Fresno Reservoir

Fresno Reservoir has been drawn down thanks to little rain and lots of irrigation.

Carolyn Anderson

The northern half of Montana seems to be in an entirely different climate zone than the southern half when it comes to water flows this summer.

Rivers like the Milk along the Hi-Line are tracking below normal flows for this time of the year. Storage levels in Fresno and Nelson reservoirs, which supply irrigators and communities along the Milk River, are near expected minimum elevations for the season, according to the Bureau of Reclamation.

“Above average temperatures coupled with below average precipitation has led to irrigation demands remaining higher than normal and reservoir storage levels lower than normal,” the Bureau said in a press release.

Fresno Reservoir is 30 feet below the full storage level elevation of 2,575 feet while Nelson Reservoir is nearly 12 feet below the full storage level of 2,221.6 feet.

“Irrigation operations are starting to ramp down for Milk River Project beneficiaries about a month sooner than normal,” the Bureau said. “Both reservoirs are expected to gain storage after the middle of August into October as water from the St. Mary River Basin continues to be transferred to the Milk River Basin.”

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Afterbay Dam

Afterbay Dam, seen here discharging a flow of about 11,000 cubic feet per second in April, spills water into the Bighorn River, which has seen continuous high flows this spring.

Down south

Turn your gaze southward, though, and it’s a completely different story. Flows into Bighorn Reservoir, which straddles the Montana-Wyoming border, set a new record between April and July — 2.9 million acre feet of water that was 270 percent of average.

Streamflows into Bighorn Lake during July were 249 percent of average as runoff from record snowfall in Wyoming continued to pour downstream.

The nearby Yellowstone River at Miles City was running at 10,300 cubic feet per second compared to the average of 8,330 cfs for this time of the year. That’s because the Yellowstone benefits from the Bighorn, Tongue and Clarks Fork of the Yellowstone rivers which collected some of that Wyoming snow this winter.

In the middle

The Missouri River Basin is positioned between the two extremes. Missouri flows into Fort Peck Reservoir are very close to average.

"With the exception of the Fort Peck to Garrison reach, runoff into the Missouri River mainstem reservoirs during July was below average due to the continuing drought conditions in the upper basin," said Jody Farhat, chief of the Corps' Missouri River Basin Water Management Division, in a press release. "Runoff into Garrison was 124 percent of average, due to runoff from the remaining (Wyoming) mountain snowmelt. July runoff ranged from 20 to 90 percent of average in the other reservoir reaches."

Garrison Dam backs up Lake Sakakawea, just over the Montana border in North Dakota, and benefits from the Yellowstone River’s ample flows as it joins the Missouri River just upstream. The difference in inflows to Sakakawea versus Fort Peck can be seen in what the Army Corps is releasing from the two dams.

While Garrison was pumping out 34,500 to 33,000 cfs in mid-July, Fort Peck was averaging 9,900 cfs. That will be increased to 10,000 cfs this month with Fort Peck Reservoir expected to drop another 2 feet in August.

So while boaters at Fresno Reservoir are being cautioned that launching could be difficult, folks accessing Bighorn Reservoir are still dodging driftwood because it’s full to the top of its conservation pool and 60 feet above the minimum launch elevation at Ok-A-Beh Marina.

Meanwhile the Bighorn River, a prized trout fishery downstream of the reservoir, is flowing at 4,200 cfs, about twice its normal rate for this time of the year. Yet the Bureau is forecasting it will drop to about 2,700 cfs by September.

The differences show the yin and yang of Montana water flows this season, with a state strongly divided. What the next couple of months hold for precipitation and weather is anyone’s guess, but the Old Farmer’s Almanac is predicting “September and October will be cooler than normal, with slightly above normal rainfall.” The National Weather Service, on the other hand, is predicting a pretty normal weather pattern over much of the eastern part of the state and below normal in the west for the next eight to 14 days.

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Montana Untamed Editor

Montana Untamed editor for the Billings Gazette.