Dam releases to peak June 8, bureau says
Ample year of water should see reservoir fill by mid-July.
By Angus M. Thuermer Jr., Jackson Hole, Wyo
May 20, 2009
The runoff has started.
Water flows out of Pacific Creek tripled during the weekend from near 1,000 cubic feet per second Friday to 3,000 cfs early this week.
In the Snake River Canyon, flows climbed to 12,800 cfs and tributaries began to rise. The signs of spring drew attention to proposed releases from Jackson Lake Dam outlined last week by federal hydrologists.
With a snowpack registering 109 percent of the 30 year average, Jackson Hole residents can expect ample releases from the dam in Grand Teton National Park and the reservoir behind it to fill by mid-July. Bureau of Reclamation Operations Manager Mike Beus, from Burley, Idaho, told a gathering of interested Snake River users that the largest releases from the dam would be 6,000 cfs about June 8.
Releases from the dam, at 4,160 cfs on Friday, will be increased in coming weeks and will taper off following the peak flows that are expected to last only days. By July 2, outflow from the dam is expected to be down to 3,000 cfs, according to a graph Beus provided to about 40 people who attended his briefing.
The Snake River reservoir system that includes Jackson Lake is “reasonably full” at 78 percent of capacity, Beus told the audience.
According to Beus’ graph, Jackson Lake Reservoir should fill by mid-July and will be drawn down to hold 650,000 acre feet by late September. The lake level affects some marinas that have closed in past years when water was lacking.
Beus’ presentation suggested no such problems this summer.
When the Snake River’s flows will peak below the dam will depend on natural runoff as well as the releases. On Tuesday, Jackson Lake Reservoir was holding 76 percent of its capacity of 847,000 acre feet of water.
Beus said the amount of water in the snowpack above the Snake drainage is similar to what was present last year at this time. In 2008, the runoff was extremely late, rapid and efficient, he said.
Water content of the snowpack this year is high at high elevations, Beus said. Also, there is more water concentrated in the snow this year at the southern end of the drainage – in places above Star Valley – compared with last year and less in the north around Yellowstone National Park, Beus said.
Sometimes the best statistical efforts at prediction and management are confounded by nature and irrigation demand, Beus admitted. Irrigation demands from Jackson Lake Dam two years ago resulted in high flows late in the season, upsetting anglers who seek low, steady flows in August and September.
Last year, it was June runoff that was high and late compared with the 30-year average. The stark difference between the two years illustrated the difficulties he faces, Beus said.
“We erred on one side ... we erred on the other side,” he said of the two years.
The bureau is constrained by water rights, irrigators’ demands, laws mandating it provide flood control, requirements for winter releases to sustain aquatic river life, and the complexity of an integrated system of reservoirs.
