Dakota County is upgrading the spillway capacity of the Byllesby Dam in order to meet Federal Energy Regulation Commission requirements.
Federal Energy Regulatory Commission regulations require that the dam meet standards for 100 percent of a Probable Maximum Flood, the most severe possible flood calculated by combining information about precipitation, geology and water management strategies. To reach the worst-case scenario of a Probable Maximum Flood, the Cannon River Watershed area would need to receive the equivalent of 24 inches of rain over a 10-square mile area in six hours.
Although the dam is structurally safe, the spillway capacity did not allow the Byllesby Dam to safely pass the Probable Maximum Flood. Construction of the spillway improvements includes adding a gate structure and modifying the existing spillway. Construction will continue into late 2014.
An architectural drawing shows a model of the proposed new spillway and surrounding area. The drawing includes a pedestrian bridge crossing the Cannon River that will be constructed and financed by the Department of Natural Resources.
Spillway construction activities will not affect the lake level or downstream river flow.