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Where is the Rapidan Dam and what is its history?

Rapidan Dam had been in poor condition before failure
Rapidan Dam had been in poor condition before failure 01:31

RAPIDAN TOWNSHIP, Minn. — On Monday, officials in southern Minnesota announced the Rapidan Dam experienced a partial failure and remains in "imminent failure condition."

Residents in low-lying areas of the Minnesota River Valley are advised to closely monitor the situation and prepare for the possibility of evacuation.


County officials said Tuesday the breach threat has diminished, but they are still concerned because the dam was built on sandstone bedrock that can further erode.  

Where is the Rapidan Dam?

The Rapidan Dam is located on the Blue Earth River in Rapidan Township — about 14 miles south of Mankato. 

The dam is situated just north of the County Road 9 Bridge. 

What is its history?

When it opened in 1910, the dam doubled the power capacity of Mankato's electric system, according to the Blue Earth County Historical Society.

The dam was constructed using hollow reinforced concrete.

The dam was owned and operated by Northern States Power Company until 1970 when it offered the dam and surrounding 750 acres to Blue Earth County "as an environmental corridor of wildlife habitats, recreation areas and nature trails," says the county's historical society.

However, in 1972, the historical society says the county board ruled the dam should either be rebuilt or torn down since the dam would cause "considerable damage" to the surrounding areas should it collapse.

To address safety concerns, engineers added more concrete reinforcement to the Rapidan Dam in the 1980s. The National Society of Professional Engineers called the renovations "one of the top engineering feats of 1986."

In 2002, emergency repairs were made after discovering extensive undermining of the dam's foundation.

Blue Earth County says that over the years, Rapidan Dam has experienced regular flooding, with the second-highest flood on record happening five years ago, in 2019. That flooding caused "significant damage to the dam's structure and usability," and it has not been able to produce energy since then, according to the county's website.

A study from 2021 identified two feasible solutions for the dam: repair it or remove it.

From 1999 through 2022, a report found that the dam actually lost Blue Earth County over half a million dollars.

Officials for Blue Earth County formed the Future of Rapidan Dam to "identify the community's needs and concerns and use their input" to make the best decision regarding what to do with the dam.

A dam repair feasibility report commissioned by the county in 2021 estimated it would cost at least $15.1 million to repair the dam. However, the county reports it would likely lose over $4 million on the dam over 40 years. Removing the dam was estimated to cost at a minimum of $65.2 million.

In August 2022, Blue Earth County presented the final engagement summary, which found the majority of people surveyed supported the dam being repaired.

Ultimately, since both options had significant costs, neither were implemented.

In March 2023, Blue Earth County began the Federal Energy Regulation Commission licensure exemption surrender process for the Rapidan Dam since the dam is no longer generating hydroelectricity.

The National Inventory of Dams rated the dam in poor condition as of April 2023, classifying its hazard potential as "significant."  

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