Forecasters with Northern Water say Lake Granby could spill as early as next week, but full reservoirs on the Front Range and high runoff have left officials on the fence over possible pumping through Grand Lake.
Typically, reservoirs on the Front Range fill by May, which lowers Lake Granby enough to accept additional water during runoff season, said Kara Lamb with the U.S. Bureau of Reclamation.
But flooding on the East Slope in September, coupled with additional precipitation and runoff, have kept Carter and Horsetooth reservoirs, the Colorado-Big Thompson Project’s main draw points for Front Range water users, too full to accept much water.
Add above-average runoff on the Western Slope to the equation, and there is a fair amount of uncertainty whether the Alva B. Adams Tunnel will have anywhere to transport water if and when Lake Granby spills.
“There could be a little pumping to help with the spill situation,” said Brian Werner with Northern Water. “It’s dependent on this side of the mountains, and if there’s any room to put any water, so demands really haven’t started, and like I said, we’re full everywhere.”
There’s a possibility that pumping could be halted until Labor Day, Werner said.
For Grand Lake residents, pumping can mean the difference between pristine clarity and a cloudy lake.
Last year, reduced pumping saw the clarity of natural Grand Lake increase, while the shallower Shadow Mountain Reservoir became more turbid.
“If there is not going to be any pumping, I think that what happened last year in both Grand Lake and Shadow Mountain is a good idea of what to expect,” said Esther Vincent, water quality manager with Northern Water.
Vincent added that, in the case of reduced or no pumping, Shadow Mountain Reservoir may get some reprieve this year due to runoff. The increased flow from runoff could keep the water temperature in Shadow Mountain Lake lower, which would inhibit algae blooms that would normally cloud the lake.
“When there’s no movement of water, the water gets pretty still,” Vincent said. “The surface water gets quite a bit warmer. That’s optimal conditions for algae to grow.”
But it’s all tentative at this point, said Andrew Gilmore with the Bureau of Reclamation.
“Essentially all of the reservoirs over here are almost full,” he said from Loveland, “so if we’re going to be running the tunnel, it’s going to be 250 cfs or less, so half of its capacity or less,” Gilmore said. “And it really depends on if there is any space over here at all.”
Lake Granby nears limits
As of July 3, Lake Granby was at 2.6 feet from capacity, with levels rising around a third of a foot per day.
Werner, of Northern, said if the lake does spill, forecasters expect it to do so between July 10 and July 14.
“Our forecaster, who I just talked to, said we’re still 50-50 on whether we’re going to spill,” Werner said.
Spilling is not a very common occurrence for Lake Granby. The last time the lake spilled was in 2011, and before that it was in 2000. The large amount of snowpack has led to above-average flows this year, and reservoirs on the Front Range are already near capacity. Specifically, Carter Lake is at 99 percent full, while Horsetooth Reservoir is 99.2 percent full, according to the Bureau of Reclamation’s website.
The Farr Pumping Plant has not been pumping since May 18, Werner said.
There was only 84 cfs of flow through Adams Tunnel on Thursday, July 3, according to Lamb.
There is also a small amount of water flowing through the Lake Granby spillway due to maintenance on the spillway’s valves.
For now, officials will be monitoring weather over the Fourth of July weekend to determine if additional water can be sent to the Front Range.
“You’re balancing your water supply, and this year, we have an above-average snowpack and an above-average water supply to balance,” Lamb said.
Hank Shell can be reached at 970-887-3334 ext. 19610.