Image: In the distance can be seen the size of the landslide that has plugged the Chilcotin River southwest of Williams Lake. / Govt. of BC

By Fraser Valley Today Staff

Environment

Water expected to begin passing over the Chilcotin River landslide soon

Aug 4, 2024 | 5:39 PM

VICTORIA — New modelling suggests water will start to move over overtop the landslide along the Chilcotin River Sunday (Aug. 4) or early on Monday.

In a news release Sunday afternoon, provincial authorities say in the best-case scenario, it is anticipated that it will take 12 to 24 hours for the trapped water to pass through the blockage. The scenario predicts flows below typical spring flooding peaks on the Fraser River but significantly higher than usual on the Chilcotin River. If the water moves through in 12 hours, the Chilcotin River could see peak flows over 10 times greater than a typical freshet peak. In the 24-hour scenario, peak flows could be over seven times greater.

The worst-case scenario involves rapid drainage of the lake behind the dam within one hour, resulting in significantly higher flows on the Chilcotin River and flows at or below freshet peaks on the Fraser River. This could cause the Chilcotin River to rise to 21 meters at the Farwell Canyon bridge and 12 meters at its confluence with the Fraser River. Peak flows could reach Lillooet in 12 to 13 hours. Updated modelling data has been shared with communities that could be impacted.

Since the landslide on Tuesday, there has been minimal water seepage through the natural dam, causing the water level behind the dam to rise steadily at approximately 18 centimeters per hour.

With the assistance of external experts, the government is maintaining around-the-clock surveillance. The monitoring effort includes aerial survey flights and the collection of additional technical data to better understand the evolving situation.

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An evacuation order issued by the Cariboo Regional District remains in effect, and people are urged to stay out of the Chilcotin River valley between Hanceville and the Fraser River confluence. Residents are also advised to avoid the banks of the Fraser River. Ongoing planning aims to address potential debris impacts related to public safety.

The landslide incident also poses risks to spawning salmon and other fish species. The Province is collaborating with First Nations and Fisheries and Oceans Canada to establish an incident command structure, mobilize resources, and plan mitigation efforts.