Warmer temperatures with some snowmelt and near- or wetter-than-normal precipitation in much of the Columbia River basin in March led to some early runoff but overall resulted in higher April-Sept. water supply forecasts and a better outlook for stream flows in the basin that will aid juvenile and adult salmon and steelhead migrations this summer, according to a NOAA water supply briefing last week.
Snowpack improved in the eastern basin along the west side of the Rocky Mountains, as well as in the upper Snake River during March, said Henry Pai, senior hydrologist at the NWRFC, during his online presentation, Thursday, April 3. However, while central Oregon snowpack still has “incredible amounts” with snowmelt that is causing some flooding in the Malheur River basin, snowpack along the east side of the Washington Cascade Mountains continues to be much lower than normal.
Upper Columbia River basin water supplies improved the most in March, but are still lower than normal, with the forecast at two of storage dams in British Columbia on the rise. Duncan Dam, on the Duncan River, is now predicted to be 88 percent of normal (1.973-million-acre feet, April – Sept.), a rise of 10 percentage points from the dam’s March forecast. The forecast at Mica Dam on the Columbia River rose 5 percentage points to 88 percent of normal (10.76 MAF, April – Sept.).
The forecast at Hungry Horse Dam, on the South Fork of the Flathead River in Montana, rose to 92 percent of normal (1.738 MAF April – Aug.), up 12 percentage points, and at Libby Dam, on the Kootenai River, also in Montana, the water supply was up 7 percentage points to 79 percent of normal.
Grand Coulee Dam is at 90 percent of normal (55.48 MAF, April – Sept.), which is up 9 percentage points from the March water supply forecast.
The Dalles Dam forecast is for a 90 percent of average water supply (85.186 MAF, April – September), a five-percentage-point increase over the 85 percent of average forecast in early March, The Dalles water supply is the culmination of all water available upstream in the Columbia and Snake rivers. The forecast at the dam has improved from a low 82 percent in February to 85 percent in March and now to 90 percent of average in April.
The most impressive water supply forecasts are in central Oregon where the forecast for Crooked River at Prineville is at 147 percent of normal, up 4 percentage points from March, the Grande Ronde at Troy is at 112 percent of normal, down 6 percentage points, and the Owyhee Dam is at 113 percent of normal, but is down 18 percentage point as the snowpack has begun to melt. In addition, the Umatilla River at Pendleton is at 98 percent of normal, down 9 percentage points.
The April 1 Oregon Water Supply Outlook Report by the USDA Natural Resources Conservation Service said that overall, snowpack across much of the state is above normal, with near normal snowpack in the Hood, Sandy, and Lower Deschutes Basin. Low-elevation (below 4,000 ft) snowpack on the western side of the central and northern Oregon Cascade Range remains below normal.
The “onset of snowmelt has begun at the larger basin scale across Oregon, with anomalously warm temperatures in the second half of March initiating the melt season. The rate of snowmelt has been particularly rapid in southeastern Oregon in the Blue Mountain Range, contributing to flood conditions in parts of Harney County. Snowpack in this region and across much of eastern and southern Oregon have been robust this year, which bodes well for water supply but does and has presented, in some cases, enhanced flood risks moving into the melt season.”
The April Oregon Water Supply Outlook Report is here: https://efotg.sc.egov.usda.gov/references/public/OR/WSOR_April_2025_OR.pdf
The least impressive water supply is in an area the U.S. Drought Monitor is showing is in moderate to abnormally dry drought: that’s on the east side of the Washington Cascades. The water supply for the Okanogan River at Malott is at just 62 percent of normal, up 5 percentage points, and the Methow River near Pateros is at 59 percent of normal, up 8 percentage points.
“Although precipitation for March was above normal for all major basins in the state, there’s been little improvement to pervasive and persistent deficits in both water year-to-date (WYTD) precipitation and snowpack across much of the central and northern Washington Cascades,” the April 1 Washington Water Supply Outlook Report by the NRCS said. “Snowpack is below 70% at several monitoring sites near Washington Pass (SR20) and near the I-90 corridor. Snowpack within in the Upper Yakima Basin has degraded as percent of normal, notably in the Wenatchee Mountains, since March 1. Since the typical timing for peak snowpack for all major basins is here or very near, drastic changes to conditions are becoming less likely.”
It added that snowpack is near to slightly above normal across the southern Washington Cascades and the northern Blue Mountains in eastern Washington. Water supply forecast for the Upper Yakima has fallen and water supply shortages should be expected for the Yakima Basin, despite forecast improvements for reservoir inflows in the Naches Basin. The water supply outlook remains below normal for the Okanogan, Wenatchee, and Chelan basins.
The April Washington Water Supply Outlook Report is here: https://www.nrcs.usda.gov/sites/default/files/2025-04/WSOR_April_2025_WA.pdf
The upper Snake River basin April water supply forecasts are mostly above normal, with American Falls at 103 percent of normal, up 6 percentage points, and Lucky Peak at 119 percent of normal, up 7 percentage points. Still below average is Dworshak Dam on the North Fork of the Clearwater River at 92 percent of normal (2.281 MAF, April – July), but up 2 percentage points over the March forecast. Water supply at Lower Granite Dam on the Snake River is at 98 percent of normal (22.841 MAF, April – September), down 3 percentage points.
“The near to above normal snowpack across many basins in Idaho bodes well for a good water supply season this water year,” the Idaho Natural Resources Conservation Service’s April report says. “Of course, what happens during the spring and summer will strongly influence whether there is enough water to go around, but with this year’s snowpack and the expectation reservoirs will fill, conditions are setting water users up for success. The only areas of concern for water supply are in the Coeur d’Alene-St. Joe, Big Lost, Little Lost and Birch-Medicine Lodge-Beaver-Camas basins where the snowpack is below normal.”
The April Idaho Water Supply Outlook Report is here: https://www.nrcs.usda.gov/sites/default/files/2025-04/borid425.pdf
The Idaho Water Supply Outlook report says that the snowpack has peaked in all river basins across Idaho. “Cool temperatures at the end of March halted the snowpack melt that began around March 24 in all basins, but the upcoming warm and dry weather is almost guaranteed to continue widespread melt. April could still bring some cooler weather that slows down the snowpack melt rate, but right now, it looks like winter has ended and spring has officially arrived in Idaho. Thanks to the robust snowpack, water supply looks good across most of Idaho.”
The U.S. Department of Agriculture’s NRCS, snowpack report includes this information:
1. Salmon River basin: 114% of normal, unchanged from March 1).
2. Clearwater River basin: 97% of normal, up 2%.
3. Umatilla River basin: 158% of normal, up 11%.
4. Walla Walla River basin: 115% of normal, up 7%.
5. Upper Grand Ronde basin: 125% of normal, up 2%.
6. Imnaha River basin: 125% of normal, up 5%.
7. John Day River basin: 163% of normal, up 5%.
8. Upper Deschutes River basin: 121% of normal, up 10%.
9. Methow River basin: 75% of normal, down 5%.
10. Wenatchee River basin: 74% of normal, down 7%.
11. Upper Yakima River basin: 76% of normal, down 8%.
12. Lower Yakima River basin: 108% of normal, up 24%.
13. Klickitat River basin: 112% of normal, up 19%.
14. Mt. Hood drainage (Hood, Sandy, Lower Deschutes): 92% of normal, up 7%.
11. Coeur D’ Alene basin: 86% of normal, up 4%.
12. Western Montana (five sub-basins): 90% of normal, up 2%.
13. Henrys Fork-Teton River, ID, basins: 99% of normal, unchanged.
14. Big Wood River, ID, basin: 111% of normal, down 5%.
15. Salmon Falls, ID, basin: 125% of normal, up 9%.
This information was compiled by Kyle Dittmer, hydrologist with the Columbia River InterTribal Fish Commission, and included in his April and spring weather outlook/climate report released April 3.
Temperature and precipitation over the next 90 days is forecasted by NOAA’s Climate Prediction Center as equal chances of above or below average.
NOAA presentation slides are here: https://www.nwrfc.noaa.gov/presentations/html/wy2025_ws/WS_Briefing_Page_20250403.pdf
April water supply information is here: https://public.crohms.org/tmt/documents/WSF/WSF_WY25_04.pdf
For background, see:
— CBB, March 15, 2025, Columbia Basin Water Supply Forecast, April-September, Remains Below Normal, Coming Precipitation Could Help, https://columbiabasinbulletin.org/columbia-basin-water-supply-forecast-april-september-remains-below-normal-coming-precipitation-could-help/
— CBB, February 7, 2025, Though Doesn’t Feel Like It Right Now, Columbia Basin Water Supply Forecast For April-September Still Dropping, https://columbiabasinbulletin.org/though-doesnt-feel-like-it-right-now-columbia-basin-water-supply-forecast-for-april-september-still-dropping/
— CBB, January 20, 2025, Columbia River Basin Snowpack A Mixed Bag So Far, Water Supply Forecast At Dalles Dam (April-August) Now 89 Percent Of Normal, https://columbiabasinbulletin.org/columbia-river-basin-snowpack-a-mixed-bag-so-far-water-supply-forecast-at-dalles-dam-april-august-now-89-percent-of-normal/