VANCOUVER — The mountain pine-beetle infestation that has swept across British Columbia’s Interior, leaving more than eight million hectares of forest dead or dying, is creating a growing flood threat throughout the Fraser River watershed.

And a massive salvage logging operation that is now under way — in an attempt to harvest the insect-killed trees while they still have commercial value — is exacerbating the situation, warned the Forest Practices Board in a report released yesterday.

Steve Chatwin, manager of special projects for the Forest Practices Board, a government advisory body, said that a sophisticated computer model, being used for the first time, was able to project stream-flow conditions following a pine-beetle infestation, as well as after the insect-killed wood had been harvested.

Under each scenario, the size and frequency of floods jumped dramatically over baseline data.”Floods will be bigger [and] . . . a former 20-year flood will become a three-year flood,” Mr. Chatwin said.

Studying the Baker Creek watershed, near Quesnel, the Forest Practices Board determined that peak flow in the stream would be 61-per-cent higher after the forest in the watershed had been killed by pine beetles.

When the computer model was adjusted to indicate what would happen once the insect-killed forest was logged in a salvage operation, the peak flow jumped even more, to become 92 per cent higher.

Under each scenario, the peak high-water mark advanced, occurring about 15 days earlier than normal, as the snow pack melted sooner and faster once the forest cover had died off or had been removed by logging.

“The results indicate the peak flow changes following MPB attack may be dramatic,” the report states. “A forest management dilemma is that any salvage harvesting of attacked trees will lead to even higher peak flow changes. Not harvesting leaves the stands vulnerable to fire, which would have a similar hydrological impact as harvesting. . . . ”

Mr. Chatwin said the Forest Practices Board is recommending that watershed-by-watershed hydrological assessments be done throughout the pine-beetle-infested forest to minimize the damage from logging.

Under the provincial Liberal government, the practice of requiring hydrological assessments in all watersheds was dropped.

Mr. Chatwin said it would cost an estimated $30,000 to do a hydrological assessment on a typical watershed, and he suggested that, given the damage that could result from floods, that would be money well spent.

Bruce Fraser, chair of the Forest Practices Board, said “it is like flying blind” to log beetle-killed forests without first assessing the hydrological impact.

And Mr. Fraser said the study obviously raises concerns about what might happen in the Lower Fraser Valley if watersheds farther upstream, where the pine-beetle infestation is intense, all experienced more intense floods.

He said the implications of the study are obvious, but the computer model did not make projections about the broader Fraser watershed.

“What the long-term impacts are for downstream on the Fraser is something we have no information about,” Mr. Fraser said.

He said given what they found on Baker Creek, however, a much broader study should obviously be done to determine just how big the flood threat is in the Lower Mainland.

“The impact of the mountain pine beetle has to be modelled for the entire Fraser,” he said.

The report explained that when the forests are killed by pine beetles, the needles fall off within a few years, allowing snow to fall more directly to the ground and providing less shade.

The result is a deeper snow pack that melts more quickly in the spring. Salvage logging speeds up that process by removing all the forest cover.

It takes 40 years for replanting to stabilize a watershed.

The computer model, which measured some 6,000 variables in the forest, was developed and run by Dr. Younes Alila of the University of British Columbia faculty of forestry and Dr. Charles Luo of CH2MHILL Engineering.

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