Tiber Dam, Embankment dam in Liberty County, Montana

The Tiber Dam is an earth-fill embankment dam on the Marias River in southern Liberty County, Montana, impounding Lake Elwell, also called Tiber Reservoir. It is 4,300 ft (1,311 m) long and 211 ft (64 m) high; a dike added to the reservoir's southern rim near the dam between 1967 and 1969 is 17,000 ft (5,182 m) long and 60 ft (18 m) tall. Its main spillway is controlled by three gates.

Construction of Tiber Dam on the Marias River began in 1952 and was completed in 1956, creating Lake Elwell. Between 1967 and 1969 a dike was added along the reservoir's southern rim near the dam because the spillway was settling. The spillway was then rehabilitated from 1976 to 1989.

The dam and its reservoir serve as a gathering place where locals and visitors enjoy fishing and water activities together. People come here to spend time on the water and experience the outdoors as part of their regular recreation.

The dam is an earth-fill structure 4,300 ft long and 211 ft high; a 17,000 ft dike, 60 ft tall, runs along the reservoir's southern rim near the dam. The main spillway is controlled by three gates, with an auxiliary spillway and outlet works handling smaller releases. Access is via Liberty County roads south of Chester.

The dam creates a body of water with varying depths and shoreline characteristics across its expanse. This diversity in water conditions and landscape allows different recreational activities to thrive in the same location.