A look at the history of Dillon Reservoir

Dillon townsite prior to construction of Dillon Reservoir via Denver Water

Here’s a report from The Summit Daily (Deepan Dutta). Click through and read the whole thing. Here’s an excerpt:

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…

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.

Aside from the dam, constructing the reservoir itself was a herculean endeavor itself. Given that the entire purpose of the reservoir is to impound water for use elsewhere, the reservoir needed to be lined and segregated from the ground [water].

That’s why a steel liner was installed to ensure the water stayed in the reservoir and didn’t get contaminated. The liner – a quarter-inch thick, highly polished steel – was pieced together at the bottom of what is now the reservoir in 30-foot long pieces.

There’s 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 through a 15-foot wide fixed-wing gate, which can be opened and closed to regulate water flow into the Lower Blue River.

Denver Water employees Rick Geise and Nate Hurlbut assisted in setting the plug, which helps prevent chunks of ice and snow from falling into the spillway. Photo credit: Denver Water

When fall comes and the reservoir is lowered, the spillway is no longer in use. Mather explained that since cold water 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 ice and debris build-up.

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 pipe across the Continental Divide and down 174 feet of elevation to the eastern portal in Grant.

Mather said the construction of the tunnel began one month to the day before she was born, on 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.

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