The dam that hems in the Cannon River, forming the biggest lake in the south metro area, needs an update if it's going to stand up to a worst-case flood.

The Byllesby Dam, on the border between Dakota and Goodhue counties near the town of Randolph, is safe and holds water just fine, but the Federal Energy Regulatory Commission has said it should be upgraded to handle 100 percent of a "probable maximum flood."

Even one-quarter of such a flood has never been seen there. To reach the worst-case scenario, the area would need to get the equivalent of 24 inches of rain over a 10-square-mile area in six hours.

"There's a ridiculously low possibility that that's ever going to happen," said David Swenson, Dakota County's water resources director. "But it's still a good idea to do [updates]."

Dakota and Goodhue counties, which share ownership of the dam, would split the estimated $4.8 million cost of the upgrade. The counties will apply for a dam safety grant from the Minnesota Department of Natural Resources to cover up to 50 percent of the bill.

A study that will reevaluate the magnitude of a worst-case flood -- last calculated in the 1980s -- will be finished in June and the counties will base their construction plan on that new measurement, starting work in 2011.

Dam updates could include changes that would allow water to flow over the perimeter dam, just north of the hydroelectric main dam, and higher gates on the top of the dam to better control water flow.

The hydroelectric Lake Byllesby Dam was constructed in 1910 as Henry Byllesby expanded his electrical empire, Consumers Power Company. It was donated to Dakota and Goodhue counties in 1969 and in the early 1980s began producing hydroelectric power for about 2,400 homes.

Previously, the dam needed to meet standards for 50 percent of a probable maximum flood. The federal commission increased the requirement to 100 percent because of additional development downstream.

The calm waters in the reservoir, 3.5 miles long and covering 14,800 acres, draw boaters each summer, and people often fish at the base of the dam. It also draws visitors to the Lake Byllesby Regional Park.

"The dam is safe and can handle levels we've seen over the last 100 years, but it's just a precaution," Swenson said.

Katie Humphrey • 952-882-9056