Look back: Why Horsetooth Reservoir was nearly drained

It's hard to imagine now, with snow melt starting to fill Horsetooth Reservoir, but the man-made lake that quenches Fort Collins' thirst was once little more than a mud puddle.

The year was 2001, and Horsetooth Reservoir and its four dams had just turned 50. They were over the hill.

And their signs of age were hard to ignore, even cloaked in the arcane language of bureaucratic reports: Increased seepage. Heightening foundation pressures. A lack of protective measures to prevent internal erosion.

In the worst of all possible scenarios, water seeping through the dam cores could make them disintegrate, causing what bureaucrats call a “rapid failure.”

In other words, a dam could break, swiftly and without warning.

If that happened, "all or parts of the communities of Fort Collins, Timnath, Windsor, Greeley, Kersey along with other towns in Larimer and Weld counties, would be severely damaged or destroyed,” read a Bureau of Reclamation study cited in a June 2000 Coloradoan article.

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The economic impact of such a disaster: $6.4 billion.

The story of the $77 million repair project that nearly drained Horsetooth is one worth telling as Fort Collins approaches its rainiest season, just a few months removed from the high-profile emergency at the Oroville Dam in California. Extreme rains threatened that dam's spillway and raised the possibility of the structure failing and unleashing an uncontrolled torrent of water on residents.

Nearly 15 years later, as mountain snowpack thaws and Horsetooth is nearly 90 percent full, let's reflect on the two-and-a-half-year revamp that shielded Fort Collins from the possibility of a dam disaster.

The project

No one argued that failure was imminent for Horsetooth's dams.

They were still passing U.S. Bureau of Reclamation inspections. They behaved the way they were supposed to, even absorbed some of the blow of Fort Collins’ historic 1997 flood, when more than a foot of rain pounded the city in 31 hours.

But in the geeky, complex world of dam design, you prepare for worst-case scenarios. And besides, a dam failure wasn't unheard of in Northern Colorado: The Lawn Lake dam failure of 1982 killed a man and sent a deluge of water through downtown Estes Park.

So it was time to give Horsetooth a makeover.

"What makes it urgent is that there are so many people and so much population downstream from the dams that we need to take action," Bureau of Reclamation engineering and construction liaison Beth Boaz said at the time.

Horsetooth’s four dams – Soldier Canyon, Spring Creek, Dixon and Horsetooth – were outfitted with clay cores to make them waterproof.

Here’s the thing about old dams, though: They leak. Water is really good at finding the easiest path of resistance to head downstream.

By the 2000s, water was starting to seep through the Horsetooth dams. Not a lot. But it was enough to get the Bureau of Reclamation’s attention. And, in the unlikely scenario that an earthquake hit Northern Colorado, the Bureau of Reclamation folks weren’t certain the aging dams would remain intact.

"They were built in the 1940s with 1940s technology," Northern Water Conservancy District spokesman Brian Werner said. "We needed to bring them into the 21st century."

More: Stout: The lost town at the bottom of Horsetooth

After some deliberation, the Bureau of Reclamation, which served as the project leader, decided to blanket the dam’s clay cores with 10-foot layers of sand and fine gravel that would keep them dry. They’d build fortifying buttresses on each dam’s downstream side.

First, they’d have to nearly drain Horsetooth.

The Bureau of Reclamation restricted water levels to 5,360 feet – 70 feet below full – during construction. At various points, Horsetooth was down to “dead pool” storage – about 7,000 acre feet, or roughly 5 percent capacity.

Northern Water general manager Eric Wilkinson, who was born and raised in Fort Collins, can still remember how strange it looked.

“Obviously, you don’t ever want to see an empty reservoir,” he added. “But personally, I thought it was impressive -- the work that was done and the time frame in which they carried it out.”

They kept just enough water in Horsetooth to meet users' needs during the irrigation season, storing the rest in the western reaches of the Colorado-Big Thompson Project.

Workers found all sorts of stuff on the floor of the mostly bare dams and reservoir. Dinosaur fossils. A 1950s Porsche of mysterious provenance. The remnants of at least one building foundation from Stout, the quarry town that once touched the south end of Horsetooth Reservoir.

And they found another reason for the seepage at Horsetooth Dam.

Most of the surface below Horsetooth is made of sandstone. But there’s also a seam of limestone called the Forelle formation, which runs in starts and stops from the Bellevue area all the way to Carter Lake.

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The geological uprising that created the scenic hills east of the reservoir made some of that brittle limestone become crinkly. Grout injected in the cracks long ago wasn’t standing up to the test of time.

So they dug down into the Horsetooth Dam, injected more grout and added a clay blanket to the upstream side of the dam. They put a layer of thick plastic on top of that.

By November 2003, the work was basically done. The reservoir was finally starting to fill by that spring. Water sloshed across long-exposed banks and boat ramps re-opened. As Horsetooth filled, a historic drought came to a close.

The people of Northern Colorado had their reservoir back.

But someday, Horsetooth will have to dry up again. The modernized dams will eventually become outdated.

Wilkinson won’t say when that will happen.

“Nobody can really know that,” he said. “But right now, it’s state-of-the-art. It’s performing very well. I think monitoring of the dams in the last 15 years has shown that they did a good job.”

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How Horsetooth is different from Oroville

What happened at the Oroville Dam in February is unlikely to occur at any of the Horsetooth Reservoir dams, according to officials from the Bureau of Reclamation and Northern Water. That's partially because of the modernization project, but there are also a few other reasons. 

One is magnitude: The amount of water released through the Oroville Dam in 24 hours is more than Horsetooth Reservoir could hold at full capacity.

Also, Horsetooth Reservoir's relationship to its dams and water sources is different from Lake Oroville's relationship with the Oroville Dam and the Feather River, which the dam impounds to form the lake. No river runs into Horsetooth Reservoir; gravity transports water there from other reservoirs in the Colorado-Big Thompson Project.

The Bureau of Reclamation in June 2016 finished a comprehensive review of Horsetooth Reservoir's dams. All passed inspection.

“The Way It Was,” a podpast podcast

As the Coloradoan's newest podcast project, "The Way It Was" will cover Fort Collins history topics. New podcast episodes will launch on SoundCloud, iTunes and Stitcher the second Thursday of every month.

The first episode — about Stout — is available to listen to now. It includes an interview with an unofficial historian of Horsetooth, a 40-year-old taped interview with a farmer that once called Stout home and information on a remnant of the old town you probably didn't know still existed.

Listen now at noconow.co/stout.