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Thu March 28, 2013
Snow Melt Impact on Wisconsin Rivers and Streams Yet to Unfold
What’s up with this weather? Last spring was warm and the summer extremely dry. Although this year’s cold snap “appears” to have broken across Wisconsin, temperatures remain below normal, and our mounds of snow are only slowly shrinking.
WUWM Environmental Reporter Susan Bence spoke with meteorologists and hydrologists pondering what might follow, particularly near the mighty Mississippi.
While it was extremely low last year, flooding is always a big concern come spring. The Mississippi bumps along Wisconsin’s western edge, as the river meanders an astounding 2,300 plus miles from Minnesota into the Gulf of Mexico. It all begins as a mere trickle – that soon flows through six reservoirs.
Right now, the U.S. Army Corps of Engineers is drawing down those lakes in northern Minnesota. Spokesperson Patrick Moes says the annual task helps reduce the chance of flooding.
“That process should be complete in the coming weeks. The whole purpose is to drop the lake level within each of the lake level within each of the reservoirs enough to allow the spring melt to fill the lake back up and then protect downstream .from any catastrophic flooding,” Moes says.
Moes says in order to gage how much water to release, the Corps must calculate the amount of snow on the ground – and its water content.
“Earlier this month we sent a team out to do snow surveys to determine how much water is contained within the snow pack within the Mississippi headwaters to determine how much water is contained within the snowpack. And then we sent another team on the exterior part of Minnesota to verify those numbers as well,” Moes says.
The Corps passes along its findings to the National Weather Service. Senior Hydrologist Diane Cooper says its technicians add data.
“They’re running the models right now. We look from a climatological perspective of what’s the typical number of years we have flooding problems during this period; we do three month outlooks and we kind of do chunks to look at risks if flooding in that three month time frame,” Cooper says.
Cooper says scientists conclude that “at this moment” the upper Mississippi faces a “normal” risk of flooding.
“And so that’s a key part of the message; we’re looking at a normal risk,” Cooper says.
However, she admits Mother Nature recently tested the prediction by throwing in a couple storms – snow and rain.
“The rain melted the snow; it was able to percolate into soil. So, even though we are in extreme drought still, the top layers of the soil got saturated and then we had the cold air move back in – so all that water in the top layer of the soil froze and froze solid,” Cooper says.
It’s called concrete frost. Cooper says it would not take much rain to run off the frozen mass and cause flooding.
“The unfortunate thing about this situation is because that water hits the ground and it’s not able infiltrate into it, we’re concerned that from the long term perspective, this wonderful snow pack may not help the drought hardly at all,” Cooper says.
Weather service Hydrologist Brian Hahn has already seen several Wisconsin rivers overrun their banks.
“The Sheboygan River at Sheboygan, Fox River at Burlington, Crawfish River at Milford and the Rock River at Newville,” Hahn syas.
According to Hahn, who monitors south central and southeast portions of the state, no river system is at higher risk than others.
“The next place that floods basically depends on how the precipitation falls; and how rapidly or how slowly the snow melts; and how rapidly or slowly the frost comes out of the ground,” Hahn says.
On Friday, the National Weather Service will release its spring outlook for river conditions throughout Wisconsin and beyond. From what Hahn has observed...
“Right now it looks like most of it is going to runoff; so you can say the river levels will have improved, but I’m not so sure about the ground moisture,” Hahn says.
That’s Hahn’s 40-year hydrology career talking.