It has been a record-breaking spring for Bighorn Reservoir, and it looks like it could continue.
In March, the 72-mile long lake that straddles the Montana-Wyoming border recorded 306,700 acre feet of water pouring in, the highest on record and 216 percent of average. Then April roared in with nearly double March’s numbers, 603,400 acre feet of water, 411 percent of average — another record-setter.
An acre foot is the amount of water it takes to cover one acre in a foot of water.
All of that water has been gushing in at a high rate to make room for lots of snow still sitting in the mountains of Wyoming — snowpack that’s still 165 percent of average.
Bighorn Reservoir is already 73 percent full in spite of record-breaking outflows. Right now Yellowtail Dam, which backs up the lake’s water, is releasing 13,000 cubic feet per second of water into the Montana portion of the Bighorn River. That’s close to the May 10 record flow of 13,300 cfs set in 1947, before the river was dammed.
Snow continued to build in the Bighorn River basin until the end of April, and only this month has the mountain snowpack begun decreasing, and quite quickly. The South Fork of the Shoshone River, which feeds Buffalo Bill Reservoir outside of Cody, Wyoming, was running at 2,190 cfs while the average for this time of year is 412 cfs. The North Fork of the Shoshone, which also fills Buffalo Bill, was pumping out 3,570 cfs compared to an average of 1,450 cfs.
Buffalo Bill was lowered to make room for the runoff — it’s now 55 percent full. Water that was released early from Buffalo Bill and Boysen reservoirs are the reason that Yellowtail Dam has been releasing more water than usual. Boysen, which captures water from the Wind River (which turns into the Bighorn River at Thermopolis, Wyoming) is 63 percent full. The Shoshone and Bighorn rivers flow into Bighorn Reservoir near Lovell, Wyoming.
Even with all of that water flowing into Bighorn Reservoir, it will be early June before the lake is high enough to make the boat ramp useable at Horseshoe Bend Marina, on the south end of the lake near Lovell. That ramp needs a lake elevation of 3,617 feet. Right now the lake is at 3,603.4 feet, but the inflow — 12,336 cfs — is nearly equal to what the dam is dumping.
With so much snowpack still to melt, the Bureau of Reclamation is predicting that Bighorn River flows below Yellowtail Dam could hit 14,000 cfs in May and 14,500 cfs in June. If June is a wet month in the Bighorn Basin, that outflow could jump to 17,000 cfs.
Such an outflow would top the mark set in the wet spring of 2011 when the Bighorn hit 15,700 cfs — a year of record flooding. It wouldn’t set a new record, though. That belongs to 1967, the year Yellowtail Dam was completed, when the river hit 25,300 cfs.
For now, the strong flows in the Bighorn Basin are expected to continue into July.
“Based on the May 1 water supply forecast and the planned releases out of Boysen and Buffalo Bill reservoirs, the May through July runoff into Bighorn Lake is expected to equal 2,454,200 acre-feet (259 percent of average),” according to the Bureau of Reclamation’s monthly report.
Streams that feed the Bighorn Basin aren’t the only ones higher than normal. The Yellowstone River at Billings is flowing at 24,100 cfs, almost triple the average of 8,230 cfs for this time of year, according to the U.S. Geological Survey’s streamflow website. Downstream at Forsyth, the Yellowstone is at 35,600 cfs compared to an average of 12,600 cfs. That’s still far below flood stage, which is 14 feet. As of Wednesday the river’s stage at Forsyth was almost 7 feet.
The Clarks Fork of the Yellowstone, Stillwater and Boulder rivers were all more than double their average flows for this time of year, as well.