Rock arch rapids and other features moderate river flow when a substandard dam is removed and natural river flow is restored.
When a flood damaged the Lake Shady dam beyond practical repair, Olmsted County saw an opportunity to return the upriver lake to a riverine, wetland, and upland bionetwork. The project needed to restore the river’s ecological health and natural beauty without impacting high water elevations during future flood events while also creating new recreational opportunities for the community.
Our design called for demolishing the existing concrete dam and constructing an 800-foot-long series of 10 rock arch rapids to withstand the tremendous stresses caused by an estimated 100-year storm event flow—approximately 2,800 cubic feet (79 cubic metres) per second. Using specially quarried four- to six-foot boulders, the arches were built under low river flow and frozen conditions to preserve the lakebed and support heavy equipment.
The project restored 1.05 miles (1.69 kilometres) of river channel with 120,000 cubic yards (91,700 cubic metres) of saturated sediment, 11 riffle structures, and 2,665 linear feet (812 metres) of woody debris structures to stabilize banks and improve fish habitat. The scenic overlook adapted from a modified dam abutment structure and a 1,100-foot (335-metre) portage way encourage a variety of encounters with this reach of the Zumbro River now restored to its natural beauty.
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