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To keep lakes clean, keep leaves out of streets

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The Mississippi River
Bare trees are reflected in the Mississippi river as fallen leafs float on Sunday, Oct. 19, 2014.
Bridget Bennett for MPR News 2014

When you're raking your yard this weekend, water pollution experts offer some advice: Keep those leaves out of streets and storm drains.

Leaves are full of organic matter and nutrients like phosphorus. When they get blown or washed into the street, they end up in the city storm drain, which usually leads to a lake or river.

"Once they're in the water, those leaves — with all the nutrients and organic matter — can help grow algae," said Cathy Rofshus, spokesperson with the Minnesota Pollution Control Agency. "We don't want algae in our water, because then the water can be unfit for recreation like boating and swimming."

About half of the 180 lakes in the Twin Cities watershed studied by the MPCA don't meet state water quality standards because of excess nutrients.

Leaves can be a significant source of those nutrients — as much as 60 percent in some lakes, Rofshus said.

In 2016, a U.S. Geological Survey study found that leaves and other organic matter left behind in the fall contributed 56 percent of phosphorus in stormwater. That number dropped to 16 percent when leaves were cleaned up before a rainfall.

Rofshus says there are better things to do with those pesky leaves: rake them, compost them or mulch them.

"The leaves are really good for the soil in your yard and garden," she said. "So if you can mulch them in place it can help your grass grow next spring. If you can compost them, that's good too."

She also recommends cleaning out any plugged storm drains and picking up any leaves left behind by street sweepers.

"Whatever we can do to help out is good for our water," Rofshus said.