A mixed bag of weather in the Upper Snake River region following heavy February snowfall is challenging water managers as they balance the need to make room in reservoirs for snow melt with the need to recharge the aquifer.
A plan to temporarily shut off outflows from southcentral Idaho’s Milner Dam — shortening the aquifer-recharge season — is an example.
Supplies of irrigation water should be plentiful, thanks to continued above-normal snowpack, speakers said May 8 at an Upper Snake River Advisory Committee multi-site meeting hosted from Boise.
But a warm March, wet April, cold early May and anticipated mixed summer conditions make managing reservoir levels tougher. Exact irrigation demand is unknown, partly because summer forecasts call for temperatures and rainfall to be normal.
Brian Stevens, supervisory civil engineer at the U.S. Bureau of Reclamation’s Upper Snake Field Office in Heyburn, said a recent sharp cool down slowed snowmelt flow into Palisades Reservoir on the Snake River near the Idaho-Wyoming border. Flows are expected to increase again by late May.
Reclamation has drawn down Palisades in anticipation of high spring runoff. The current forecast calls for runoff at 107% of normal. The bureau anticipates the reservoir being full by late June.
Reclamation plans to shut off outflows from the Milner Pool reservoir near Burley May 14 after reducing them gradually May 6-10, he said. That will allow for the maximum amount of water to be stored in American Falls Reservoir upstream for irrigation use later this year.
“Outflows from Milner Dam may resume in June when American Falls reservoir fills,” Stevens said in an interview. American Falls is not quite full — in part because Reclamation made room for high inflow in April as Palisades was releasing high outflow from upstream — but “when high runoff from the Henry’s Fork watershed occurs in late May, we anticipate American Falls filling.”
Wesley Hipke, who manages Idaho’s aquifer recharge program, said May 8 that he had planned to keep recharging for another 3 1/2 weeks but now will stop May 14, when Milner outflows cease.
For the recharge season that started Oct. 22, losing about three weeks would reduce the amount of water returned to the aquifer by around 10%, he said.
Snow-water equivalents are well above normal in most of the area’s river basins.
“It has been a good water year, and again it is dependent on what happens this summer and what the demand is,” Hipke said. “We are going into the main irrigation season in a pretty good place as far as the amount of water supply that we have.”