Spill, baby, spill: Lake Granby likely to flow over dam within 10 days
June 18, 2010
GRAND COUNTY – Sitting just 5 feet from full on Thursday, Lake Granby was on course to spill.
The last time the lake (also known as Granby Reservoir) had water flowing down the spillway was in 2000.
Such an event of greater than 75 cfs flowing at the Near Granby Gauge would be a long-awaited gift to the Colorado River between the Lake Granby Dam and Windy Gap, said Jon Ewert, DOW aquatics biologist in Hot Sulphur Springs.
That section of the Colorado has been deprived of flushing flows because of the dam built for reservoir storage.
“It would be beneficial to that stretch,” Ewert said, “it would move sediment out and clean out riffles (stretches of gravel where fish like to spawn and where insect production is).”
“What we will do, when a spill is imminent, is max out the release from the bottom of the reservoir,” said Water Northern Conservancy District spokesperson Brian Werner.
The reason for the controlled release is to avoid water going over the top of the spillway, he said. “We’ll do everything in our power to minimize that.”
Northern plans to institute a call list for downstream property owners to warn them of the possible spill with increased river flows, Werner said.
The Granby Reservoir spilled each year from 1995 to 2000. In each of those years there were controlled releases through the bottom of the dam with about two days of water going over the spillway, according to Werner. Spills have lasted from a couple of days to 10 days.
Flows in the river from a spill can exceed 400 cfs.
In 1996 and 1997, water flows at the spill exceeded 2,000 cfs.
“We don’t anticipate getting anywhere near that high this year,” Werner said.
With runoff peaking last week, the Northern Water Conservancy District was moving water out of Shadow Mountain Reservoir into Granby Reservoir, releasing by as much as 5,000 cfs for a period of 12 hours.
In the Colorado-Big Thompson system, Shadow Mountain Reservoir helps to maintain a constant surface elevation in natural Grand Lake and is a conduit between Granby Reservoir and Grand Lake.
Heightened runoff in Grand Lake inlets caused that lake to rise, which by Colorado-Big Thompson decree is not allowed to fluctuate by more than 1 foot.
As far as Granby Reservoir’s chances for spilling, “We’re 99 percent sure it’s going to spill,” Werner said.
The lake is rising about .5 feet per day, which means the lake could spill within about 10 days.
“If at the start of the year you said we were going to spill, we would have laughed,” he continued. “It shows what can change in a water year in just a couple of months.”
Pumping at Willow Creek is planned to be minimal, with one of the pumps turned off today, June 18. Northern plans to cut the other pump when “absolutely assured Granby spills,” Werner said.
Northern is not sending water through the Adams Tunnel with Front Range reservoirs full.
Windy Gap, which pumped 6,700 acre-feet, has pumps shut off.
– Tonya Bina can be reached at 970-887-3334 ext. 19603 or e-mail email@example.com.