Dam demolished on Forty Mile creek
Thursday, Jun 05, 2014 06:00 am
Forty Mile Creek in Banff National Park is flowing freely again for the first time in more than 60 years.
The Town of Banff has torn down a huge chunk of the concrete dam on the mountain creek, the source of the town’s drinking water until a giardia outbreak in the mid-1980s.
Officials say the creek has carved a new path and is running unimpeded for the first time in decades since last Friday (May 30). Fully removing the dam would have cost up to $6 million, whereas the plan to partially remove the dam put the price tag at closer to $1 million.
“We’ve demolished a dam in a national park. We have taken out three sections of the dam and the river has already found a new path around it,” said Adrian Field, the Town of Banff’s engineering manager.
“The creek is going through a gap in the dam, we have the potential for fish connectivity for the first time in 65 years and the Town’s liability of owning an aging piece of infrastructure has been removed.”
The dam on Forty Mile Creek was built between 1916 and 1949 in various stages to supply drinking water and water for firefighting to the Town of Banff.
The concrete dam, located about two kilometres north of Banff between Stoney Squaw and Cascade Mountain, is about eight metres high and 50 metres wide, with the capacity to hold about 18,000 cubic metres of water and sediment.
The Town of Banff wanted to partially remove the aging reservoir to reduce liability for routine inspections and maintenance, but was also keen to restore fish connectivity, wildlife habitat and return the dammed creek to a more natural system.
A 2009 Parks Canada inspection recommended that the dam, which the Town of Banff inherited as part of the 1990 incorporation agreement, be closely monitored and that it be rebuilt or decommissioned because of poor concrete conditions.
In addition, taking down the dam and restoring habitat is a key goal in Parks Canada’s management plan for Banff National Park. It was also a dream of Charlie Pacas, a renowned Parks Canada aquatics specialist who passed away last year.
Field said the dramatic floods of June 2013 provided an ideal opportunity to drain the dam over a six-week period last year and remove much of the sediment that had built up in the reservoir during high water.
He said the Town of Banff got the go-ahead from Parks Canada in April to start the decommissioning work, which proved extremely difficult because last year’s flood washed out the access road and bridges, and new ones needed to be temporarily put in.
Field said they initially knocked the dam down to the level of sediment, then spent about 10 days trucking out about 4,000 cubic metres of sediment from the reservoir before getting final permission to knock three sections of dam down to foundation level in mid-May.
The work needed to be done before high waters came, said Field, as high water would have resulted in sediment and trees washing downstream and blocking the low level outlet.
“This would have led to an increase in the water level behind the dam and additional problems for the demolition work. It was touch and go. There were a number of time crunches,” he said.
“We had this window between the snow disappearing and the water level coming up in order to get this work done. Almost to the day, the river level started coming up and the outlet started to block and we had to move quickly to get the dam down in time.”
Four species of fish are known to be found within Forty Mile Creek, including bull trout, westslope cutthroat trout, mountain whitefish and brook trout.
It is recognized there is potential that short-term effects to individual fish and fish habitat may occur, particularly during the first freshet or first large flood following the partial dam removal, but the ecosystem is anticipated to improve in the long-term.
According to the environmental assessment for the project, the positive effects of providing a new tributary for Bow River bull trout spawning outweighed short-term negative effects of sediment on fish during the construction and decommissioning stages.
While there is opportunity for native species like bull trout to access new habitat and bolster their populations with the dam gone, the assessment also noted there’s a chance for non-native competitive species to move into areas currently dominated by native species.
That said, the assessment concluded the cold thermal regime of Forty Mile Creek creates an environment preferential to native bull trout and westslope cutthroat, and noted introduced brook trout have not outcompeted bull or cutthroat populations upstream of the dam.
The report said not only have non-native rainbow trout never been caught in Forty Mile Creek, the creek does not provide the environmental conditions for rainbow trout to survive, such as warmer water temperatures.
Wendy Francis, program director for the Yellowstone to Yukon (Y2Y) Conservation Initiative, said partial removal of the dam is a good news story and the risks are low compared to the long-term potential ecological benefits.
She said Y2Y completed a detailed report on ecological integrity of the Upper Bow River watershed in 2008, and one of the study’s recommendations was removal of the dam on Forty Mile Creek, restoration of habitat and aquatic connectivity to the Bow River.
“There’s all kinds of impacts associated with having that dam on that creek for all of these decades, not the least of which is preventing free movement of native fish species up and down the creek,” she said.
“Restoring the ability of native fish to move up and down is a really good thing.”
Chad Townsend, the Town of Banff’s environmental services co-ordinator, said the Town is happy with the outcome, noting there have not been many successful dam removals in Canada.
“Most of the sediment is probably out now and has been flushed away, and the creek effectively shifted over last Friday,” he said.
“The turbidity in the creek is quite high at the moment, but it’s at the time of year where natural turbidity is higher and so fish would be adaptive to this.”
The province picked up the majority of the $1 million price tag through flood relief funding. The Town of Banff contributed a small amount of cash and Parks Canada has provided in-kind support, including sediment monitoring for the duration of the project.
Field praised the work of all involved, including the contractor, Bremner Engineering and Construction, machine operator Brian Sloan and site supervisor Jeff Williams. He also noted all of the hard work Charlie Pacas had put into the vision of demolishing the dam.
“I am thrilled we managed to get this project done,” he said. “It’s been a long time coming.”