Rainstorms cause flooding in the western Canadian province of British Columbia
Brandyn Desjarlais of River Wrangler Sportsfishing ferries people and donations between Chilliwack and Mission with his friend Dayton Bardos, after rainstorms lashed the western Canadian province of British Columbia, triggering landslides and floods, shutting highways, on the Fraser River near Mission, British Columbia, Canada November 18, 2021. Reuters

Canada's British Columbia province on Thursday warned residents to prepare for flooding when rains eventually return after a prolonged drought exacerbated by climate change that has raised concerns about long-term damage to ecosystems ranging from glaciers to salmon rivers.

The usually rainy western province has experienced weeks of record-breaking warm fall temperatures and minimal precipitation in central and southern regions.

Vancouver, the largest city in British Columbia, received less than a sixth of its average rainfall in August and September and has received only 0.2 millimetres so far in October, according to Environment Canada. Across the province more than 150 maximum daytime temperature records were broken in September alone.

The drought is the latest sign of a changing climate in British Columbia, which last fall was lashed by atmospheric rivers that unleashed catastrophic rainfall, leading to flooding and landslides.

With forecasts showing rain could return by the end of next week, the provincial government said it would monitor river conditions closely in coming weeks.

"Dry soils can increase runoff and river flows," Emergency Management BC said in a news release.

The exceptionally long dry spell in Canada's most biodiverse province has sparked concerns about how different species will respond. Biologists are particularly concerned about salmon, which are critical to ecosystem health and culturally significant to indigenous First Nations.

Salmon migrate from the ocean to rivers to spawn at this time of year, but this month social media pictures showed thousands of dead salmon in a dried-up stream in central British Columbia.

Jason Hwang, vice president at the Pacific Salmon Foundation, said many salmon populations were delaying their journey upstream. But warmer water increases the risk of them becoming diseased and there were questions over the knock-on effects of spawning later than usual, such as when salmon eggs hatch.

"The scale of the effects of this weather pattern and climate change effects is something that we have not seen before. This is way beyond normal," Hwang said. "It affects their whole fresh water life history."

In the alpine, glaciers that would usually be accumulating snow at this time of year are still melting. While the dry fall is less damaging than the heat dome that engulfed British Columbia last summer, it is still not good for glaciers' long-term health, said Brian Menounos, a professor of geography at the University of Northern British Columbia.

"You have to think about these glaciers like a bank balance," Menounos said. "Anytime it's negative, it's bad because you accumulate these losses through time."

The drought is also impacting British Columbia's vast forests, which are under increasing pressure from the impact of a warming climate, said Ken Lertzman, professor of forest ecology and management at Simon Fraser University.

He said species like the western red cedar, which is important to both First Nations and the forestry industry, are in decline as a result.

"In general one stressful year will not have a long-term impact," Lertzman said. "Unfortunately right now it's a stressful fall coming on top of other stressors, largely climate change related."