Local historian tells the tale of the Dillon Reservoir | SummitDaily.com

Local historian tells the tale of the Dillon Reservoir

The Dillon Reservoir as seen in July 2019. Local historian Sandra Mather gave a lecture about the history of Dillon Reservoir at the Frisco Historic Park and Museum Thursday as part of the town of Frisco’s Lunchtime Lecture series.
Elaine Collins / Summit Daily reader

Editor’s note: This story has been updated to correct details about Roberts Tunnel, Dillon Reservoir, the spillway and the mechanism at the bottom of the dam that sends water into the Blue River.

FRISCO — The history of Summit County spans 158 years, with many intertwining tales of exploration, conquest, fortune and heartbreak. But in 1961, 100 years after the county was founded as one of Colorado’s 17 original territories, the construction of the Dillon Reservoir officially began; a singular event which would change Summit County forever.

There is a long tale about the lead-up to the reservoir’s construction itself, a history told in rich detail by local historian Sandra Mather at the Frisco Historic Park and Museum Thursday afternoon as part of the town of Frisco’s Lunchtime Lecture series.

The lecture, titled “Dillon, Denver and the Dam,” took place in the old Historic Park Chapel behind the museum, where every pew was filled and the audience spilled out of the door. Mather, a former president of the Summit Historical Society who has written 20 books about Summit County’s rich history and has a doctorate in physical geography, spoke to the capacity crowd on why the reservoir was built and the numerous challenges it faced being built.

“If you look at the Dillon Reservoir, it’s beautiful; but for some people, it brings back some very painful memories,” Mather began. “For some it’s a symbol of newness, change, opportunity and progress. For others, it symbolizes the death of a very old and comfortable way of life.”

The reservoir’s need was first realized in 1907, when the city of Denver realized it would require a lot more water as it grew. In 1913, Denver Water started buying water rights around Summit County, seeing the area’s natural geography as ideal for a reservoir.

“This was a huge catchment area,” Mather said. “You had a confluence of three streams, the narrowing of the valley north of Dillon, you would have gravity flow through the tunnel across the Continental Divide, and all were very important.”

Unfortunately, many benefits that were found in geography were lost to the local geology. There were numerous challenges in trying to find a place to put the dam, and once it was found a whole lot of earth-moving had to be done to artificially strengthen the foundation and ensure water would not start leaking under the dam.

Before constructing the dam itself, a core trench was dug 90 feet deep under the entire length of where the dam now stands, down to the bedrock. Another trench was dug into the bedrock itself, and then giant holes were dug into that trench 300 feet deep and filled with concrete. Suffice to say, the dam built on top of that foundation is well reinforced.

When the dam was finally completed in 1963, it stood 231 feet tall, 5,888 feet long and over 580 feet wide. Twelve million tons of fill was used to build the dam, with most coming from borrow pits in the reservoir area.

There is also the matter of managing overflow. That job goes to a morning glory spillway, which is basically a giant cement funnel at the dam’s maximum capacity height of 9,017 feet. All overflows fall into this spillway, which features fins at the top to prevent a whirlpool at the top, which would create air bubbles that can deteriorate the spillway’s cement.

Overflow water runs straight down the gullet of the spillway, which is 15 feet wide at its narrowest part before turning 90 degrees and running into the Blue River.

The river’s regular flow is controlled through a separate 15-foot wide fixed-wing gate at the bottom of the dam, which can be opened and closed to regulate water flow into the Lower Blue.

When fall comes and the reservoir level is lower, the spillway is no longer in use. Mather explained that since cold air sinks, the spillway can get iced up inside, damaging the concrete. To prevent this, Denver Water uses a crane to lift a giant “plug” — a 6-ton steel disc — and lower it into the spillway, preventing damage.

Mather described another key component of the entire reservoir system, the Roberts Tunnel. The 23-mile long tunnel, which when built was the second largest in the world, takes water from the reservoir in the West through a 10-foot wide tunnel under the Continental Divide and down 174 feet of elevation to the eastern portal in Grant. The entire tunnel is lined with a quarter-inch thick, highly polished steel.

Mather said the construction of the tunnel began one month to the day before she was born September 17, 1942. Construction of the tunnel officially ended two months to the day after Mather graduated from college, when the eastern portal opened 22 years later, on July 17, 1964.

The tunnel was initially supposed to be called the Blue River Tunnel, and then Montezuma Tunnel. The tunnel was finally formally named after Harold D. Roberts, a lawyer who secured Congressional approval for the reservoir, along with snapping up many of the water rights required for the reservoir.

Roberts was also responsible for going after properties behind on their taxes and buying up foreclosures that resulted in the entire town of Dillon being moved for the fourth time in its history, giving him a more ignominious reputation here in Summit.

The total cost of the tunnel was $46 million in 1961 dollars. Mather said she did the math, and that it came out to about $25 per inch of the tunnel’s length. When the reservoir is full, the tunnel can carry 680 million gallons of water in 24 hours.

At the moment, however, the tunnel is shut off. Mather explained that the Dillon Reservoir is a reservoir of last call for Denver. That means that the reservoir, which provides 37% of the Front Range’s water supply, is kept at full capacity for as long as possible, and then starts to flow through the Roberts Tunnel when needed.

While the reservoir was being built, there was an entire chapter of the story of the reservoir Mather told involving the razing and moving of the entire town of Dillon to make way for the reservoir, along with Summit County’s somewhat contentious history dealing with Denver Water. That chapter and the rest of the history of the Dillon Reservoir will be told in a follow-up story later this week.

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