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GRAND LAKE — Zipping across Grand Lake in an outboard motorboat on a warm day in late September, Mike Cassio eases off the throttle as he nears a break in the shoreline and points out one of two tributaries that feed Colorado’s largest and deepest natural body of water. This one is called the North Inlet. It winds about a dozen miles northeast into Rocky Mountain National Park. “If you go up here,” Cassio says, “this river is pristine. It’s unreal how beautiful and clear that is.” 

Cassio, president of the Three Lakes Watershed Association, a nonprofit that advocates for protecting the local environment, revs the boat back up to speed and continues east. The best view on Grand Lake faces this direction: A spectacular contrast of the calm water set against the face of 12,000-foot Mount Craig. As Cassio approaches the eastern shoreline, he points out a house he says belongs to the family who owns Hallmark Cards. “They’ve been up here forever,” he says.

Cassio eventually cuts the engine and the boat drifts toward the second of the two tributaries, this one the East Inlet. Same story: Snowmelt flows down from the craggy depths of the park, feeding into the lake. “Again, it is pristinely clear,” says Cassio, who has a house on the lake. “Right here, we can see the bottom, and we’re at, basically, 13 feet.”

This is how Mother Nature drew it up. A natural lake, left behind by a receding glacier, perched at 8,369 feet, framed just so by some of Colorado’s most prized wilderness. However, a remarkable feat of engineering, the Colorado-Big Thompson project, disrupted the balance of this ecological system. A massive federal Bureau of Reclamation project that moves water from the Colorado River headwaters in Grand County to the northern Front Range, C-BT supplies drinking water to more than a million Coloradans and supplemental irrigation to 615,000 acres of farmland. 

Getting this critical water to communities on the northern Front Range impacts communities on this side of the mountains. One of the characteristics of the C-BT system that has frustrated Grand Lake residents from just about the moment the bureau completed the project in 1956 is that C-BT at times operates by pushing murkier water marred by weeds and algae into Grand Lake, turning, as locals describe it, what would otherwise be a pristine mountain pool into something that more resembles a bowl of split pea soup. 

Locals consider the clarity of this lake an important community asset, something to be enjoyed not just by those who live here but everyone who comes to visit. What’s more, they contend that as long as people have thought about the significance of this plumbing project there were also those who recognized the value of a crystal clear Grand Lake and sought to protect the water. 

Canton “Scally” O’Donnell, 92, grew up in Grand Lake and was around for the pre-C-BT days. “What I remember as a youngster is we could go out and see a boat that had been sunk,” O’Donnell said. “Somewhere in the range of 60 feet of water; we could always see it.” 

In the first picture above, clear waters near East Inlet inside Grand Lake, where a fresh mountain stream flows into the body of water. In the second picture, the water quality of Shadow Mountain Reservoir near the town of Grand Lake is much more murky. (Hugh Carey, The Colorado Sun)

Getting the word out

After pointing out the clearer spots near the North and East inlets, Cassio pilots the boat out into the middle of the 1-mile-wide lake to highlight the comparatively cloudier water. “This green water is not a normal thing,” he says. “It shouldn’t be this green.”  

For years, multiple agencies have worked on addressing the clarity in Grand Lake, and, by some measures, it has improved. This summer, however, the Three Lakes Watershed Association brought new attention to the issue when it presented to the state legislature’s Interim Water Resources and Agricultural Review Committee. The association asked the state to independently review not just the clarity in Grand Lake but also whether the entire Western Slope C-BT system might be in need of a facelift. “The system was designed in 1937,” Cassio said. “It’s old and needs an update.”

The water committee did not act on the association’s request, but Cassio says the hearing helped get the word out. “We’re trying to gain some momentum up here to make everybody understand what’s going on.” 

One person who took notice was Rep. Julie McCluskie, whose district includes Grand County. Recently selected to serve as Colorado’s next speaker of the House, McCluskie toured Grand Lake earlier this year. “Getting out on a boat was a great opportunity to look at the water — it was green and murky,” McCluskie said. “I was struck by the fact that that is abnormal; our typical waters here in the mountains are crystal clear, fresh off the mountains.” 

McCluskie said she plans to look at options for improving Grand Lake clarity and the C-BT system during the upcoming legislative session. “When we’re talking about the water supply to the Front Range, the system that was designed decades ago, it really does deserve our attention, and I think with both federal and state focus and resources there may be some things we can do in the short and long term.” 

Kirsten Heckendorf holds her hat while boating on Grand Lake in late September in Grand County. The lake is surrounded by Rocky Mountain National Park to the north, east and south. (Hugh Carey, The Colorado Sun)

Preserve the scenic attractions

A main component of the C-BT project, the Alva B. Adams Tunnel, begins at the edge of Grand Lake, under the surface of the water. The tunnel, a giant pipe 9 feet, 9 inches in diameter, draws water from a spot between the two inlets. Buried 3,800 feet beneath the Continental Divide, it runs about 13 miles toward Estes Park, dropping roughly 100 feet along the way. From there, the Northern Water Conservancy District, which jointly operates C-BT with the bureau, stores the water in numerous Front Range reservoirs and delivers it to its municipal and agricultural shareholders. 

Moving water through the Adams tunnel requires pumping it from west to east, reversing its normal course through Grand Lake. Northern Water collects C-BT water in Lake Granby, its main Western Slope storage reservoir and Colorado’s third-largest water body. That water is then pumped to Shadow Mountain Reservoir, a much smaller reservoir situated between Lake Granby and Grand Lake. After that, gravity does the work; the water flows through a narrow channel, into Grand Lake, through the Adams tunnel and on to Mary’s Lake and Lake Estes, eventually passing through and generating power at five hydro plants.

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One of the main challenges in maintaining good water clarity in Grand Lake is that Shadow Mountain Reservoir is particularly shallow, about 22 feet at its deepest point. (Cassio jokingly referred to it as “Shallow Mountain.”) For that reason, the reservoir is prone to heating up and spawning algae. That water then moves into Grand Lake, which is much deeper, and the warmer, murkier water from Shadow floats on top. 

“Warm water is lighter than cold water,” Esther Vincent, director of environmental services at Northern Water, said. “It’s almost as if you have two completely separate pools of water — the water that comes in from Shadow only mixes with the top of Grand Lake. There’s not much dilution factor.” 

Recognizing the C-BT system would have an impact on both sides of the Continental Divide, a 1937 federal document authorizing the project, known as U.S. Senate Document 80, lays out five operating guidelines. Number two on the list reads: “To preserve the fishing and recreational facilities and the scenic attractions of Grand Lake, the Colorado River, and Rocky Mountain National Park.” Locals have long considered the clarity of Grand Lake as part of the “scenic attractions” referenced in Senate Document 80. 

Notably, there’s a way to measure clarity in a water body. In the late 1800s, an Italian astronomer created an instrument called a Secchi disc, a version of which is still used today. The contraption resembles a dinner plate attached to a rope. The scientific method of measurement using the Secchi disc is fairly straightforward. Essentially, someone lowers the disc into the water until they can’t see it anymore. That’s the Secchi depth.

This green water is not a normal thing.
It shouldn’t be this green.

— Mike Cassio, president of the Three Lakes Watershed Association

Mike Cassio boating on Grand Lake in late September Cassio is part of the Three Lakes Watershed Association to advocate to cleaner water quality in Grand County. (Hugh Carey, The Colorado Sun)

There’s one official Secchi reading for Grand Lake from before the C-BT system was fully operational. A professor took the measurement in September 1941. It registered at 9.2 meters, about 30 feet of clarity. Today, Secchi readings are regularly taken on Grand Lake and typically range roughly between 2 meters and 5 meters, about 7 feet to 16 feet. 

That initial 9.2 meter reading should be taken in context, Vincent said. “Clarity in pristine lakes is not a flat line,” she said. “It changes throughout the season.” For instance, runoff in the spring can carry sediment and other material into a water body. Vincent said clarity tends to be best in the fall, which is when the 1941 reading was taken. Other variables can affect a Secchi reading, too, she said, such as the amount of glare on the water or the measurer’s eyesight. 

But, Vincent said, “it still tells the story there’s been a loss of clarity post construction of the project.” 

A goal for clarity

In 2008, a group of local stakeholders proposed to the state water quality control commission setting a 4 meter standard for Grand Lake clarity. Eventually, in 2016, the bureau, Northern Water, Grand County, the Northwest Colorado Council of Governments, and the Colorado River District entered into a memorandum of understanding regarding Lake Granby, Shadow Mountain Reservoir, and Grand Lake, collectively referred to as the three lakes system. The agreement includes clarity goals for Grand Lake. From July 1 to Sept. 11 each year the target average Secchi depth is 3.8 meters with a minimum depth of 2.5 meters. 

Since 2016, Northern Water and the bureau have worked on finding ways to manipulate the timing and management of its C-BT operations to help with the clarity. Those efforts have led to some improvement during those summer weeks that are crucial to tourism in Grand County, Vincent said. “It’s getting better because we’re working at it,” she said.

For instance, Vincent said, they wait as long as they can after runoff in the spring and early summer to resume pumping, giving any sediment a chance to settle before moving the water. They’ve also experimented with different pumping patterns and amounts to try to find a sweet spot where the water doesn’t get too stagnant in Shadow Mountain, she said.  

Katherine Morris, Grand County’s water quality manager, said the success of the agreement, known as “adaptive management,” has been a pleasant surprise. “We have realized results that are far better than what I thought could be achieved just through operations.” 

The agreement lasted five years and the parties have since extended it for another five years. It now runs through the end of 2026.

But pursuing some kind of more ambitious structural change to the way the C-BT system operates could produce even better results, Morris said. 

“If we were to undertake something that changed the plumbing we could be playing on a completely different ballfield. And we just don’t know how good clarity could be; I think it could be quite good,” she said. “Grand Lake should be a state gem. I don’t want a lake where somebody comes and visits and says, ‘this looks all right.’ We want a lake where people come and say, ‘wow, that’s gorgeous.’”

Cassio said part of the reason for the recent push by the Three Lakes Watershed Association is that the group feels the success of the adaptive management process is reaching its limits. “We believe that they have done the most they can do within the constraints of the system,” he said. 

Cassio was also critical of the fact that there is no enforcement mechanism for the clarity targets in the agreement. “The goal was set but there’s no teeth in it,” he said. Last year, the minimum Secchi depth goal qualifier was met, but the average depth goal qualifier was not. But those clarity results cannot, for instance, lead to the state officially designating Grand Lake an impaired water body. 

Still, the parties agreeing on the adaptive management process has been a critical step, Vincent said. “Do we hit that goal every year? No. Is it better than it used to be? Yes,” she said. “This is an important, valuable process. It does make things better — it’s made the operations much more intentional and deliberate.” 

A rower takes his boat across Grand Lake on Sept. 19 in Grand County. The lake near Rocky Mountain National Park is the largest and deepest natural body of water in the state. (Hugh Carey, The Colorado Sun)

Other fixes

Heading into next year, the Three Lakes Watershed Association has both short- and long-term goals in mind for improving the situation on Grand Lake. 

Buying a giant aquatic weed harvester to regularly scrape through Shadow Mountain would help, Cassio said. He said it’s also possible to install aerators in the reservoir to help limit the spread of algae blooms in water that gets moved to Grand Lake. 

Cassio said he hoped the state could help track down federal infrastructure money for these things. “Let’s spend some money in the short term to try to keep Shadow as clean as possible,” he said. “The short-term stuff is easy enough as long as there’s a funding source.”

Changing the way water moves from Lake Granby through Shadow Mountain to Grand Lake would be a much bigger undertaking. About a decade ago, the Three Lakes association commissioned an engineering study to examine possible alternatives. Cassio said that one $70 million option looked at bypassing Grand Lake by connecting Shadow Mountain directly to the Adams tunnel. But that wouldn’t fix the problems in Shadow Mountain Reservoir itself, Cassio said. 

Cassio said the association is not advocating that water stop moving to the Front Range as part of any potential structural changes, rather that the water be moved in a way that does not impact Grand Lake. He said the dream fix, as far as he’s concerned, would be a new tunnel connecting Lake Granby to the Front Range, leaving the Adams tunnel as more of a backup, emergency system. The Three Lakes group has not studied the feasibility of that, he said. 


Jeff Rieker, Bureau of Reclamation area manager for the eastern Colorado area office, said the commitment and collaboration among the partners to working on this topic has been positive. “Reclamation fully appreciates the complex interrelationships that influence Grand Lake clarity,” he said in an emailed statement. He said he thinks more can be done within the context of the current agreement.

During the past two decades, the Bureau of Reclamation has reviewed a variety of potential structural alternatives, including how to get water around or otherwise past Grand Lake, Rieker said. However, he said, for many of those alternatives there were concerns about feasibility and overall effectiveness. 

Rieker said the bureau will continue to focus on the work outlined under the agreement. “Through this collaboration, if a structural alternative that is suitable for additional consideration is identified, we will then determine which appropriate formal federal process is required to ensure that the alternative receives a thorough review,” Rieker wrote.

For its part, Cassio said, the Three Lakes association will continue to push for improvements to the C-BT system on the west side of the Continental Divide, near the Colorado River headwaters, an area important to so many water users. “Everything that goes west down the Colorado River starts there,” he said, “and everything that goes east to the Front Range starts there.” 

Chris Outcalt covered Western water issues for The Colorado Sun until December 2022. He began his journalism career in New Hampshire, then moved West and became a reporter at the Lafayette News. He also was an associate editor at 5280 and a reporter for the Vail Daily. His freelance work has appeared in The Atlantic, Wired and Atavist. He is a recipient of the Livingston Award.