Geohazards

Watercourses and ecosystems in the steep Coast Mountains have evolved to handle the typical high volume of rainfall and snowmelt of the region—and to rebound from occasional more powerful events such as atmospheric rivers (sustained, multi-day rainstorms in which a narrow plume of subtropical moisture funnels into a focused area of the coast). However, human activity (e.g., logging, agriculture) and the occupation of floodplains in concert with the more extreme weather of climate change have combined to exacerbate the effects of such events on the landscape, leading to periodic flooding.

In the northern part of the Fire & Ice Aspiring Geopark, the town of Pemberton is particularly vulnerable, located as it is in a narrow valley surround on all sides by steep, glaciated peaks. Here, as the Lillooet River drains icecaps and heavily glaciated mountains including the Mt. Meager volcanic complex, it is joined by Millar Creek, Ryan Creek, the Green River and the Birkenhead River before flowing into Lillooet Lake. Although the Lillooet River has seen several prehistoric outburst floods due to glacial and volcanic activity, the handful of notable historic events is largely due to a water table that lies close to the surface across the entire valley, and the diking and channeling done to accommodate the town and its agricultural base cannot accommodate the most severe events. In recent years evacuations due to flooding have become more common in Pemberton—including an unprecedented late June 2021 snow-and glacier-melt flood.

The town of Squamish, at the southern end of the Fire & Ice Aspiring Geopark, is both a target for heavy coastal rainfall and the terminus of two great glacier-fed rivers draining wildly precipitous terrain—the Squamish and the Elaho. Thus, also has to contend with the occasional threat of flooding and evacuation. Although Pemberton and Squamish have been on alert at different times for different reasons, the atmospheric river event of October 2003 saw both towns face simultaneous washouts and evacuations.

Atmospheric rivers have become more common in recent years. Between September and November 2021, no less than eight hit the B.C. coast, the first seven causing episodic flooding and minor damage in various places. So severe was the widespread damage to infrastructure following the final event of 15-18 November 2021, however, that rail lines, an oil pipeline, and all highways were shuttered between Vancouver and the rest of B.C. due to damage. This situation—no access of any sort from the Coast to the Interior for weeks—was unprecedented in B.C.’s history.