Tiber Dam is a zoned earthfill structure located on the Marias River, approximately 20 miles southwest of Chester, Montana. The dam has a crest width of 30 feet, a crest length of 4,300 feet, and structural height of 211 feet. The upstream face is protected by a 3-foot layer of riprap and is sloped at 3:1. a zone of previous material consisting of selected sand, gravel and cobbles, covers the downstream face. A 1977 Safety of Dams modification raised the dam crest 5 feet to an elevation of 3,026.00 feet above sea-level. Total reservoir storage capacity is 1,555,898 acre-feet at an elevation of 3,020.20 feet.
An earthfill dike occupies a saddle on the south rim of the reservoir near the dam embankment. This dike has a crest width of 25 feet, a crest length of 17,000 feet, and a maximum structural height of 60 feet. The upstream face is protected by a 3-toot layer of riprap, while the downstream face is grass covered and has an intermediate 10-foot-wide-berm.
Tiber Dam was originally constructed from 1951 to 1956. Because of excessive spillway settlement, the spillway was diked off and an auxiliary outlet works was constructed from 1967 to 1969. To restore the original operational functions, a construction contract for rehabilitating the spillway was issued in 1976 for completion in 1979, followed by a second contract for raising the dam and dike 5 feet during a construction period between 1979 and 1981.Both abutments of Tiber Dam are located in the Telegraph Creek Formation that is composed of gypsiferous, poorly cemented sandstone and firm shale. Both formations are of Upper Cretaceous Age. Glacial till, loess and terrace deposits of Holocene Age can be found on top of the Marias River Valley Walls covering sandstone or shale deposits.