Officials in British Columbia say if a landslide damming the Chilcotin River in the central Interior gives way it could lead to a flood surge for hundreds of kilometres.
A government statement says provincial personnel are assessing the risks on the ground and by air at the slide south of Williams Lake.
It happened sometime late Tuesday or early Wednesday, and the statement says if the blockage gives way, a sudden release could cause "rapid rises" in water levels along the connecting Fraser River all the way to Hope, 150 kilometres east of Vancouver.
People have been warned to stay away from the banks of the Chilcotin River downstream of the slide as well as the Fraser River north of Hope.
B.C.'s River Forecast Centre has issued a flood warning for the Chilcotin north of the slide, saying the debris is "creating a lake" that currently extends about seven kilometres upstream.
The forecast centre says the eventual breach of the landslide debris could lead to an "outburst flood" with a surge of water rapidly flowing downstream.
If that happens, it says the surge could reach the Fraser River within hours, and while flooding south of the Fraser Canyon would likely be less severe, substantial increases in flow could extend to the mouth of the river in Metro Vancouver.
Emergency Management Minister Bowinn Ma says the landslide material is piled 30 metres high, completely blocking the Chilcotin.
"The downstream riverbed is completely exposed. The risk of ecological impact is also significant," she said in a Facebook post on Wednesday.
Evacuation orders span 107 square kilometres along the Chilcotin, with the Cariboo Regional District saying the slide poses an "immediate danger to life and safety."
The district says 60 properties are covered by the orders, including 12 homes with an estimated 13 residents.
The provincial statement says the Ministry of Emergency Management is working with communities to co-ordinate response operations, and the BC Wildfire Service has dispatched helicopters to help with assessments and potential rescues.
"The province is prepared to take additional actions to keep people and communities safe in the event of flooding, such as deploying sandbags, sandbag machines, gabions and tiger dams to communities if needed," it says.
