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It took decades and a lot of head-scratching, but a long-standing Mississippi River backwater in Washington County is now history.

Several government agencies teamed up to replace a deteriorating earthen dam leading to Grey Cloud Island Township with a bridge that restored the river’s free flow. The new bridge, now “substantially complete,” will eliminate floods that sometimes trapped residents and will allow the river to flush an unsightly slough of stagnant water, said Nathan Arnold, a Washington County project engineer who worked on the $1.6 million project.

“Before we opened it up there was no movement in the channel,” Arnold said Monday.

The new bridge also leaves ample room for boats to pass underneath, he said.

Grey Cloud Island, near Cottage Grove, is among the smallest townships in Minnesota and falls within the Mississippi National River and Recreation Area, a unit of the National Park Service.

The Mississippi River borders it on the west, while the back channel loops around on the north and east, widening on the south into two small backwater lakes before reconnecting with the river.

The channel crossing dates to the early 1900s, when a wooden bridge spanned the channel. In 1923 the bridge was replaced with culverts and filled over, then paved as part of County Road 75.

Until a massive flood in 1965 and another in 1969, the culvert worked well. To keep residents from being isolated by floodwaters, emergency work was done to raise the road. As a result, culverts became blocked and milfoil and other invasive plants infested the channel, which is a little more than 2 miles long.

Resembling a swamp, the channel was so choked with weeds that boaters couldn’t use it, said Matt Moore, director of the South Washington Watershed District.

“We literally did see some changes overnight by changing the flow,” Moore said.

The channel’s restoration was a critical improvement for the Mississippi because “in those backwater areas, that’s where all the habitat is,” he said. “They’re really vital pieces of the river ecology.”

Washington County Commissioner Karla Bigham, whose district includes Grey Cloud Island Township, said the bridge project ensures that emergency vehicles can cross onto the island during high water.

Project funding was shared by the watershed district, Washington County and a state clean water grant.

Staff writer Jim Anderson contributed to this report.

Kevin Giles • 651-925-5037