Water thick with sand and sediment has fouled a fish-bearing creek after heavy rain washed out an active construction site on Burnaby Mountain last week.

The silty water poured into Stoney Creek from a tributary, killing spawning Chum and Coho salmon and smolts, said John Preissl, a local streamkeeper.

Salmon had been absent from the once polluted creek for half a century, but returned about a decade ago, thanks to decades of rehabilitation work by foundations, government agencies and hundreds of volunteers, Preissl said. Before the accident, streamkeepers had counted nearly 200 salmon in the creek.

“We had our best Chum run that we can remember since the salmon have been coming back,” Preissl said.

“It’s just really sad.”

Preissl estimated hundreds of smolt were lost between Thursday, when the water first entered the creek, to Monday, when the water’s visibility had begun to improve. Sediment in water smothers salmon eggs and packs into the gills of smolts and spawning fish, Preissl explained.

“They’re wiped out. We don’t know if they’ll be coming back from about 1,000 feet below where the washout was.”

The sediment entered the creek in a heavy rainstorm, which came as a construction crew was rehabilitating a culvert that runs directly underneath Gaglardi Way and a Kinder Morgan pipeline, according to the City of Burnaby.

James Lota, an assistant director of engineering with the city, said staff first noticed the slope was eroding last year and they had rushed to complete a design for repair, find a contractor and get the necessary permits. But by the time all that happened, spawning season was underway.

“There’s normally a ‘fish window,’” Lota said. “The best times to work in these places is the end of summer and we recognize that. But because of the condition of the culvert and the slope we were really just trying to stop any damage to Stoney Creek and that’s why we wanted to get in there now before the winter.”

Before the contractors were finished the job, their bypass pumps failed and the slope under the pipeline started to wash into the creek, Lota said.

Preissl said Kinder Morgan crews brought in a crane with a sling to suspend the pipeline while the contractor figured out how to fix the slope.

Ali Hounsell, a spokeswoman for Trans Mountain, said company crews and consultants began to monitor the scene as soon as they were notified about the washout. The company uncovered a section of the pipeline after speaking to city engineers, but finding no damage, the pipeline continued to operate as normal, Hounsell said.

Preissl and other streamkeepers plan to take water samples and count salmon as crews continue their cleanup work.

mrobinson@vancouversun.com