Officials to ramp up Gunnison River flow through Black Canyon
The U.S. Bureau of Reclamation announced Monday that flows in the Gunnison River through the Black Canyon will be increased to 3,883 cubic feet per second for a 24-hour period projected for Tuesday.
Increasing (or ramping up) the flows by about 500 cfs per day should meet the May 18 target, said Dan Crabtree, lead hydrologist for the Bureau’s Grand Junction office.
One cfs is equal to 646,317 gallons per day.
The peak flows and accompanying shoulder flows are components of the Black Canyon reserved water right. That 2008 settlement includes annual peak flows and shoulder flows — tied to natural inflow — plus a year-round base flow of 300 cubic feet per second.
Collectively, these elements were considered to mimic natural flows prior to the construction of the Aspinall Unit dams and deemed critical to the health of the park and the Gunnison River.
Once the peak flows are finished, the river will be ramped down by 400 cfs per day to around 800 cfs for the rest of the summer, Crabtree said.
The peak flow and its timing were decided once the May 1 runoff forecast for the Gunnison basin was available, Crabtree said.
In spite of a dry winter, “the forecast was for near normal inflows to Blue Mesa,” Crabtree said. “The reading just barely reached the normal line, but that’s what we go with.”
Dry soil conditions, left over after a particularly dry November, may absorb some of the melting snow and affect the actual runoff, Crabtree said.
“But the forecast calls for 560,000 acre feet of inflow, which is close to normal,” he said.
The peak flows will be produced by increasing flows from Blue Mesa into Crystal Dam and then opening the outlets on Crystal.
That will boost flows to around 4,150 cfs. It take an additional spill of around 800 cfs to make the peak flow target.
While it’s not preferable to see water go over the dam instead of through the hydropower generators, there’s little choice when it comes to meeting the Black Canyon water right.
“We always want to use the water in the most efficient way possible,” Crabtree said, “whether it’s for hydropower, storage against future drought or for recreational purposes.”
Total releases from Crystal Dam will be around 5,000 cfs. The amount above the 3,883 cfs target flows account for irrigation demands through the Gunnison Tunnel.
Crabtree said the flows should drop to 700–800 cfs by May 26.
Anglers headed to the East Portal or down any trail to the river should be aware of the higher flows, which might raise river levels as much as 5 feet in places.
Floating anglers should just enjoy the ride, said fishing guide Marshall Prendergast.
“The guides actually like those higher flows because there’s less rowing and more excitement,” said Prendergast, who guides for Bob Burke’s Cimarron Creek fly shop in Montrose.
“It’s more of a problem in those big-water years like we’ve have the last two” when flows were in the 6,000–7,000 cfs range, he said. “This year it’s projected we’ll have about half what we saw last year, so it’s not going to be a problem.
“I’m headed down tomorrow, hoping to get in one more trip before the big flows.”
One impact of peak flows is to lower the water temperature and this could delay the insect hatches, Prendergast noted.
“Remember that prior to the dams we had flows down there upwards of 20,000 cfs and that killed bugs and the trout,” he said. “We’re not killing them anymore, now we’re just delaying the hatch.”
One of the arguments behind sending peak flows down river is to remove some of the gravel, mud and sand deposited in lower water years, particularly those drought years earlier this decade.
It doesn’t take long for a sand bar to develop, then grasses to sprout and finally small bushes and trees start to take over.
“Then, you really need high flows to move that out of the river,” Prendergast said.
But not too high. Bureau officials will monitor the high flows to make sure they don’t reach the critical flood stage at Delta.
Last year, a sudden surge of hot weather pushed the Cimarron River higher than expected, which in turn increased the Gunnison.
This year, with cool weather keeping Silverjack Reservoir just over half-full and the Cimarron River low, officials decided to go ahead with the peak flow and not wait for the Cimarron to come up.
“Rather than try to time it with the Cimarron, we decided to go ahead,” Crabtree said. “Silverjack may not fill until later this month or even June, and if irrigators in Bostwick Park start to take water from the Cimarron, that affects flows, too.”