Flooding in parts of British Columbia’s Interior region has been exacerbated by the 2017 and 2018 fires that left the soil and ecosystem with a limited capacity to absorb water, experts say.
Prof. Lori Daniels from the University of British Columbia’s Faculty of Forestry explained that snow in unburned forest is captured in treetops and evaporates back into the atmosphere, while snow that accumulates on the ground melts and seeps into the soil.
“In an intact forest, there’s trees and branches and plant parts that are all organic and have material, and act like sponges and absorb the melting snow so that there’s a lot of water being held both on the surface of the soil and infiltrating down into the soil particles,” Daniels said.
But she said a forest ravaged by fire does not have canopy tops or leaves, twigs and other organic matter on the floor to absorb the water.
City of Williams Lake councillor Scott Nelson described the floods as the “single-largest” the area had seen in about 200 years.
The city has issued an evacuation alert for the Green Acres Mobile Home Park affecting 86 homes. He said the city is asking residents to ration water use as crews are on their way to fix a ruptured sewer line in the River Valley.
The water has pushed up the creeks, turning them into rivers that eat out the sides of the banks and pull down dirt and stumps creating a mud bog that flows down the stream, he said.
“We’re looking to get through this but it’s simply a terrible, terrible situation,” he said. “The water has risen in our lakes probably four or five feet.”
The B.C. River Forecast Centre is maintaining a flood warning for the Chilako River, Cache Creek, Cariboo and Chilcotin areas.
Chris Keam of the Cariboo Regional District, meanwhile, said Monday that 48 properties are either under evacuation alerts or orders.
Daniels explained that fires in the region in 2017 and 2018 were very intense and burned much of the organic matter, in some cases down to white ash and mineral soil. Water runs over this bare land, filling streams and causing flooding, she said.
Compounding the problem is this year’s 20 to 35 per cent increase in snow packs as measured on April 1, she said.
“There’s lot of extra snow out there in the landscape so when we get these warm spells like we are experiencing, and that snow begins to melt rapidly, the fact is that the soil just cannot absorb the melting water it would have prior to the forest fires.”
Prof. David Scott, research chair of the University of British Columbia’s earth, environmental and geographic sciences department, said the soil becomes water-repellent after intense fires.
This slows down movement of water through the soil and it may mean that it runs over the surface, increasing the flooding risk and soil erosion.
Chair of Cariboo Regional District Margo Wagner said it’s impossible to tell how many hectares of land is under water and the impact on the agricultural industry.
“I can tell you it will be considerable because when you have this flooding, especially when it comes over a riverbank it’s not clean water,” she said.
Last year hayfields had over 30 centimetres of water, carrying silt, stones and debris, Wagner said. But when the water receded, the gravel and debris still clung to the hay, she added.
City of Williams Lake Mayor Walt Cobb said he’s lived in the area for 65 years and has never seen flooding like he is seeing this year.
“I guess it’s a combination of issues, but the water is as high as I’ve ever seen in all the time,” he said. “And we’ve been told that the worst is yet to come.”