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Yahara River lakes at manageable level heading into winter after summer overflow and flooding, Dane County officials say

December 17, 2018

The water levels in the Yahara chain of lakes are just a few inches above the average for this time of year, according to Dane County Land and Water Resources Department assistant director John Reimer, putting the county in a good position to avoid the weeks-long overflow and flooding seen over the summer.

Historic rains on Aug. 20 drained into lakes Mendota, Monona, Waubesa and Kegonsa, overwhelming the lakes. Homes and businesses on the Isthmus and along some of Lake Monona’s shorelines were left to rely for weeks on sandbags and water pumps to prevent flood damage on their properties.

One of the reasons water levels remained so high at the end of the summer was the rate of water flow through the Yahara River, Reimer has said. Aquatic weeds and debris that collected at railroad bridge trestles had slowed water flow to a crawl, meaning the water couldn’t drain from the lakes. The county deployed several weed harvesters and worked to remove much of the debris to open up those waterways.

Reimer said the lakes remain 3 to 8 inches above the 40-year winter average, so a large volume of water is still moving through the system even though lake levels are far lower than they were in September.

“Since we’re a little bit higher, our flow rates are also a bit higher,” Reimer said.

Even when the lakes freeze over for the season, water will still move through the chain of lakes, Reimer said, because water underneath the frozen surfaces still runs through the Yahara River.

Though water levels are above average, the water is at a manageable level and doesn’t indicate any greater concern for flooding heading into the spring than any other year, Reimer said. The amount of precipitation — either snow, ice or sleet — and the thaw patterns are big variables though that could raise concern.

If Madison sees a lot of accumulating snowfall in the coming months that thaws all at once in the spring, it could have the same effect as a major rainfall, Reimer said, because the water would be draining into the watershed all at once. Frozen ground wouldn’t absorb much of that melted snow either, so it would all flow into the lakes.

If snow accumulates, but melts at different times or different speeds, the water can gather into the lakes slowly and be moved downstream more quickly through the Yahara River, keeping the lake levels stable.

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