Dane County Board members wrestling with the aftermath of extensive flooding caused by the immense Aug. 20 rainstorm on Thursday heard from the department charged with maintaining the water levels of the Yahara River lakes.
The county’s Land and Water Resources Department is focusing on improving the flow of water through the chain, said Assistant Director John Reimer.
The seasonal minimum and maximum lake levels are set by the state Department of Natural Resources, but those maximums have been more regularly breached in recent years because of heavier rainfalls. When the storm clouds passed over western Dane County Aug. 20, Lake Mendota was already more than a foot above the summer maximum.
Reimer told Dane County Board members Thursday that outflow of water from the lakes has compounded the problems from heavy rains and increased surface water runoff caused by urban developments. Multiple choke points have been created by built-up sediment, bridges over the river and aquatic plant growth, slowing water movement and extending the time it takes to lower lake levels after a storm.
“We’ve got these huge pulses of water coming in, and it takes longer to get out,” Reimer said.
Because of the choke points and the current outflow, the county often keeps two of its three dams on the chain fully open, he said. The Babcock dam at the outflow of Lake Waubesa and the La Follette dam at the outflow of Lake Kegonsa need to be open to maintain lake levels, and because they are always open, they cannot be used to speed up the flow.
If the blockages were cleared, those dams could typically be closed to some degree in normal weather conditions, then opened when water needed to be moved out of the upstream lakes at a quicker rate.
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Board Chairwoman Sharon Corrigan said board members will look at short-term flood mitigation efforts and improvements to better deal with heavy rains as it considers the 2019 budget. Corrigan is also the lead sponsor on a resolution introduced Thursday that will convene a technical work group to create recommendations to prevent future flooding.
The main method the county currently uses to open the choke points is plant harvesting. The county has 11 harvesters, which remove truckloads of weeds from the river, Reimer said. The weed growth in the river and lakes is caused in part by nutrient runoff from farm fields that then fertilize aquatic plants.
The county has found that the water flows 2½ times faster when excess plant life is removed from the waterways.
About 700 dump-truck loads of aquatic plants were removed by harvesters this year, Reimer said.
To remove the sediment, the department would need to dredge about 2 feet from the bottom of the river — although that depth depends on the section of the river — but the department does not currently have a permit to dredge. The department would also need to assess the material that would be dredged for toxins that could be released, which would affect what dredging methods are used.
Reimer said it is critical for downstream waters to be at their minimum-allowed levels because those lakes would then have the flexibility to hold more water from overwhelmed lakes upstream. The county controls this with its three dams and coordinates with the city of Stoughton, which manages the next dam downstream. That coordination began last year, Reimer said, and helped the flooding situation this year.
It’s still going to take time — possibly into next spring — to get the lakes back within their permissible ranges, Reimer said, but the flows have opened up.
“Water levels are going down nicely right now, and we’re hoping it can keep going this way,” he said.
If the weather stays predominantly dry, it could still take about two months for all of the lakes to go down to their summer maximum levels, Reimer said.