In the foothills outside Longmont, Colorado, tucked high in a narrow valley, sits an ugly, cement slab. It's the size of a train car and juts out into North St. Vrain Creek, a shallow alpine stream that serves as the city's main drinking water supply.

A tiny sign greets hikers as they pass the structure. It reads: "Chimney Rock Dam." A small arrow points to the right.

What the sign doesn't tell you is how that cement slab ended up there.

It doesn't mention the Ku Klux Klan's rise to power in Colorado nearly 100 years ago. It doesn't include how the Klan took over Longmont and pushed hard for the splashy public works project. The sign fails to mention how day laborers started setting cement in the creek, and how this cold slab of cement eventually played a part in kicking the Klan to the curb.

It's bitterly cold outside when Erik Mason and I walk up the road that curves alongside the creek. It's within the city's watershed preserve, downstream from the much larger, completed Button Rock dam. The preserve encompasses much of the headwaters of North St. Vrain Creek.

Mason is a local historian and a curator at the Longmont Museum . The snow crunches under our feet as we get closer to the dam site - or what's left of it.

The dam doesn't block the creek completely, water still flows past. That's because it's unfinished, left in the same state since 1927.

"You can see a lot of the structure that normally would be concealed in a finished dam that is still exposed because it was essentially abandoned mid-construction," Mason said, pointing to the rusted pipes that poke out from the cement on all sides.

"Do people call it the KKK dam?" I asked.

"I actually don't think too many people know the story," Mason said. "People may know that the KKK was in Longmont. But that they had this strange public works project is probably a lesser-known aspect of the KKK's history in Longmont."

Luke Runyon / KUNC

The Klan in Colorado rose to power in the 1920s. Targeting Catholics, Jews and Mexicans, they swept into office with anti-immigrant rhetoric. They held rallies on the Front Range, clad in white hoods and robes.

A small news item in the June 1923 Longmont Ledger, one of the city's newspapers at the time, noted a speech given by a KKK lecturer visiting from Atlanta:

"He gave what might be called a strong patriotic address, claiming that loyalty to the Constitution was the first principle of the Klan. Whether any of his listeners were sympathizers of the Klan or not could not be told, but all applauded his patriotism to the flag. Several hundred people heard the address which lasted nearly two hours."

"People may know that the KKK was in Longmont. But that they had this strange public works project is probably a lesser-known aspect of the KKK's history in Longmont."

The Klan's growing political power in Colorado lent itself to running for elected office. In Longmont, that meant the city council and school board. In 1925 KKK members gained a majority of Longmont city council seats and dismissed longstanding city employees like the city engineer and the fire chief, replacing them with Klan sympathizers.

When it came to governing, the Klan majority had a narrow focus: water security.

Once in control of the council, they started looking for an eye-catching infrastructure project. They wanted something big, something made of cement, something to show the people of Longmont they were getting the job done.

"Essentially they were trying to find a way to employ their friends," Mason said.

Bags of cement started piling up at the site. KKK sympathizers were placed in jobs overseeing the build. To complete the job the council opted for day laborers, a way to make sure foreign workers weren't tapped for the build, Mason said. But the work was slow - and the initial cost of $85,000 quickly ballooned to $350,000.

Luke Runyon / KUNC

The rising cost, combined with a growing anti-KKK sentiment in the city, is what killed the dam, Mason says.

A group of locals unhappy with the change the KKK brought called themselves the Visible League, and they took out an ad in the Longmont Daily Times. It read that if the KKK wasn't kept in check, it would only lead to "hatred and distrust" in town.

"Citizens are threatened," the ad continued. "A campaign of persecution has been started."

By the time the next election rolled around in 1927, voters ousted Klan members from the council.

"People really, I think particularly in that first council election, weren't paying attention," Mason said. "And it wasn't until they got into office and started doing some of these crazy things that people realized that this was a really bad idea."

The new council saw the escalating costs and poor design and put the kibosh on the dam's construction. Left with 7,000 sacks of unused cement, the bags were hauled into town for a different public works project: road paving.

"... It wasn't until they got into office and started doing some of these crazy things that people realized that this was a really bad idea."

But walking past the dam you'd never know it. Mason sees it as a monument to the power of civic engagement - and to our reluctance to engage with uncomfortable racist history.

"People don't like negative history and they want to think that everything was great in the past and that there weren't the problems we have today," Mason said. "Reality is that people have been struggling and fighting against each other for as long as there have been people."

History is supposed to teach, Mason said. How else are we supposed to avoid making the same mistakes over and over again?

"This project reminds us that we always need to be vigilant."

This story is part of a project covering the Colorado River, produced by KUNC and supported through a Walton Family Foundation grant. KUNC is solely responsible for its editorial content.

The Longmont Museum is an underwriter with KUNC.