Partners working together to manage Nechako levels
The flood danger on the Fraser River has been reduced, now that a large amount of freshet water has moved through the watershed.
Part of the reason the Fraser’s flood levels stayed below the crisis line during last week’s rush of melt-water was careful management of the Nechako River water flow coming in from the west. The Nechako is in large part managed by human decision making since the headwaters at Ootsa Lake are controlled by aluminum smelting company Rio Tinto as part of their Kenney Dam power supply facilities.
The Nechako takes in large amounts of water from Francois, Fraser and Stuart lakes via rivers like the Chilako, Nautley, Stuart and other waterways, but it’s mainstay supply is from the Ootsa source.
By holding back on the volume of that discharge, Rio Tinto was able to mitigate the flooding south of the Prince George confluence of the Nechako into the Fraser.
However, hot weather happened again this week, bringing more water down, and the Ootsa Lake reservoir is also a body of water that can theoretically grow too great to hold back. If, hypothetically, the Ootsa water had to be drained off and the Fraser levels were very high at the same time, it could be a flood event.
“The Nautley River, which drains a fair chunk of the unregulated part, has been flowing quite high, and largely in response to that, our understanding is that Rio Tinto has eased off on flows from the reservoir to maintain lower flows that go through Vanderhoof (and on to Prince George),” said River Forecast Centre head David Campbell last week. “Those outflows at the moment are very low and we have seen a drop through Vanderhoof to levels quite a bit below concern, and a similar trend as you get on down into the Prince George area.”
“As part of our role in managing the reservoir, we are committed to working in a collaborative manner with other decision makers and regulators,” said Rio Tinto’s Justus Benckhuysen. “This year, our hydrology and reservoir operations team worked together with the River Forecast Centre of B.C. and the District of Vanderhoof to establish a forum where all parties could have access to consistent and up to date to facilitate flood preparedness decisions.”
There are regulations governing the holding and releasing of water from Ootsa Lake and the levels that must be maintained for the Nechako River. In order to maintain the balance, said Benckhuysen, staff has been monitoring all the factors in the buildup of mountain snow snf the tempera- tures this spring to base decisions on long-term plans.
“Over the past two months, Rio Tinto increased spillway discharge several times in order to maintain a low risk of flood,” he said. “This spring, given the high level of snowpack in the Nautley watershed, particular attention was given to monitoring precipitation and snowpack conditions. We worked closely with the River Forecast Centre, the Water Comptroller, to review current data and make decisions about gate changes and spillway releases to mitigate flood risk, while considering other key interests.”
Despite the early success of this planning, there is still a possible flood threat, depending on the combination of high temperatures and additional precipitation.
“Certainly my understanding is, as there is pressure from the Nautley side it becomes harder to release without causing more problems in Vanderhoof,” said Campbell. “In terms of the snowpack, we’ve still got a fair amount in the higher elevations that could certainly put pressure on later, but it’s my understanding that (Rio Tinto’s release volumes are) being managed to those considerations.”
“When managing a high water event like this year we are making daily calculations and decisions to minimize the probability of flood, and minimize the magnitude of flood if it occurs,” said Benckhuysen. “The amount of water being released into the Nechako River has been higher than the usual minimums since early March, except for a short time when minimums were in place while the downstream areas were at very high natural flow. The spillway is now discharging 250 (cubic metres per second) and could be higher than the usual amount of 49 m3/s until July. While the risk of flooding appears to have lessened, we remain vigilant in our role of monitoring conditions and sharing information with the public.”
Published reports on Rio Tinto’s effect on Nechako River flows can be accessed by emailing NechakoReservoirUpdates@riontinto.com to register for reservoir updates.
Real-time data along the Nechako River (and also the Fraser) can be accessed on the River Forecast Centre’s website.
After dropping steadily since May 19 from a peak of about 9.1 metres at the South Fort George monitoring station, the Fraser is now on the rise again, going from 8.8 up to almost 9 metres in the past two days.
The Nechako River’s monitoring station at Isle Pierre just west of Prince George, meanwhile, saw flows of about 3.65 metres on May 1. That swelled to about 4.35 on May 13, then subsided steadily to a trough of about 4.1 metres on May 20. It has upticked slightly but mostly maintained that level since then.