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In the late 1800s, Denver built its first treatment plant, Kassler, located at the base of Waterton Canyon by the present-day Chatfield Reservoir. Kassler, which used the English slow-sand filter process, was the first of its kind west of the Mississippi River.

Kassler’s underground infiltration galleries were built in 1890, the filter beds added in 1906, and the reservoir and pump station were built in 1972. A whole town was built around the plant to operate and maintain the facility. At the height of its operation, Kassler delivered more than 50 million gallons of treated drinking water to the residents of Denver.

By 1985, Kassler ceased operations because it was unable to keep up with demand. Now the buildings and nearby facilities are used for educational purposes.