Much good reporting has been done about the unprecedented flooding in the Yahara lakes.
People say they want lake levels lowered, especially in Mendota to temporarily store runoff that comes from its large watershed. But rewriting Department of Natural Resources rules to lower Mendota’s summer levels isn’t the answer. Getting rid of excess water from the downstream lakes is the problem. If Waubesa empties, then Monona drains and Mendota can release water faster. Currently, water can’t drain out of the lower three Yahara lakes fast enough due to poor river channel hydraulics with numerous pinch points and flow restrictions.
I have studied the lakes for over four decades and served on two Yahara Lake Advisory Groups dealing with lake levels. Lake levels routinely are lowered in fall to prevent shoreline ice damage. Fishery interests want refilled lakes by April and boaters want high water by May. By June, water levels in all lakes are supposed to be within the summer operating range set by DNR — a range of 6 inches for each lake. However, a rainy spring and/or summer pushes lake levels above maximums, making it difficult to lower the lakes in fall.
Dane County has an extensive lake level and river flow monitoring network operated by the U.S. Geological Survey. Data from September 2017 through August 2018 identified reasons for the flooding. Heavy rains in fall 2017 kept lake levels so high that by January the lakes were still two feet higher than winter minimums. Then big runoff events (from rain on frozen ground) occurred in January and February raising all four lakes to summer maximum in March. A rainy May and June kept lakes exceedingly high all summer until the Aug. 20 rainstorm produced the catastrophic flooding.
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Increasing the flow of water through the Yahara lakes system is key to managing lake levels and preventing future floods. Looking at the problem from the bottom up, the Stoughton dam must be opened enough to let Kegonsa drain. The Yahara Lake Advisory Group’s 2012 report unanimously recommended that operation of all Yahara River dams be centrally coordinated by Dane County to maximize efficiency. River flow rates near the Stoughton dam, which is operated by the city of Stoughton, were not always high this summer.
But the key bottleneck is moving water from Waubesa to Kegonsa through a long river channel that only drops 1.5 feet. This is especially tricky as the river passes through Lower Mud Lake which can be choked with aquatic plants (weeds) in summer. Weed harvesting in the channel downstream of Waubesa has been a primary lake management objective for decades. A former county public works director used to say the four most important things for managing lake levels were: “weeds, weeds, weeds and weeds!”
River weed cutting was certainly done in earnest during the recent flooding. But increased aquatic plant and filamentous algae growth in the lakes and rivers, possibly due to zebra mussels clearing the water with more underwater sunlight for plant growth, produced a clamoring for weed harvesting in the upper Yahara lakes this summer. Weeds apparently were also so dense in parts of the 5-mile river channel between Kegonsa and Stoughton that for the first time the county had to cut weeds to increase river flows.
The county’s 11 weed harvesters can’t be everywhere at all times. Thus, whenever lake levels exceed the summer maximum, priority should be given to keeping the river channels weed-free. This means harvesting in the lakes for recreational purposes would take lower priority — something that would not be popular with shoreline home owners and lake users. Dane County should consider expanding its weed harvesting fleet, especially if supported by funds from shoreline communities.
Lastly, the hydraulic pinch points in the Yahara River channel need to be addressed. This could mean: (1) dredging Monona’s outlet channel, (2) increasing the railroad bridge’s river flow capacity into Waubesa, and (3) channel modifications through Lower Mud Lake especially where water re-enters the river’s confined channel at a tight river bend. Just downstream of that bend is the Native American fish weir of channel rocks, an archaeological site needing protection. A diversion system would have to be built to by-pass the weir.
Another more radical solution would be an emergency system of big pumps deployed to move water from Waubesa to the river above Kegonsa. These recommendations fixing the downstream flow restrictions and pinch points will be expensive. Once completed, Mendota could be operated in the middle of its summer range (3 inches below maximum) without changing current lake level rules. It would also provide some temporary storage of runoff without operating the lake like a reservoir with widely fluctuating summer lake levels, something that would be very unpopular.
In addition to these fixes and changes, increased infiltration in the Mendota watershed would reduce runoff entering the lake. Farmers could be offered financial incentives to build water retention/infiltration basins which would not only reduce the volume of runoff entering Mendota, but would also reduce phosphorus pollution entering the lake — that’s a two-fer.
While some people deny the recent flooding was caused by climate change, trends of weather statistics are irrefutable. Southern Wisconsin is experiencing a changing climate as the region is getting wetter with more frequent big rainstorms. To be a resilient community, we need comprehensive, equitable solutions to prevent future catastrophic flooding in the Yahara lakes.
Lathrop, of Middleton, is an honorary fellow at the UW-Madison Center for Limnology: firstname.lastname@example.org