Record-low snowpack leaves its mark as Idaho enters a new water year
Idaho’s new water year kicked off on October 1, but in order to understand potential impacts as we head into a new water year, we first need to look back at this last water year.
What is a water year?
A water year isn’t like a normal calendar year. It starts on October 1 and runs through September 30. This 12-month period is used by hydrologists and scientists to measure and analyze precipitation, snowpack, and streamflow, helping them assess the state’s water resources.
Scientists use the water year because it tells the full story of our water resources from year to year. A calendar year would essentially split data in half. By October 1, rivers and reservoir carryover storage across the region typically reach their lowest annual levels, just before the next winter’s snow and rain begin to build up again. In the West, precipitation typically begins accumulating as mountain snow in the late fall (October and November) and melts during the spring and summer. A calendar year, which runs from January to December, splits this single, continuous winter cycle in half.
An important note. A new water year doesn’t necessarily mean a clean slate. How much water is left in reservoirs, soil, and groundwater can affect the year ahead. How much water remains in storage on Sept. 30 can influence how much water is available during the following year, especially if the new winter brings below-normal precipitation or snowpack, according to the USGS.
So now that we understand why a water year is used instead of a calendar year, and how those conditions can carry into the next water year, let’s dive into the data.
‘Doing pretty good based on how our water started this year’: A recap of Idaho’s 2026 Water Year
During this past water year, Oct 1, 2025 to September 30, 2026, Boise recorded 12.21 inches of precipitation, about 6% above the 1991-2020 average of 11.49 inches, according to National Weather Service data. During that same time, the Mahleur-Owyhee-Boise River Basins received 15.5 inches of rain for the water year, which is 96% of normal and a .7% decrease from its average annual rainfall, which is 16.2 inches.
But precipitation totals don’t tell the whole story. Much of the precipitation that fell during the fall and winter came as rain rather than snow because of unusually warm temperatures, as BoiseDev reported. In the Boise Basin, snowpack was only about 76% of normal around the start of 2026. Idaho hit a new record-low snowpack on February 8, marking it as one of the 10 lowest snowpacks on record, according to the Idaho Department of Water Resources.
That’s important because mountain snowpack acts like a natural reservoir. Snow stores water in the mountains and releases it as it melts in the spring and summer. So even though the Boise area received plenty of precipitation, less of that water was stored as snow for later use.
Winter brought above-average temperatures and moderate precipitation. December through February was the second-warmest period on record in Boise, dating back to 1877, according to the National Weather Service. Those conditions led to what’s known as a “wet snow drought,” where warmer temperatures cause precipitation to fall as rain instead of snow and also cause existing snow to melt quickly.
On March 30, Idaho reached its annual peak snowpack, but in many basins below 7,000ft, levels were at record lows, as BoiseDev reported. Reaching peak snowpack means the mountains have reached their maximum snow accumulation for the season. From there, we transition into the spring melt and runoff period, when the snowpack becomes a critical water source.
By spring, the lack of snowpack translated into less water running off the mountains. Despite a record amount of rain in April, Idaho declared a statewide drought emergency on April 13, after nearly all of the state’s basins were classified as being in drought conditions. In southwest Idaho, April-September runoff forecasts were below normal, with some locations expected to receive less than half of their normal runoff, according to the Idaho Department of Water Resources. At an Idaho Water Users Association meeting in May, experts noted that we were sitting at a good starting point, as the Boise River reservoir system was sitting at 97% full, but expressed concern about how a dry, hot summer would affect the region, including meeting irrigation demands and increased wildfire risk.
Summer 2026 brought early snowpack exhaustion, accelerated reservoir drawdowns, and unseasonably warm temperatures to the Boise Basin. By August 6, the Boise River reservoir system (Lucky Peak Reservoir, Arrowrock Reservoir, and Anderson Ranch Reservoir) was 66% full, and the annual drawdown at Lucky Peak Reservoir began around August 10 to August 18 (initiated by the U.S. Army Corps of Engineers and local partners), roughly a week ahead of the normal schedule and dropping by about 1.5 feet daily. Ryan Hedrick, the Middle Snake Water Operations Manager with the Bureau of Reclamation, noted in an August 18 meeting that despite a record-low snowpack, “(The Boise Basin) is doing pretty good based on how our water started this year. The Boise Basin is sitting at … 95% of average. And with that, irrigation will likely shut down in late September to early October.”
Ultimately, at Lucky Peak Dam, the Boise River recorded about 1.36 million acre-feet of runoff during the 2026 water year — roughly 74% of the 1991-2020 average, according to data from the Northwest River Forecast Center. As of October 2, Lucky Peak Reservoir was 16% full, which is normal for its winter pool levels. Storage for the reservoir sits at just over 71,000 acre-feet, compared to over 78,000 acre-feet at this time last year, according to data from the U.S. Army Corps of Engineers.
New Water Year, current drought conditions, and testing groundwater levels
The below-normal runoff comes as Idaho enters a new water year, with drought conditions still affecting much of the region. More than 50% of Ada County is currently experiencing moderate drought conditions as of September 29, with over 14% of the area classified as being in severe drought. Statewide, over 68% of the Gem State was experiencing some level of drought, according to the National Integrated Drought Information System.
While current drought conditions don’t predict what will happen in the new water year, the conditions at play now will carry into it, and they could deepen depending on how much precipitation we receive this fall and winter and how much of it accumulates as mountain snow. This matters especially for us here in southwest Idaho, where snowpack provides a significant portion of our water supply during spring and summer.
Right now, the Idaho Department of Water Resources is measuring over 400 groundwater wells to get a fresh assessment of groundwater levels in the Treasure Valley and Payette Valley region. These measurements are in addition to the roughly 200 wells they monitor within the region to get a look at current groundwater levels. The last large-scale measurement effort in the region took place in 2001, according to IDWR.
“This is essentially a snapshot of the aquifer,” said Ryan McCutcheon, IDWR Project Manager, in a press release. “Well owners’ participation is crucial, as private wells are a valuable source of information, and the more wells we can measure, the better our picture of the aquifer’s current state and its long-term and seasonal trends.”
Looking ahead, as El Niño is strengthening, forecasters with NOAA predict a greater than 90% chance of a very strong event for the U.S. for the fall and winter of 2026-27. Meaning there is the potential for warmer temperatures and drier weather in the coming months, which could in turn lead to less mountain snow and higher drought risks.
For now, the outlook for the 2027 water year will heavily depend on what happens over the coming months. And I will be watching and keeping you updated.