It said a dam wall at the 19th-century Toddbrook Reservoir above the town had been damaged by days of heavy rain.
Police said a Royal Air Force Chinook helicopter was shifting 400 tonnes of aggregate - a mixture of sand, gravel and stone - onto the reservoir wall to reinforce it.
Emergency crews are now racing to save a damaged reservoir, and thousands of residents have been evacuated amidst fears the town could flood.
The dam towers above the town of Whaley Bridge, and thousands of people have been evacuated from their homes for an unknown length of time due to the risk of flooding.
"We just hope that everyone's safe, that's the main thing, and that people have their homes to go back to", he said.
The Chinook will also be dropping aggregate into other parts of the reservoir today, to stem the flow of water going into it.
Julie Sharman, from the Canal and Rivers Trust, said it was "a critical situation" but added that the weather had improved and the water levels had reduced by 20cm.
She said strong winds on Wednesday had caused the dam to overflow, which she described as a safety feature and nothing to be concerned about.
This work was done in conjunction with expert structural engineers, who have been advising the emergency response since yesterday afternoon.
"We would ask for residents to continue to heed police advice and stay away from Whaley Bridge and further updates will be delivered as soon as we have them".
The Derbyshire Police said residents of Whaley Bridge should leave the town after Britain's Environment Agency said the incident "currently poses a significant threat to life".
"The operation loading the front face of the dam with the Chinook helicopter is still in progress".
"The majority have been able to find accommodation with family and friends".
They are attempting to get the reservoir's water level down, to reduce pressure on the wall and allow repairs to begin.
But on Thursday a section of the dam suddenly broke, sparking "severe, danger to life" warnings from the Environment Agency.
The sleepy Derbyshire town is facing total destruction as a hole in the Toddbrook Reservoir dam threatens to burst and collapse.
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