Miles of phosphorus-laden muck must be removed from streambeds that lead to Madison’s lakes.

It’s the most promising and dramatic way to improve the health of Mendota, Monona and the other lakes in the Yahara chain, which has been fouled by heavy weeds and algae most summers.

Dane County Executive Joe Parisi’s plan to vacuum the polluted sediment from the bottom of streams deserves support from the County Board and public. The sludge has built up over more than a century from manure and other nutrient-rich material washing off barnyards and fields.

Parisi is proposing spending $12 million over four years to remove 870,000 pounds of phosphorus from 33 miles of waterways leading to Madison’s lakes. The result — when combined with other efforts to keep new sources of manure, soil, leaves and other organic material out of the water — should be cleaner lakes, which are vital to the region’s identity, quality of life and economy.

The muck is more than 2 feet thick in some stream sections. This includes part of Dorn Creek, which leads to Governor Nelson State Park and Lake Mendota. The phosphorus that settled in Dorn Creek long ago steadily leaches into the water at what officials believe are potent concentrations.

Because Dane County, working closely with farmers northwest of Lake Mendota, has done a good job of protecting the lakes from additional manure, going after the slurry that’s already in the streambeds makes sense because it won’t be quickly replaced.

Parisi told the State Journal editorial board this week that many farmers upstream of Lake Mendota have lowered the phosphorus leaving their operations to about half of what’s allowable. And the county has calculated that removing the muck from streambeds may be the most cost-effective way to make further progress.

Dane County should closely monitor the impact of removing the worst sections of sludge to gauge whether additional dredging is worth the expense.

Significantly, Parisi’s plan has the backing of the Clean Lakes Alliance, a diverse coalition of private and public groups and individuals.

Besides going after the pollution already lodged in streambeds, Parisi’s county budget seeks to help smaller agricultural producers safely store manure on land to avoid harmful discharges. It helps fund further conservation practices, such as buffer strips at the edge of fields, and it would partner with local communities to reduce urban stormwater runoff.

The Madison region has made a lot of progress at protecting its lakes in recent years. But the miles of sludge that have sat at the bottom of streambeds can’t be ignored. The County Board should approve Parisi’s request to clean the streams. A century of foul sediment won’t disappear in our lifetimes on its own.

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