Flooding on the Blue Earth River breached the Rapidan Dam Monday morning and it's now in an "imminent failure condition," officials said.

Le Sueur County residents in low-lying areas of the Minnesota River Valley are advised to closely monitor the situation and potentially evacuate, according to the county's emergency management office. The agency initially said the dam had failed, but later updated that the dam had been breached. 

The National Weather Service has issued a flash flood warning for areas downstream until 10:30 a.m. Tuesday.

The Blue Earth County Sheriff's Office says debris started accumulating on Sunday at the 114-year-old dam, located about 10 miles southwest of Mankato, placing it "in imminent failure condition."  

Monday afternoon, county officials announced there had been a partial failure on the west abutment. The dam is still intact and there are no plans for mass evacuation.

Part of the river has been diverted around the west side of the dam where water continues to flow, the sheriff's office says.

Dam operators alerted officials at about 10:36 a.m. Monday the river had significantly eroded earth around the west side of the dam on the south side of Mankato, the National Weather Service says.

Pieces of an Xcel Energy building and other county buildings are adding to the debris in the water.

Gov. Tim Walz released a statement on the dam Monday afternoon. 

"Our agencies are in close contact with Blue Earth County and other local officials regarding the Rapidan Dam near Mankato. Emergency management is on the ground and acting quickly to ensure the safety of Minnesotans as the situation develops," Walz said. 

Officials also say the County Road 33 Bridge and County Road 90 Bridge are being closely monitored for debris passing downstream and may be closed if needed.

The Blue Earth County Sheriff's Office says they are also aware of power outages and are working with Xcel to restore them.

The failure is expected to cause the Minnesota River at Mankato to crest just below major flood state on Tuesday morning, the National Weather Service says.    

The Blue Earth River begins in northern Iowa and meets with the river's west branch in Faribault County in southern Minnesota, according to the Minnesota Pollution Control Agency. From there, it flows 108 miles north past the cities of Blue Earth, Winnebago, and Vernon Center to Mankato, where it enters the Minnesota River.

There are 21 cities in that watershed, of which Mankato and Fairmont are the largest.

County officials plan to hold a press conference on Tuesday at 9 a.m. to share more information about the situation.