A voluntary evacuation order for 135 homes in the Cherry Brook area caused by the threat of a dam breaching will become mandatory on Saturday.

Halifax Regional Municipality will make the order mandatory when repairs start on the Lake Major dam.

Cherry Brook is on the Dartmouth side of the Halifax Harbour, about a 15-minute drive from the city.

The homes to be evacuated are near the Salmon River, which flows from Lake Major. The affected homes are located in North Preston, Cherry Brook, East Preston, Westphal, Lawrencetown and Cole Harbour.

Officials are trying to get the word out to residents now. Search and rescue teams will go door to door to notify residents.

Residents with questions are asked to call 311, the city's information line.

On the east side of the Little Salmon River, the evacuation centre is the East Preston Recreation Centre at 24 Brooks Dr. On the west side of the river, the evacuation ​centre is the City Heights Church in Westphal at 946 Main St. Both centres will open Saturday at 8 a.m.

Officials said they are worried about flooding if the dam breaches.

"If it were to break, we're talking about a lot of water, so we want to make sure that people are a safe distance if something were to happen," said Jennifer Stairs, a city spokeswoman.

The work is only expected to take one day to complete and will begin at about 9 a.m. and should be completed by 5 p.m.

The repair work will consist of placing some rock fill near a fish ladder close to the Lake Major dam. The fish ladder is a passageway for fish to bypass the dam.

4 bridges to be closed

Four of the six bridges in area will be closed and barricaded by police. Those include the bridges on Lake Major Road, Salmon River Drive, Crane Hill Road and Ross Road.

The bridges that will remain open are the ones that run over highways 107 and 207.

"Even if you don't live in the area, if you're travelling through the area tomorrow … you'll see some impacts, so we want to make sure people have that information ahead of time," Stairs said.

Once the repair work begins, the risk of flooding will increase.

"There isn't any serious concern at the moment, but we're trying to be precautionary here and get people out ahead of time, if at all possible," she said.

Crews discovered a problem with the dam when they were doing routine maintenance.

Carl Yates, the general manager of Halifax Water, said that design work for the full replacement of the dam was already underway.

"It's actually one of our high-priority projects on our books to do," he said.

The chair of the Lake Major Watershed Advisory Board said he didn’t know any details of what was happening Friday evening, but his group had been briefed by a Halifax Water official in the fall about the aging dam.

"The discussion was that it was an older dam and that the dam was obviously going to be in need of repair or replacement sometime in the near future," Sparks said.

There was no sense of any immediate problems, he said. The group was told studies were being done to determine whether the dam should be replaced, repaired or moved.

Lake Major provides water to the greater Dartmouth area.