What we know about the Rapidan Dam on the Blue Earth River

The Rapidan Dam near Mankato is no longer in danger of “imminent failure” but the 114-year-old structure on the Blue Earth River was still battered by the raging water that slammed debris and sediment into it.

State officials on Tuesday said they expect the Rapidan Dam to survive the deluge that’s soaked much of Minnesota for the last few days.

Here’s everything to know about the dam that’s been endangered by catastrophic flooding in southern Minnesota.

Rapidan Dam is about 6 miles southwest of Mankato as the crow flies on the Blue Earth River. It was originally built to provide power to the city — the dam doubled the capacity of Mankato’s power system when it was first erected.

The Rapidan Dam was built in 1910. The facility was operated back then by the Northern States Power Co., according to the Blue Earth County Historical Society, and was constructed using a novel method at the time. Builders used hollow reinforced concrete to increase the structure’s durability.

In 1970, Northern States Power (now part of Xcel Energy) transferred ownership of the dam to Blue Earth County. That same year, the unincorporated community of Rapidan opted to skip the Fourth of July, instead celebrating Rapidan Dam Days four days earlier so that Minnesotans from neighboring communities could come and watch the fireworks.

Two years later, the Blue Earth County board ruled that the dam should either be destroyed or rebuilt because it would cause “considerable damage” to the surrounding area if it collapsed. The dam was one of the first facilities to be rebuilt with funding from the Public Utility Regulatory Policies Act of 1978, according to the historical society.

The Rapidan Dam is 87 feet tall and 475 feet wide

Not much. Rapidan Dam hasn’t provided electricity to the city for years — a 2019 flood damaged the facility so badly that county officials began to assess whether it was worth repairing enough so that it could generate electricity again. A 2021 study estimated repairs at $15 million and removal at $82 million.

County officials determined the profit they could make on whatever electricity the dam would generate was not worth the investment. Last year, they initiated the process of handing the dam’s operations over to the federal government.

The National Inventory of Dams has determined the dam poses a “significant” hazard potential for anyone working on the structure or people and wildlife downstream.

Still, the dam doesn’t “hold back a significant amount of water,” officials said Tuesday morning.

Since the 2019 flood, the Blue Earth River has fully flowed through the Rapidan Dam. That storm sent 32,500 cubic feet of water through the structure every second. At its peak, the water flowed at 34,800 feet per second during the most recent storm, officials said.

State officials projected that a water flow of 42,000 cubic feet per second would make the water in the Blue Earth River rise about 2 feet within two miles downstream. A larger threat, officials said, looms in the built-up sediment that would be unleashed downstream if the dam collapses.

No. Back in 1965, officials say, a 500-year flood sent more than 43,000 cubic feet of water per second toward the structure.