A hydrologist answers your questions about flooding in Minnesota

A building sits surrounded by flood water
A yard surrounded by flood water in Bayport, Minn. on Tuesday.
Kerem Yücel | MPR News

We’re in peak flooding season, Minnesota. 

Some flooding patterns stay pretty consistent year over year, but as weather events get more extreme, we can expect flood season to be affected, too. A historic year for snowfall means a huge snowmelt. 

One surprising twist? “We are very fortunate that we had a drought,” said National Weather Service Twin Cities hydrologist Craig Schmidt. “If we had this huge snowpack coming into a non-drought year, we would have had a lot bigger flood.”

We asked what questions you had about this year’s flooding season in Minnesota and took them to Schmidt for answers. Here’s what he told us.

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The following is a lightly edited transcript of a conversation with Schmidt on Thursday.

When is flooding expected to be the worst in Minnesota?

We're probably at it right about now. It’s pretty much this weekend. That’s when we’re finally going to have the last of the snow melt working its way through and the last of the water from the last couple of rainstorms all working their way through. Water is going to run high for a few weeks after this, so if we were to get another big heavy rain event, it could bring it right back up again.

How do I find my local flooding forecast? 

The best flood forecasts come from the National Weather Service on water.weather.gov. That’ll bring up a big national map, and then you can zoom into your area. From there you can pick your point, get the hydrograph and see the forecasts.

How far out do those forecast?

Our operational river forecasts go out five to seven days. We have probability guidance that goes out about 10 days, and then we have seasonal outlook that we do in the spring.

How can I check river gauges? 

That would be on the same NWS site. The National Weather Service doesn’t actually own the gauges. We use the data from all different agencies like the Minnesota Department of Natural Resources and the U.S. Army Corps of Engineers. We collect all of those and put them onto one map.

How has the flood forecast changed with the rainy weather we’re currently getting?

Some rises – like the Redwood River, Cottonwood River and Crow River – all of those were where the rain was the heaviest. We had to put out some new warnings and raise the forecast there. 

A little less so on the Mississippi and the St. Croix. The rain caused those to bump up maybe a half foot or so. Once a river becomes as large as the Mississippi is, flowing through St. Paul, you’ve got so much water there that adding the amount we got from the rainfall is just like a drop in the bucket. It doesn’t cause as much of an effect.

Is the concept of ‘x-year floods’ becoming less literal with climate change? Or does it really mean that we can expect that sort of event once every x years? — Lucia, Minneapolis

The idea of the 100-year flood doesn’t mean that it happens just once every 100 years. What it means is that you have a 1 percent chance of it occurring in any given year. So you could have it happen twice in five years. But we are seeing more extreme weather, more extreme rainfall events and more extreme snow, so those probabilities and those return intervals do need to be adjusted and rethought, I’d say. 

The scientists that do that sort of thing are recalculating them all the time, but it takes a lot of work and a lot of time to do that. The last update we had on those was like 2014. There’s been a lot of calls to have that done again, and I think they’re working on a batch of them right now.

Does ground thaw influence flood conditions? Is that something that’s monitored? — Lauren, Hopkins, Minn.

We refer to this as “frost depth” most of the time. It’s how deep the ground gets frozen each winter. The shallower the frost depth, the quicker it thaws once we start warming up. And the quicker it thaws, the faster the snowmelt is able to get into the soil and get the soil wet again if it was dry. 

It definitely has an effect. If it stays frozen – say you’ve got 3 feet of frozen soil once you start warming up – all of that water just ponds up and runs on the surface and floods everything. But if it gets into the ground first, it can help recharge the soil and recharge groundwater before it moves to the rivers. It causes less of a flood.

Is that something we saw this year?

We had a really shallow frost depth. That was actually helpful for our flood situation this year. Had we had a typically deep frost, it would have been a worse flood probably.

How do dams and those who operate dams influence flooding and flood conditions? — Pete, Hudson, Wis.

For the most part, a lot of dams are built for the purpose of flood control. The only time that's bad is if you end up with an event that gets so big that overtops the dam, and then you end up with an uncontrolled situation. Thankfully, none of that around here this year.

How do current floodwaters help to replenish our underground water supplies? Or do floodwaters mostly pass through on the surface? — Ginger, St. Paul

Floodwater is what’s extra. It kind of depends on how fast the snow melts. If it melts too fast, then it overcomes the ability for the soil to absorb it. A lot of times the floodwater is what the soil couldn't grab hold of. It’s not really the flooding that would influence the groundwater, but just the fact that we had so much snowpack. All the water that was snowpacked – a lot of that, as it soaks in, is definitely going to help the groundwater whether we have flooding or not.

Why do people need to worry about floods when they happen every year? — Steve Corl, Hector, Minn.

Every year in this part of the country, your snowpack has to melt. Once enough snow melts and enough of it gets into the rivers, it starts coming out of the banks and starts flooding and causing damage to roads – and if it’s bad enough, homes and businesses. 

Right now it’s making a lot of people take detours and take twice as long to get to work. It doesn’t affect everyone, but it certainly affects a lot of people and makes life a lot harder. It costs people a lot of money and costs cities, states and the federal government a lot of money to repair all the damage that happened from flooding.

Are there long-term environmental concerns from flooding?

A lot of times flooding can put more pollutants into the ecosystem. There are impacts where you get erosion – fast-moving water in an area that doesn’t normally have water like that, the banks change. Flooding can eat the bank away to where suddenly a building could be in danger of falling in. And then the other one is runoff from farm fields. That's not happening right now because nothing's planted yet, but when we have flooding in the summertime, that takes the chemicals or whatever is in farm fields.

When will the St. Croix River in Stillwater crest? And at what height? — Laura, Hudson, Wis.

The middle of next week – Tuesday or Wednesday at about 90 feet. That’s the current forecast.

When will the Mississippi River crest? — Heath, St. Paul

At St. Paul, we’re expecting that to crest closer to Wednesday or Thursday next week at 19 feet.

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