Most of the 12- to 15-inch deluge that drenched parts of western Dane County flowed away from Madison toward the Wisconsin River.

Only about 4 inches fell across the lakes’ watershed, but that 4 inches swelled Madison’s lakes enough to flood streets and homes. Even before the rain fell, Lake Mendota was nearly a foot higher than the official maximum. For most of the summer the lake was 8 to 9 inches higher than it should have been in part because underwater plants and delays at a small dam hindered efforts to move excess water downstream.

Staff used ten mechanical weed harvesters to remove over 270 loads of aquatic plants in an effort to increase the water flow out of the chain of lakes. That doubled the flow of water, so more water is leaving the Yahara chain than what’s entering, allowing lake levels to stabilize and lessen the impact of the already flooded areas.

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Harvesting of aquatic plants occurs over an area of 485 acres of water and 64 miles of shoreline in the Yahara lakes. Following last August’s heavy rains, a team of Dane County aquatic plant harvesters worked the river corridor and successfully doubled the flow of water moving south out of the Yahara Chain of Lakes. Last year, 1,847 loads of aquatic plants were removed, resulting in around 8,496 tons of material leaving the system. About 691 loads (or 3,178 tons) were removed for flood mitigation.

In addition to flood mitigation, aquatic plant removal can also cut down on the amount of phosphorus found in the Yahara Chain of Lakes, which can increase the frequency and extent of hazardous algae blooms. About 4,851 pounds of phosphorus was removed from the chain of lakes system in 2018. It’s estimated that about one pound of phosphorus can produce up to 500 pounds of algae.

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