The reservoir contains around 1.3m tonnes of water.
“The plan is to try and stop the dam breaking, clearly. And so a huge amount of effort is going into that,” Mr Johnson said to a group of residents when he visited the town on Friday evening.
“The Chinook’s been over putting in the aggregate and putting in the sandbags to try and stop it bursting. They’re pumping out huge amounts of water.”
“We’ve had an exceptional weather event, we must make sure that this dam can cope with it in the future,” he added.
“That will mean a major rebuild, clearly.”
The reservoir’s spillway partially crumbled on Thursday after heavy rain fell in the area.
The RAF Chibook has dropped around 400 tonnes of sanbags in an effort to repair the dam.
But officials said it was unclear how long the crisis would go on for. The reservoir’s water level has dropped by half a metre, but needs to drop by several more metres to be considered safe.
“We will be putting plans in place for residents to return to their home to pick up very vital things they need along with their animal welfare,” said Kem Mehmet of Derbyshire Police.
“This is very controlled, I must stress that, because this is still life at risk.”
Police have closed railway lines in the Whaley Bridge area over the risk of potential flooding.
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.
Meanwhile, clean-up operations are under way across parts of the North West hit by heavy rain. Residents in Poynton in Cheshire were also evacuated on Wednesday night.
If you would like to see how the emergency response unfolded live, please see what was our live coverage below:
Good morning and welcome to The Independent's live blog coverage of the Whaley Bridge flooding.
An RAF Chinook has been drafted in to a Derbyshire town in efforts to stop a reservoir collapsing after it was "badly" damaged following two days of heavy storms.
Thousands of Whaley Bridge residents have been evacuated in what police call "an unprecedented, fast-moving, emergency situation".
Toddbrook Reservoir, which contains around 1.3 million tonnes of water, has seen "extensive" damage during the flooding and images appear to show a huge hole in the dam wall.
Speaking on Thursday evening, deputy chief constable Rachel Swann, chairwoman of the Local Resilience Forum, said: "At this time the future of the dam wall remains in the balance and I would remind people of the very real danger posed to them should the wall collapse."
Colin Drury, our North of England correspondent, has been speaking to residents of Whaley Bridge who fear dam collapse will see their homes disappear in the blink of an eye.
Over 6,000 residents have been evacuated from the town after two days of heavy rain saw water topping the overflow flow wall at the nearby Toddbrook Reservoir.
Here’s his report:
Derbyshire Fire and Rescue Service has been working through the night to help stem the flow of water into the reservoir.
A helicopter was sent from RAF Odiham in Hampshire to assist at Toddbrook Reservoir, a Ministry of Defence spokesman told PA.
"It will drop one-ton bags of aggregate – a mixture of sand, gravel and stone – into Todd Brook,” he said.
Videos shared by Shirebrook Fire Station show the Chinook laden with the aggregate as it hovers above the dam wall.
Derbyshire Police have posted a series of updates on Twitter about their efforts to secure the dam wall.
They say the RAF Chinook has helped to move 500 tonnes of aggregate to the reservoir to reinforce it. And 16 high volume water pumps have been installed in order to reduce water levels.
It could be a “at least 24 hours” until the risk of the dam collapsing is ruled out, the chief executive of the Canal and River TrustRichard Parry said.
“We clearly don't know the nature of the failure, we've not had the opportunity to examine it, but we're operating in a very precautionary way with the other agencies,” he told BBC Newsnight.
“Our first priority is to draw down the water and it's very important that we do keep everyone out of the area until that is done.
“It will be at least 24 hours, it could be longer, it really depends on how much progress we can make overnight and into tomorrow morning.”
A video, captured by David Brocklehurst, shows the RAF Chinook arriving at the reservoir as the sun rises.
Derbyshire's Chief Fire Officer, Terry McDermott said the level of water behind the dam wall seems “to be starting to reduce now”.
He said there are 150 firefighters at the reservoir, with 10 high-volume pumping crews from around the UK.
Speaking in nearby Chapel-en-le-Frith, Mr McDermott said: "It does seem to be starting to reduce now. I think a lot of that is because the amount of water going into the reservoir has slowed down."
But he said engineers remain "very concerned about it".
"The structural engineer is saying if we don't do something there will be a problem," he added. "It's not going to go away on its own. It's absolutely necessary, the activity that's going on at the moment."
A Muslim-faith charity has brought bags of supplies for people evacuated from Whaley Bridge overnight.
Penny Appeal brought the provisions to residents who had spent the night in emergency accommodation, Charles Lawley tweeted.
Julie Sharman, chief operating officer of the Canal and River Trust, said the reservoir level had gone down by about 8in (200mm) overnight but that the situation was “still critical”.
"And, until we're beyond that critical situation, the risk is a material risk and that's why we've taken the action we have," she added.
Earlier today, Ms Sharman told BBC 4’s Today programme: "The operation loading the front face of the dam using the Chinook helicopter is in process and is going to go on for most of the day here.
"Additional pumping is going in and the good news is that the inflow to the reservoir has reduced considerably.
"We've lowered the level of the water in the reservoir by 200mm. We are obviously aiming to get that down considerably more."
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