Water levels at the Toddbrook Reservoir in Whaley Bridge have been reduced by half a metre since Thursday but the damage to the 180-year-old structure remains at a "critical level".
The Government was warning on Saturday that there is still a threat to life in the town and urged residents to remain patient.
It warns there could be damage and disruption from floodwater and lightning strikes.
Evacuated residents spent another night away from their homes on Friday and police warned it could be several days before they are allowed to return.
Prime Minister Boris Johnson promised that the damaged reservoir would have a "major rebuild" as he met locals at nearby Chapel-en-le-Frith High School, which is being used as an evacuation centre.
Describing the damage to the dam as "pretty scary", he said: "The plan is to try and stop the dam breaking, clearly. And so a huge amount of effort is going into that."
An RAF Chinook and around 150 firefighters using high-volume pumps appear to have partly stabilised the reservoir's spillway, with further pumps brought in by officials on Friday.
Mr Johnson, who arrived by helicopter, said he thought they had to get the level of the water down about eight metres, although there was some discussion with the surrounding officials about whether this was the exact figure.
He added: "I was talking to one of the villagers from Whaley Bridge who said that he remembered something like this happening 50 years ago.
"We've had an exceptional weather event, we must make sure that this dam can cope with it in the future.
"That will mean a major rebuild, clearly."
Environment minister Therese Coffey warned that the situation remained in a "critical stage" on Saturday as she issued a message to residents.
"I cannot emphasise enough how patient they need to be," Dr Coffey told BBC Radio 4's Today programme.
"Of course this is a distressing and worrying time for them, but if that dam were to breach it would lead to a loss of life if there were people there. There's no doubt about that."
Those evacuated from their homes were allowed to return briefly on Friday evening to pick up any vital items or pets.
The operation allowed just one person from each household to return for a maximum of 15 minutes.
Derbyshire Police said that any residents who re-entered Whaley Bridge would be doing so at their own risk and that the threat to life remained high.
Around 1,000 people were evacuated from the town but most found their own accommodation with family and friends, according to Derbyshire County Council.
Police have closed railway lines in the Whaley Bridge area over the risk of potential flooding which is due to continue into the weekend.
The reservoir is on the north-west edge of the Peak District National Park and was built in 1831, according to experts, although the Environment Agency records it as being built in 1840-41.
According to a 2011 Environment Agency report on national dam incidents, Toddbrook "has a history of leakage".