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Rock should stabilize 150-year-old dam

By Gretta Becay

news@postbulletin.com

Working through winter’s cold weather, a crew recently put about 200 tons of rip rap on the lower face of the Mantorville Dam to stabilize it.

A year ago, the Mantorville City Council began investigating repair and maintenance options for the 150-year-old dam. Last summer, the top concrete cap was removed and holes in the structure were repaired.

In late September, a city crew opened the flood gates to drain the pond behind the dam, sluice the river bottom and to inspect the dam. This work is done about twice each decade. This time, deterioration of the central limestone support for the gates, undermining of the area under the benches and the absence of rip-rap behind the dam were revealed.

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In February, the council approved an $8,500 bid from Swenke Co. from Kasson to furnish and install about 200 tons of rip rap on the lower face of the dam. The bid estimated the rock would cover the entire 165-foot length of the dam, 4 feet high and 6 feet out from the base.

In previous years, the river below the dam has been dredged from under the Minnesota Highway 57 bridge to re-situate the rock that is washed away during the warmer months. Lonnie Zelinski from Swenke explained to the council in February that this larger rock will not erode as easily and should help stabilize the dam.

Coincidentally, the day the placement of the rip rap was approved, an unseasonable warm front moved through the area, the ice behind the dam broke up and large chunks began flowing over the top.

Recent cold weather refroze the top of the pond behind the dam and the rip rap was placed Tuesday.

The dam was built after the Civil War and was rebuilt 18 years ago. The Department of Natural Resources has recommended removing it and turning the area into a rock rapids. Some community members say the dam is a landmark they’d like to keep.

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