Duncan Reservoir, also known simply as Duncan, is a privately owned water resource located in Modoc County, California, near the city of Canby. Built in 1919, this earth dam stands at a height of 19 feet and has a storage capacity of 2575 acre-feet. The primary purpose of Duncan Reservoir is water supply for irrigation and other uses, drawing from the Tr Pit River.

Managed by the Natural Resources Conservation Service, Duncan Reservoir is regulated by the California Department of Water Resources for safety and compliance. While the dam is considered to have a significant hazard potential, its condition assessment in 2017 was rated as satisfactory. The last inspection in August 2019 showed no immediate concerns, and the dam meets state permitting, inspection, and enforcement standards.

With its rich history dating back over a century, Duncan Reservoir serves as a vital water source in the region, supporting agricultural and community needs. As a notable feature in the San Francisco District, this reservoir showcases the importance of proper dam management and maintenance to ensure the safety and sustainability of water resources amidst changing climate conditions.