Much of Southern Idaho, cut through in scythe like fashion by the Snake River Plain, relies on the frozen water stored in the state’s mountains to fill its rivers. When winter ends and summer’s broiling heat arrives, it is these snowy peaks that serve as the state’s reservoir, filling the Salmon, Snake, Big Lost, Boise and other tributaries with cold, clear water.
But as the amount of snowfall declines, with scientists citing the effects of climate change as a key contributor, major problems arise for the state’s ecosystems, residents and agriculture industry. And that erosion is already underway. By the turn of the century, Idaho could see reductions of 35%-65% of its snowpack, according to a study published in Nature Reviews Earth & Environment last year. After a dry winter in 2019- 20, researchers were hoping for a turnaround. But last winter, Idaho had only 87% of its normal snowpack, and that was a bad combination when combined with the second-worst spring drought on record, said David Hoekema, a hydrologist at the Idaho Department of Water Resources.
Making matters worse was that dry soil absorbed much of that below-average snowpack, leading to only 53% of normal runoff , Hoekema said. Idaho’s dry weather in 2021 saw over two-thirds of the state in extreme or exceptional drought — the two highest categories — in early October. This month, most areas of the state are listed in moderate to severe drought, according to the U.S. Drought Monitor. As such, a wetter winter was sorely needed to fill up the state’s streams and reservoirs, but since a spell of storms in late December and early January, much of the state has seen weeks with little to no snowfall or precipitation. A ridge of high pressure along the Pacific Coast blocked storms for the past month, and the rest of February is likely to remain dry, Hoekema said.
With Idaho’s mountains in need of more snow to reach what is considered “normal” snowpack levels, Hoekema said he is “very concerned” the drought will intensify this year, he said in an email, which could affect agriculture, landscapes and residents this summer just as it did last year, when irrigation water that normally runs till October had to be cut off early. “If that materializes, it’ll lead us into a really tight water year,” Hoekema said. And those are becoming more frequent. Between the 1940s and the mid-2010s, streams throughout the state saw an average flow reduction of 20%, according to Charlie Luce, a research hydrologist at the Rocky Mountain Research Station, which is part of the U.S. Forest Service.
And during the driest years, streams saw reductions of up to 40%. Luce said the changes are associated with shifts in weather patterns consistent with expectations of climate change in the Pacific Northwest region, which have led to reduced precipitation in the high mountains. With an increase in average temperatures, Luce described the lower precipitation in dry years and the larger portion of precipitation falling as rain rather than snow as a “one-two punch” that can further restrict the state’s water supply. Luce said he projects there will be more years with low soil moisture, which also decreases runoff into streams and reservoirs.
Outlook in Idaho after a dry spell
The amount of snowfall in a given winter is not the only factor that determines how full Idaho’s reservoirs get. Soil moisture levels are also important, as well as weather patterns that come in the spring and can help flush snow down from the mountains. In the southern part of the state, last year’s skimpy precipitation left reservoir systems close to dry, meaning only a wetter-than-average winter this year would allow water managers to rest easy. The 2020-21 winter was a La Nina year — an atmospheric phenomenon that generally brings more precipitation to the Northern Rockies. This winter is also a La Nina period, and trends over the past 40 years indicate that back-to-back La Ninas get subsequently drier, Hoekema said, which does not bode well for runoff.
Hoekema said the Boise River system may be able to “slide through” with 75% of normal runoff this year, but that figure would leave the system dry come next fall. In the Snake River system, the situation is more dire, as the system likely needs 120% of normal snowpack to get through the summer. In the middle of February, the state’s snow-water equivalent, a measure of how much water is contained in snow, was lower in many places than what Hoekema predicts is needed, with portions of Southern Idaho seeing figures as low as 70% of normal. The situation is better in the Little Wood and Big Lost basins of Central Idaho, where the levels are 107% and 108% of normal, respectively, according to data from the Natural Resources Conservation Service.
Hoekema wrote in an email that late-winter snows could greatly improve the situation, but “the window for recovery is closing fast.” In the Snake River system, the likelihood of the reservoir filling up for the year “seems pretty dismal at this point,” Hoekema said. The upper Snake, in East Idaho, would need an additional 8.5 inches of snow to reach normal conditions. Since Jan. 9, the particular area in need has seen only half an inch of accumulation, Hoekema said. In the Boise system, the situation is a little less clear, he said. The system would start March with 87% of normal snowpack if it received just another inch of snow in the next few weeks. “At this point, it is really difficult to predict whether or not the Boise basin will see drought intensify,” Hoekema said. “It all depends on what happens in March.”
Agriculture: ‘Some of the worst yields … in the next 100 years’
Drought connected to climate change is expected to affect Idaho’s farming economy significantly, according to a recent report from the University of Idaho. The report, called the Idaho Climate-Economy Impacts Assessment, found that though crop yields for plants in Idaho appear to have risen over the past 60 years, the volatility among rain-fed crops is more than among irrigated crops. Last year in North Idaho, where agriculture is largely rain-fed, farmers experienced “some of the worst yields we’ll probably see in the next 100 years,” Hoekema said.