Chimney Rock Dam

Luke Runyon / KUNC

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.