High inflows forecast, Pathfinder spill expectedWritten by Christy Martinez
Casper – In 2002 Wyoming’s North Platte River system experienced 100-year record low inflows throughout the runoff period, with only 118,000 acre-feet added to the system.
This year, the Bureau of Reclamation’s (BuRec) Wyoming Area Office is forecasting 1,450,000 acre-feet of inflows to the North Platte.
“That gives an indication of how difficult it is to manage this system, when we have these large swings,” says BuRec Wyoming Area Manager John Lawson. “And they swing rather quickly, rather than being a long, extended event.”
The North Platte isn’t the only area of the state with unexpected changes – Buffalo Bill Reservoir near Cody has also become a reservoir to watch this spring.
“When we did our Buffalo Bill forecast on April 1, the snowpack was 107 percent. As of April 12, it was 121 percent. In a matter of a week and a half the snowpack has dramatically increased,” notes Lawson. “That gives us some pause with regard to our forecast, and we’ll look at increasing those expected inflows.”
Even with the lower snowpack measured the first of the month, BuRec still forecast inflows at 127 above average, or 840,000 acre-feet of water.
“In our plan we started moving up our releases out of Buffalo Bill, and we’re now at about 2,500 cubic feet per second (cfs),” says Lawson. Previously, winter releases were 350 cfs – the maximum winter flow release.
“It will appear that the reservoir is being dropped, and to some it will seem unnecessarily,” says Lawson. “This time of year, with cool temperatures, we’re not getting any inflow, but as soon as the inflow comes the reservoir will dramatically turn around.”
Lawson calls it a “stay tuned” situation.
“On April 1 we forecast 840,000 acre-feet of inflow, and in 2008 and 2009 we got 950,000 acre-feet both years, and the snowpack wasn’t as high this time of year in either one of those years,” he states. “Always the unknown are the heavy rains in late May, and particularly June.”
He adds that taking actions early on to move water from the reservoir will avoid the high releases of 2009, should the reservoir receive 950,000 acre-feet this spring.
Lawson says his agency is also making a judgment call regarding the Wind River Basin and Boysen Reservoir. While snowpack west of the Continental Divide is high, at around 130 percent, he says the east side isn’t as dramatic at 115 percent of average.
“Although the basin percentage is still rather small, it’s still above average,” he says. “We do know there’s a lot of snow in the high country, and the Big Horn Mountains below Boysen are at 121 percent of average, so when that starts running off we’ll have to be careful with releases from Boysen, because that puts a lot of stress on the river system.”
Returning to the North Platte system, Lawson says that, as of April 11, the upper basin averaged 143 percent above average snowpack – even higher than the previous week’s 139 percent.
“Last year at this time we were at 100 percent, and that tells us we’ve got a lot more snow than last year,” he adds.
The Lower Platte River Basin, from Pathfinder Dam to Guernsey Reservoir, isn’t as dramatic, but is still above last year’s 104 percent at 116 percent this spring, and BuRec forecasts 180,000 acre-feet of inflows.
“In the first of the month forecast we predicted 1,450,000 acre-feet of water in the upper basin, and we started doing our planning accordingly,” notes Lawson.
In 2010 the basin received 1,242,00 acre-feet of water, while the previous two years it gained over 950,000 acre-feet each year. The average for the basin is 714,000 acre-feet in a year.
Currently snowpack levels in the Upper North Platte River Basin are the highest on record since the SNOTELs, which measure mountain snowpack, were put in place in the mid-1980s.
Lawson says that one thing his agency notes as a trend in the higher inflow years is snowpack building later in the spring season, as well as rain.
“When we find those years when we still see the snowpack building well into May – that’s the difference,” he says. “Last year we had heavy rains, and one of the worst situations we can have is cooler weather this time of year, which results in continued building of the snowpack, which delays runoff, and then it warms up and we get rain on snow. That’s the worst situation to deal with, because it wants to come off, and it’ll come off really quickly, and that’s what happened to us last year.”
“Overall, we have 136 percent above average in storage, and we’re trying to drop that down,” says Lawson. The system was above 140 percent of average in storage at the end of February, and has been evacuating water since then.
“We’ve got snowpack at 143 percent of average, a forecasted inflow 203 percent above average, and system storage 136 percent above average – that tells us we better get as much was out of the system as possible,” he adds.
Winter flow from Seminoe Reservoir above Pathfinder Reservoir was 500 cfs, and by March 10 that had increased to 2,300 cfs. By April 15 that increased to 6,000 cfs.
“We want to make room in Seminoe, as the uppermost reservoir, and we’re building Pathfinder, because right now we’re releasing the maximum amount we can between the jet flow valves and the power plant. The only way we can release more water is to go back into a spill,” explains Lawson. “We intend to have Pathfinder near the spillway crest by the end of May.”
Currently Gray Reef, below Pathfinder and Alcova Reservoir, is releasing 4,500 cfs. After Alcova fills, another 500 cfs will be put down the river.
“Any more than that can only be done by a spill,” comments Lawson. “Last year in June we were forced to release as high as 7,000 cfs at Gray Reef.”
Besides running more water downriver, another benefit to a spill situation at Pathfinder is the added storage in the reservoir.
“When we spill at Pathfinder the reservoir has to be built up higher than the weir, and that results in storing more water,” says Lawson. “That physical storage capability is 1,016,000, but last year at one point it held 1,060,000 acre-feet. That gives us another 50,000 acre-feet of storage upstream.”
Downstream from Pathfinder, Guernsey Reservoir is releasing 4,200 cfs, 3,800 cfs of which is passing Whalen Dam and continuing to the state line and into Nebraska.
“What we’re trying to do is evacuate about 430,000 acre-feet to make room in our system,” notes Lawson. “Last year Gray Rocks Reservoir had about 50,000 or 60,000 acre-feet of storage space, and I understand that now they have about 16,000. Everything’s filled up, and we don’t have as much storage available, so that’s why we’re evacuating as much as we can.”
Moving forward, Lawson says his agency is running daily analyses to estimate what’s coming in and how much is going out. Looking at numbers from years past, Lawson warns that predictions for this year aren’t a hypothetical max plan – the system has seen them before, and even higher.
“I’m hoping this is all we’ll have, and I never thought I’d ever say that I hope we only get 1,200,000 acre-feet. A couple years ago, I would have said that’s unheard of, and now I’m hoping that’s all we get,” he says.