Upper Snake Reservoirs End Season Near Record Low
The Upper Snake River reservoir system closed 2026 with its second lowest October 31 volume in 30 irrigation postseasons.
Updated on Nov. 10, 2026 in Current Conditions
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The Upper Snake River reservoir system ended the 2026 water year with 614,815 acre-feet on October 31, the second lowest October 31 volume in 30 irrigation postseasons. Only the 2003 low was smaller, and the system remains well below its 30-season median of 1,583,795 acre-feet.
Why it matters
Oct. 31 volume serves as a benchmark for how much water carries over into the following year, and this year's carryover ended below average after a very poor winter snowpack. Irrigators across Idaho are now deciding what to plant next year with limited reservoir storage.
Reservoir records show 614,815 acre-feet in storage on October 31, 2026, against a 30-season median of 1,583,795 acre-feet. The system must reach about 120% of normal April 1 streamflow at the Heise gauge to have a chance of filling, and that forecast is not yet known.
The players
Water District 1
Water District 1 is the Idaho water management authority headquartered in Idaho Falls that administers water rights in the Upper Snake River basin.
The details
Irrigation shutoffs and above-average October precipitation kept month-end volume from falling below the 2003 low. Some operations shut off early as their water supplies ran out, reducing demand, while irrigators needed less reservoir water because of the rain. The 1921 water right must fill to 100% before any junior rights can receive new fill.
Timeline
2003: 30-season low October 31 volume recorded
2017: 30-season high October 31 volume recorded
2025: Irrigation season ended with below-average carryover
Winter 2025-26: Very poor snowpack recorded
October 31, 2026: System volume measured at 614,815 acre-feet
Seasonal Patterns
The 2026 October 31 volume ranks second lowest against the 30-season record that includes the 2003 low and 2017 high. It reflects how the system's storage swings with snowpack and carryover over multi-decade cycles.
Irrigators in the Upper Snake region should plan for limited reservoir allocations in the coming season. Residents should monitor official water district updates before making planting decisions tied to irrigation supply.
The takeaway
Low October 31 volume signals limited irrigation supply ahead, so planning around reduced storage is prudent. Monitoring the April 1 streamflow forecast gives the clearest early read on whether the system can recover.
What happens next
The April 1 streamflow forecast at the Heise gauge will determine whether the reservoir system can fill, and irrigators will make planting decisions based on that outcome.
Further reading
For broader context on storage trends, see Current Conditions across Idaho.
Source note: This article includes information reported by Capital Press.
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