Boise Diversion Dam

The Boise Diversion Dam. Water managers are cautiously optimistic about the upcoming irrigation season in southern Idaho.

Reservoir storage in southern Idaho is down this winter but snowpack is up, which means good conditions must continue for irrigators to have sufficient supplies this year.

Specialists at a Jan. 13 state Department of Water Resources supply-outlook meeting expressed cautious optimism as favorable forecasts over the next few months offset mostly dry conditions expected for the rest of January.

Irrigation demand jumped in an unusually dry, hot 2021.

The amount of water stored in the Upper Snake River’s reservoir system is 64% of the 1991-2020 average, said Jeremy Dalling, reservoir operations lead with the U.S. Bureau of Reclamation in Heyburn.

“It’s better than in October and November, but is a pretty big hole to fill,” Dalling said. It gets this low once or twice a decade.

Forecast runoff at the Heise gauge near Idaho Falls is 91% of average, he said. Reclamation would like to see it close to or just above normal, he said.

Other runoff forecasts are 80-95% of normal across the Upper Snake and 137% in the Little Wood River Basin, which was severely depleted in 2021. It was the second straight extremely dry year in the central mountains.

“There is potential we could fill the system, but we are not forecasting that at this time,” Dalling told Capital Press.

Season-long irrigation supply also will depend on spring and summer conditions. He said current snowpack is similar to that of 1993 — which was cool, wet and saw low irrigation demand — and 2004, which turned hot and dry.

In southwest Idaho, Ryan Hedrick, water operations leader with Reclamation in Boise, said the amount of water stored is about 76% of average in Boise River reservoirs and about 86% in Payette River reservoirs. Runoff is projected at 109% and 105% of normal, respectively.

Filling Anderson Ranch Reservoir, the largest and most remote of the three on the Boise River “is looking promising,” Hedrick said. “But it is awfully early to predict.”

Danny Tappa, hydrologist with the USDA Natural Resources Conservation Service Idaho Snow Survey, said snowpack is above normal in most of the state.

For example, it’s ahead by about six weeks in the Lost River Basin and a month ahead in the Big Wood, compared to long-term medians, and two weeks ahead in the Boise Basin.

Soil moisture is high, helped by heavy rain in October and snow in late December and early January, he said. That bodes well for spring runoff.

“We are off to a good start in most basins,” Tappa said.

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