Oldman River Dam | Three rivers flow through dam and downstream to Alberta’s irrigation district
PINCHER CREEK, Alta. — The Oldman River Dam would cost $900 million it if were built today.
Now celebrating its 20th anniversary of full operation, the dam cost $353 million in 1986 dollars, the year construction began on the project. The reservoir and dam trap water flow from three rivers — the Oldman, the Castle and the Crowsnest.
With a capacity of 400,000 acre feet, or 495 million cubic metres, the earth-filled dam is the largest of its kind in Alberta, said operations supervisor Robert Plante, who gave an overview of operations Oct. 12 to a tour organized by the Lethbridge-based Oldman Watershed Council.
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Sixty-five percent of annual watershed accumulation flows through this river valley from mid May to mid July, said Plante. Capturing the water allows it to be used by downstream irrigation districts during the growing season.
“The idea was to impound the water when it was not needed and use that water when it was needed most, July through September, irrigation time,” said Plante.
The Lethbridge Northern Irrigation District is a primary user of water released from the reservoir. It supplies 220,000 acres of farmland with irrigation water.
Downstream municipalities and cities also use the water, and interprovincial agreements require a certain amount to pass into Saskatchewan.
Plante said the Oldman dam has three main components: spillway, earth-filled dam and diversion tunnels. Power generation through one of the tunnels is managed by Atco, an Alberta utility company.
The company generates 32 megawatts of hydro power from 60 cubic metres per second of flow through one tunnel, said Plante.
The other tunnel has 100 cubic metres of capacity, for a combined 160 cubic metres per second if both tunnels are running wide open.
The spillway comes into use if more water than that must be released downstream. It can pass up to 8,400 cubic metres per second at full bore.
Water release and storage are dictated by the amount of snow pack in the nearby mountains each year, said Plante. Rainfall plays a major role in accumulations, but it is unpredictable.
Operators will start filling the reservoir early in years of low snow pack to ensure enough capacity for downstream users in summer. However, major rainfall in spring can wreak havoc with those plans.
“No matter what you do, you’re going to be wrong because you can’t predict rain and you can’t predict how much,” said Plante.
There are also specific mandates on water release, he added.
“We cannot pass at any time more water out than what’s flowing in. We have to operate as if we were never here.”
As well, he said under no circumstances should the dam create flooding. Outflow should equal inflow, which is constantly monitored.
The minimum flow requirement in winter is 6.5 cubic metres per second, and Plante said flow is nearing that now. That rate is required from November to March. In the remaining months, a percentage of natural flow is required to pass through.
The dam has an expected lifespan of 200 years, said Plante. Various technological instruments monitor pore pressure in the dam and seepage in the abutments.
