How a cool spring has increased the Fraser River’s flood risk

A cool spring has left BC’s rivers below their seasonal levels, but that has actually increased the risk of flooding over the coming month, forecasters are warning. And with last year’s disasters still fresh in the minds of many, government officials are urging people to know their local risk and prepare their properties in case the waters rise again.

Only a month ago, the flooding situation was relatively benign, with the snowpacks that feed the province’s rivers close to normal. But April’s cool temperatures, coupled with more snow at high elevations, has worsened the situation.

In the spring, much of the water in BC’s rivers comes from snow that accumulated at high elevations over the preceding winter. And much of it sticks around into June. Even when there is a lot of snow, rivers can generally accommodate the water that flows from a gradual, steady melt. The danger of flooding increases when that melt is compressed into a short period. A speedy melt is more likely later in the year, when the weather is warmer and extended heat waves are more likely. In other words, the lack of warm weather now is not necessarily a good thing. And the longer BC goes without a spell of decent weather in the mountains, the more the danger rises.

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Increasing risk

“The cooler temperatures through April have increased the risk of significant flooding if an extended period of heat or extreme heat occurs later in May or early June,” BC’s River Forecast Centre wrote in a bulletin last week.

Melting was slow through April, and when the River Forecast Centre releases data next week, its experts expect the figures will be significantly above normal. Automated snow detection stations across the province have shown the situation is worsening, and on Tuesday, officials said provincial snowpack indices are around 114% of normal, indicating a “moderate” risk. (The threshold for “high risk” is at the 120% level.)

But experts say this could be one of the hardest flood seasons to forecast in recent decades.

In its April bulletin, the forecast centre warned that last year’s landslides and fires increased the flood risk on many rivers, but there is a lot of uncertainty on how that would look across the province.

“Due to the significant and possible changes in river channel morphology that occurred in many areas, rivers may be at increased vulnerability to flooding at lower levels than previous freshet seasons,” the centre warned.

The rushing water has shifted the riverbeds of many watercourses that experienced flooding last November. But it has also altered the Lower Fraser River, which didn’t flood. As landslides sent huge amounts of sediment into many rivers across Southern BC, much of that sediment ended up in the Fraser and, eventually, in the river bed in the Fraser Valley. In just the area around the Island 22 boat launch north of Chilliwack, dump trucks have removed 3,600 tonnes of material.

Residents of low-lying areas in valley bottoms across BC will be watching the water to see how it behaves in the months to come. As the region at the bottom of the province’s largest river, the Fraser Valley gets more notice before a flood than many other communities.

But if a flood does occur, there is the potential for devastating outcomes. Previous floods in 1948 and 1894 left floodplains under metres of water, and today, hundreds of thousands of people live and work on those plains. Multiple reports have warned that a major flood could cause damage that would far exceed even last November’s devastating Sumas Prairie disaster.

This year’s threat is not yet out of the ordinary. But with November’s disaster still fresh in the minds of many, government officials have increased their efforts to encourage residents to be aware of their property’s susceptibility to flooding and take the appropriate precautions.

Recently, federal officials surveyed Canadians and found the vast majority of respondents were either unaware of their community’s local flood risks or unconcerned.

“It was really eye-opening,” Public Safety Canada spokesperson Tim Warmington told The Current. “It’s not something that people are looking into or are too concerned about.”

Awareness is important, he said, because there are relatively simple things that individuals can do to better safeguard their homes and properties before a flood. But many of those things need to be done in advance.

“There’s simple stuff you can do like walking around your property when there’s heavy rainfall, seeing where the water pools to see if you need to address grading around your house,” he said. Checking the caulking on seals around windows, keeping a backup battery for a sump pump, and clearing gutters and drainage can all help people avoid localized flooding.

For larger floods, like what could happen during the freshet, it helps to know where your home lies in relation to a broader floodplain. And that information is not always easily available. The federal government has promised to create a flood portal to display that information, but that is still in the works. (It’s also unclear whether that portal will provide information for all citizens, or just local governments and insurance companies.)

For Fraser Valley residents, some of the best information comes in a series of 2019 maps displaying potential flood scenarios. Those maps, created by the Fraser Basin Council, showed what parts of the region could be under water in a variety of freshet flood scenarios. You can find the maps here.

Those maps form the backbone of The Current’s ongoing series about various communities’ freshet flood risk. We wrote about Langley’s flood-prone areas in January. Stories on Abbotsford and Chilliwack will follow in the months to come.