The outburst flood: A Chilliwack River landslide could create a temporary lake

Imagine arriving at the Chilliwack River one morning to find it has seemingly dried up overnight.

Would you take a picture? Inspect the puddles for signs of life? Start looking for fish to save?

Here’s what you should do: run for higher ground.

A spontaneously dried-up riverbed would not be a sign of drought, but rather that somewhere upstream, a landslide has spilled into the valley bottom, plugging the river.

Behind that landslide, water would be creating a small lake. When the water rises high enough to overtop the new landslide, it would begin to flow over, then through, the slide debris. Within a matter of just minutes, a surge of water would pour downhill, turning the barren riverbed into a series of raging rapids.

BC saw the potential impact of such an “outburst flood” last summer, when a landslide blocked the Chilcotin River in central British Columbia. Over several days, a large lake built up behind the slide. When the lake rose high enough to breach the landslide, water began to trickle over the debris. Within a matter of hours, the water cut a course over, then through, the slide debris. The lake quickly began to drain, sending a massive pulse of water pouring downriver. Only the fact that the Fraser River was already very low, prevented significant damage downstream.

Fortunately, a landslide-caused outburst flood on the Chilliwack River would not be nearly as large as that seen on the Chilcotin. But it still could be deadly. (Editor: The name of the Chilliwack River changes to the Vedder River as it exits the Chilliwack River Valley and flows beneath the Vedder bridge. It’s the same river though.)

The threat is well-known and has worried local officials and geologists for decades. In 1962, a landslide 12 kilometres east of the Vedder Crossing bridge blocked the river. Although water cut a small course through the slide, water began to back up. To remove the risk of a temporary lake forming, army crews stationed nearby at Camp Chilliwack were called in to blow a larger hole in the blockage. That slide occurred at Ranger Run, a notoriously unstable site that remains one of the two locations that most worry geologists.

The other is near the tiny community of Slesse Park. In 1997, slides in the area blocked a large portion of the river. The water again punched a new route through the debris, but Chilliwack Mayor John Less warned that a future slide could create a larger lake behind the slide.

Scientists have finally started studying the risk of such a flood in detail and have confirmed the threat is “very high.” The most extensive analysis of the threat was completed in 2023, but just published online earlier this year. That analysis looked specifically at a blockage created by a slide at Slesse Park, a neighbourhood with a slow-moving landslide that has worried officials for decades.

You can read the entire analysis here. 

The study suggests that, in the most likely scenarios, downstream property damage could be relatively limited. The threat to human life, however, remains signficiatn.

The nature of the river’s banks, and the valley’s relatively steep gradient, means that although a landslide near the Slesse Park community could block the river, any temporary lake would likely be smaller than that on the Chilcotin. That is relatively good news for property owners both within the Chilliwack River Valley, and in Chilliwack itself. But it doesn’t necessarily make such a flood less likely to kill.

Even if a blockage was short-lived, any quick-releasing outburst flood could be deadly, particularly given the presence of anglers, casual observers, and people living in recreational vehicles along the river.

For technical reasons related to the fact that landslides tend to occur some time after a rain event, geoscientists believe a blockage is most likely to occur when the river’s flow is relatively low. That would allow the river’s banks to accommodate much of the pulsing water from a sudden outburst.

For the three most likely scenarios considered, water would collect behind the slide before overtopping the debris. The size of the landslides would impact how much water collected—and how much time it took for the water to rise above the debris, and then begin eroding a new course through it. The water could collect for up to 20 hours. Or a blockage might burst after just six hours. Once the first signs of water arrived at a barren site downriver, the river’s level would rise extremely quickly, peaking just 10 to 20 minutes after the debris blockage was breached.

Officials would be able to warn anglers and recreation users to get out of the river during the period in which water was clogged, but there would be little warning after a breach near Slesse Park.

It would take less than half an hour for outburst water to reach Chilliwack River Provincial Park, and only about 75 minutes for it to get to the Vedder Bridge.

The worst case scenario would involve a landslide coinciding with a massive rain storm and existing flood conditions. Scientists believe that scenario is relatively unlikely, both because of the rarity of such storms and because large landslides most often occur days or months after a major rain event, as water gradually makes its way into the ground.

An outburst flood that coincides with a flood could aggravate damage and threat all the way into Chilliwack—in part because such an event would already overtop the Vedder’s substandard dikes.

Addressing the slide threat is complex—and potentially extremely expensive.

At Slesse Park, a toe berm had been constructed to separate the river’s normal flow from the unstable bank. But the berm was destroyed in during the 2021 atmospheric river. At that point, it had already been damaged by a series of landslides. Engineers have suggested the creation of a new toe berm taht would contain minor landslides and prevent erosion. But it wouldn’t come cheap, with engineers pegging the potential cost at $22.6 million.

The threat of an outburst flood is now being incorporated into disaster planning work by the Fraser Valley Regional District. It is one of several landslide-related threats within the Chilliwack River Valley.

The new study only looked at the ramifications of an outburst flood at two locations where recent landslides have already occurred. Other slide threats lurk elsewhere. The report notes that there are six other “serious” landslide sites in the region, and that ongoing monitoring is needed to understand the changing risks.