As Irrigation Wanes, Nearly Full Reservoirs Prompt New Releases

Idaho’s huge spring runoff is sticking around in nearly full reservoirs and high flows in the Snake River, the state Department of Water Resources reports.

Last month, the Snake River below the Milner power plants near Burley had an average flow more than 10 times its historic norm for this time of year.

It averaged 6,700 cubic feet per second, compared with an 85-year average of 622 cfs. It flowed at 6,830 cfs on Thursday.

At Weiser, the Snake in September ran at an average 18,800 cfs, 63 percent above the 85-year median of 11,500 cfs.

The Snake River reservoirs are still largely full of water as the irrigation season draws to a close. The five upper dams currently hold back 1 million acre-feet more than usual for this time of year. Eastern Idaho was inundated by water in June as the snowmelt outran dam managers’ ability to release it.

The huge carryover and the slackening irrigation demand has prompted reservoir operators to begin higher-than-normal releases so they can be drafted to traditional winter storage levels.

The five Upper Snake reservoirs collectively are 73 percent full, 57 percent higher than the 29-year average. They now hold 2.8 million acre-feet. At this time of year, they normally contain about 1.8 million.

Palisades Reservoir is at 90 percent capacity, 40 percent above normal; American Falls, 57 percent capacity, 88 percent over norm; Island Park, 90 percent, 101 percent above average; Ririe Reservoir, 78 percent, 27 percent over; Jackson Lake, 79 percent full, 46 percent over norm.