Cure for lake’s clarity issues remain murky
Grand Lake, CO Colorado
Halfway along the canal that connects Grand Lake with Shadow Mountain Reservoir, Bill Mueller often notices something floating with the current other than boats.
Clumps of weeds make their way along pumping flows from the reservoir toward natural Grand Lake.
Sometimes they are deposited near the shore where the canal slightly bends, other times they are carried into the natural lake where they become part of an altered lake bed.
“Some of them can be 3-foot diameter clumps of weeds, sometimes it’s just a few,” said Mueller, who often spends retirement hours reading on his porch overlooking the canal.
Mueller said weeds didn’t appear until the Northern Water Conservancy District resumed pumping water this summer.
The Northern Colorado Water Conservancy District, which supplies water to users from Broomfield to Fort Collins, was pumping water at the beginning of May, then shut off the pumps at the end of May and into June while heavy natural spring runoff filled Front Range and West Slope reservoirs to near capacity.
During that runoff time, the clarity in Grand Lake decreased as high inflows brought a lot of suspended solids into the system, according to data from the Grand County Water Information Network, which records clarity readings from various parts of reservoirs and lakes.
The readings show that Grand Lake clarity increased significantly as snowmelt subsided and as lake temperatures rose, all the while water flowed in its natural direction during ceased pumping activity.
But clarity deteriorated again around the time pumps were turned back on as temperatures increased and Shadow Mountain Reservoir water flowed into Grand Lake, according to the Network.
Northern returned to pumping at 194 cfs on July 1, increased pumping, then dropped down to 225 cfs on July 18, according to Northern.
Now nearly halfway through an eight-week experimental pumping regimen, the utility is hoping slow moving water pumped at a lower rate will prevent an algae bloom in shallow Shadow Mountain Reservoir, caused by water sitting too long in hot summer.
This is the third summer where such pumping experiments have taken place. This year, because the system will be out of commission in October for replacement of the dam and walking bridge at the connection of Grand Lake to the canal, the utility is trying something different: instead of shutting pumps off for a two-week period in late summer, it is moving water at 225 cfs for eight weeks.
A 2009 study conducted by Associate Director for the Center for Limnology of University of Colorado, Boulder, James McCutchan, confirms that the shallowness of Shadow Mountain Reservoir causes high temperatures in the lake, leading to increased algae production and higher levels of suspended solids.
When pumping is turned on, this water is transferred to Grand Lake, which experts believe reduces its transparency. McCutchan’s study found that non-algal particulate matter, which may include sediment, decaying leaves, weeds and dead algae, have the most direct impact to Grand Lake clarity, materials carried into the lake from Shadow Mountain Reservoir.
“They own those water rights,” Mueller said about Northern’s ability to transfer water through the system. “I just wish we could get it there without pumping it through Grand Lake. In my opinion, it’s harmful to Grand Lake.”
Northern maintains that clarity problems in Grand Lake could be created by many other factors, such as nutrient levels from surrounding development and summer’s rising water temperatures.
A “Three Lakes Technical Advisory Team” through Northern’s multi-year “Nutrient Project” is charged with studying ongoing problems in the Three Lakes region, with an eventual look toward solutions to the problems. The team comprises representatives from Grand County, the Colorado River Water Conservation District, several watershed organizations, the EPA, Northern, the U.S. Geological Survey, Division of Wildlife, Bureau of Reclamation, the U.S. Forest Service, Northwest Colorado Council of Governments, Colorado Department of Health and Environment and the City of Fort Collins.
Presently, the Advisory Team is studying the surrounding Grand Lake watershed for potential sources of pollution.
In 2008, the state Water Quality Control Division assigned a 4 meters, or 13.2 feet, clarity standard for Grand Lake starting in 2014 – the first clarity standard of its kind in Colorado. The ruling states the 4-meter clarity mark will be the standard unless a more appropriate standard is identified.
That will be a target that Northern will need to achieve in spite of operations. For this reason, Northern has been searching for an operational alternative that might accommodate the standard.
The low flows of 225 cfs for eight weeks, less than half of the system’s full flow capacity, is intended to keep Shadow Reservoir water “fresh.” The pump regimen came from a recommendation by Northern’s Water Quality Consultant Dr. Jean Marie Boyer, who has been studying water quality at the three lakes for more than a decade. She determined the flows are low enough so not to spill East Slope reservoirs, said Jeff Drager, project manager at Northern.
“We’re not moving as much water, but enough to keep it fresh and from going stagnant,” he said. “We’re trying to find a balance that keeps the weeds down as best as we can.”
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