PNW Water supply
Notes on Pacific NW Water Supply Fall meeting at Boise State University …
High points:
2025 water year was a mixed bag in Idaho … dry in N. Idaho, good in SW Idaho including the Boise Basin and Treasure Valley, exceptional in the Owyhee Basin and E. Oregon, and below-average in the Magic Valley and Eastern Idaho.
“Where did the snow go?” was the main theme touched on by snowpack and hydrology experts … Snowpack levels close to 100% of normal did not equate to 100 percent runoff … see more details below.
The short answer: Dry October starting the water year (Oct. 1, 2024-Sept. 30, 2025), so soil moisture was dry; dry spring with below-normal precipitation as the snow melted, early snowmelt in places, and warm summer – 3rd warmest water year on record and the 4th warmest growing season on record.
Reservoir carryover is very low in Eastern Idaho, Magic Valley and Salmon Falls Cr. … much better in Boise, Payette and Owyhee.
We’re starting the 2026 water year on the wet side with above-normal precip and good soil moisture.
Initial two-week and four-week forecasts from the National Weather Service are calling for above-average precip, which could mean snow in the mountains!
Erin Whorton, NRCS Snow Survey (speaking as an individual)
Had a average winter snowpack, but spring precip was unusually dry … especially in the Snake Plain … plus higher temps in the summer …
Reservoirs peaked above normal but drained very fast in the 2nd half of the summer …
Soil moisture was dry in October …
April 1 – seemed pretty normal …
Then dry, warm summer … (Precipitation map below for the 2025 water year (October 2024-September 2025) SW Idaho and E. Oregon are the wet spot.
Brad Gillies – NW River Forecast Center NOAA
Very low streamflow runoff in the Upper Snake in 2025 … compared to what was expected.
Even more low flow in N. Idaho …
Henrys Fk example … 93% of normal snowpack on Jan. 1; 83% as of May 1, then 79% runoff observed by Sept. 1.
Snake River observed runoff was 78% at Heise, northeast of Idaho Falls. Jan. 1 snowpack was 99% April 1, 91% May 1.
