A landslide along the Chilcotin River near Williams Lake, B.C. on Aug. 1.Willie Sellars/The Canadian Press
Water that has been backed up by a massive landslide in B.C.’s central Interior is expected to overtop the natural dam late Sunday or early Monday and enter the riverbed below, according to government modelling.
While authorities had warned downstream communities along the Chilcotin and Fraser rivers to prepare for a worst-case scenario, the modelling shows that the water will likely be flushed down river without the need for major evacuations.
A government update issued late Sunday afternoon said the best and more-likely scenario is that it will take 12 to 24 hours for the trapped water to move past the blockage, resulting in flows below typical freshet (spring flooding) peaks on the Fraser River, but potentially more than seven times greater than the typical freshet peak flow in the Chilcotin River.
In a worst-case and less likely scenario, the water could drain rapidly, with water in the Chilcotin River rising to 21 metres at the Farwell Canyon bridge and 12 metres at the confluence of the Fraser River. About 12 hours later, water could reach Lillooet – the closest community, about 150 kilometres from the slide site – where peak flows in the Fraser River could be similar to, or just above, typical freshet peak flow, according to the modelling.
People continue to be warned to stay away from the Chilcotin and Fraser rivers. Evacuation orders issued Wednesday by the Cariboo Regional District, which span 107 square kilometres along the Chilcotin, remain in place.
The landslide occurred late Tuesday night, resulting in a nearly full blockage of the Chilcotin River near Farwell Canyon. The province issued an emergency alert on Wednesday, warning that sudden, potential flooding and debris flow could pose a threat to human life.