send a house toppling into the water.

Communities in northeast Minnesota are cleaning up after water poured into homes and businesses by the foot; St. Louis County declared a disaster. Fallon said the USGS’s recent actions are among the top five responses to Minnesota flooding he’s seen since he started assisting in 1993.

“How bad it is is relative to where you live on the river and how it’s impacted you. But from an aggregate standpoint, it’s definitely a top 10 flood, maybe top five, top six in places,” Fallon said. And it’s careful, consistent measure that’s made tracking impacts of climate change possible.

“Our rainfall is definitely increasing across the state,” MPR News meteorologist Sven Sundgaard said Wednesday. “In the Twin Cities, mega rains — which are defined as six inches or more over a 1,000 square mile area — have doubled in this last 24 years compared to the last quarter century, which is pretty incredible change.”

Each degree Fahrenheit of global warming equates to roughly 4 percent more water vapor in the atmosphere. This has led to a 42 percent hike in downpours in the Upper Midwest. Over the past century, the amount of precipitation in the largest rainfall annually has increased by more than an inch.

“There’s a lot of variability, ups and downs, but overall we are getting wetter,” Sundgaard explained.

While Fallon isn’t focused on predicting future climate change, he and his colleagues are “kind of the climatologists of stream gauges, and so we can tell you what’s happened.”

The Mississippi River at St. Paul is forecast to crest late Saturday after entering major flood stage Tuesday. The stream gauge there serves as a case study with 130 years of annual peak flow record data. Ten of the top 12 peaks were recorded since 1956.

“So you’d normally expect those to be evenly distributed in the first half and the second half of the record, but we’re not seeing that,” Fallon said. “We’re seeing many more floods in the last 60 years, 65 years than we did in the first 65 years.”