Duncan, also known as Twilight Lake, is a local government-owned water resource in Durango, Colorado. Managed by DAVIS ENGINEERING SERVICE, INC., this dam was completed in 1978 and serves multiple purposes including fish and wildlife pond, recreation, and other uses. With a height of 31 feet and a length of 180 feet, Duncan has a storage capacity of 78 acre-feet and covers a surface area of 7 acres, drawing water from Purgatory Creek-OS.

This earth dam, with an uncontrolled spillway and a slide gate outlet, poses a significant hazard potential but has been assessed to be in satisfactory condition as of July 2019. The dam is inspected every two years, with the last inspection conducted in July 2019. Despite its moderate risk level, Duncan remains a crucial water resource for the local community, supporting various recreational activities and providing habitat for fish and wildlife. Located just 30 miles from Durango, Duncan plays a vital role in water management and conservation efforts in La Plata County.

With state regulation, permitting, inspection, and enforcement in place, Duncan is a well-maintained and regulated water resource. The dam's emergency action plan status, inundation maps, and risk management measures are currently not available, but the dam is equipped with necessary safety features like a spillway and outlet gates to ensure the protection of downstream areas. As a key component of the local water infrastructure, Duncan continues to be a focal point for water resource and climate enthusiasts interested in sustainable water management practices.