May 1, 2026 Spring and Summer Streamflow Forecasts
Upper Missouri Basin
The high elevation snow pack in the Upper Missouri Basin as of May 1
was below average. The snow pack in the St. Mary Basin was 57 percent,
and the Milk River Basin was 55 percent.
April precipitation across the Upper Missouri Basin was below average.
Basin precipitation summaries for the Missouri Basin include: Milk St. Mary,
59 percent; Milk Canada, 42 percent; Lower Milk, 85 percent; above Toston,
104 percent; Toston to Ft. Peck, 75 percent.
April 2026 Upper Missouri Mean Precip Oct-Apr WY2026 Upper Missouri Mean Precip
Stream flow in the Upper Missouri Basin is forecast to be
below average during the upcoming Spring and Summer.
The St. Mary River is forecast to have 53 percent of average
April-September flows. Runoff is expected to range around
76 percent of average for the Missouri Basin above Fort Peck,
Montana.
Upper Missouri May 1, 2026 Spring and Summer Streamflow Forecasts
Of the four major irrigation reservoirs in Montana; Lima
Reservoir had 95 percent average storage, Clark Canyon was
holding 96 percent of average water, Gibson Reservoir had 99
percent of average stored water, and Fresno Reservoir had
49 percent of average stored water.
Yellowstone Basin
The high elevation snow pack in the Yellowstone Basin was near average
on May 1. The snow pack in the Upper Yellowstone Basin was 71 percent
of average. The snow packs in the Wind, Bighorn, and Shoshone Basins were 60,
54, and 55 percent of average, respectively. The snow packs in the Tongue
and Powder basins were 36 and 17 percent of average, respectively.
Precipitation during April was near to below average. The upper Yellowstone
River Basin received 66 percent of average precipitation while the Lower
Yellowstone River Basin reported 69 percent of average. The Bighorn
Basin received 78 percent of average precipitation while the
Wind River Basin received 69 percent of average. The Little Bighorn -
Upper Tongue Basin received 104 percent of average and the Powder River
had 74 percent of average precipitation.
April 2026 Yellowstone Mean Precip Oct-Apr WY2026 Yellowstone Mean Precip
Forecast stream flow in the Yellowstone Basin is below average
for the upcoming Spring and Summer. Stream flow for the
Yellowstone River above Sidney, Montana is forecast to be about
68 percent of average. Flow in the Tongue Basin is expected to be
45 percent of average. Flows in the Powder River are expected
to be about 25 percent of average.
Monthend storage at Boysen Reservoir was 96 percent of average.
Stored water in Buffalo Bill Reservoir was 112 percent of average.
Monthend storage was 99 percent of average at Bighorn Reservoir.
Yellowstone May 1, 2026 Spring and Summer Streamflow Forecasts
Platte Basin
The high elevation snow pack in the Upper North Platte Basin was
29 percent of average on May 1. The snow pack in the North Platte
Basin below Seminoe Reservoir was 74 percent of average. The snow
pack in the South Platte Basin was 54 percent of average.
Precipitation during April in the Upper North Platte Basin was 91
percent of average. In the Lower North Platte Basin, precipitation
was 79 percent of average. The North Platte Plains below Guernsy
Reservoir had 21 percent. The South Platte Mountains had 38
percent whereas the Plains had 31 percent of average April
precipitation.
April 2026 Platte Mean Precip Oct-Apr WY2026 Platte Mean Precip
Stream flow in the Platte Basin is forecast to be below
average during the upcoming Spring and Summer. Runoff
for streams above Seminoe Reservoir are expected to be
about 46 percent of average. Streams in the South Platte
Basin above South Platte, Colorado can expect 42 percent
of average flow. For the remainder of the South Platte
basin, flows are expected near 40 percent.
Stored water in the South Platte Basin was 93 percent of average
on May 1.
North Platte Streamflow Fcst South Platte Streamflow Fcst
