Damage appeared in the wall of Toddbrook Reservoir dam, which holds 300 million gallons of water, this morning following heavy rain.
In the afternoon, the town's 6,500 people were told to evacuate to a school in neighbouring Chapel-en-le-Frith.
Engineers and emergency services are working through the night to reduce the water level and shore up the damaged dam.
A Chinook helicopter will be used to drop aggregate and control the flow of water.
Damage appeared in the wall of Toddbrook Reservoir dam, which holds 300 million gallons of water, this morning following heavy rain.In the afternoon, the town's 6,500 people were told to evacuate to a school in neighbouring Chapel-en-le-Frith.Enginee