Tiber Dam


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.

RegionFacilities in Great Plains | Great Plains Home Page
ProjectLower Marias
Dam TypeZoned earthfill (dike closes low saddle beginning about 1 mile southwest of the right abutment of the dam).
ReservoirLake Elwell
WatercourseMarias River
LocationMarias River, 20 miles southwest of Chester, Montana, and 71 miles upstream of the confluence of the Marias River with the Missouri River. The dam can be reached from Chester, Montana by traveling south on Montana Highway 223, 10 miles to a point where the highway angles to the left; continue south on a gravel road 3 miles; turn right and go approximately 5 miles. The first turn to the right goes to the camp and the second goes to the dam.
Original Construction1952-1956
Modified Construction 1967-1969: spillway dike and auxiliary outlet; 1976-1979: spillway rehabilitation
National ID NumberMT00579
Hydrologic Unit Code

Structural Height211.00 ft
Hydraulic Height (Normal Operating Depth at Dam)196.70 ft
Top Parapet (Elevation)
Spillway Crest Elevation2,975.00 ft
Crest Elevation3,026.00 ft
Crest Length4,300.00 ft
Crest Width30.00 ft
Base Width2,500 ft
Volume of Dam Construction Materials 9,800,000.00 cu yds
Streambed at Dam Axis
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)2,993.00 ft
Top of Active Conservation Pool (Elevation)2,976.00 ft
Top of Inactive Conservation Pool (Elevation)2,966.00 ft
Top of Dead Storage Pool (Elevation)2,870.00 ft

Hydraulics & Hydrology
Total Water Storage at Elevation 1,515,522.00 acre-ft at 3,020.20 ft
Maximum Water Surface Elevation 1,515,522.00 acre-ft at 3,020.20 ft
Normal Water Surface Elevation
Spillway TypeConcrete crest and concrete-lined open channel at right abutment of dam controlled by three 22- by 38- ft radial gates.
Auxiliary Spillway
Spillway Capacity at Elevation68,467.00 cfs at 3,020.20 ft
Auxiliary Spillway Capacity at Elevation5,845.00 cfs at 3,020.20 ft
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation 1,605 cfs at 3,020.20 ft
Diversion Capacity at Elevation
Drainage Area4,375.00 sq mi
Hydrometeorological Report (HMR)
Probable Maximum Flood (PMF) Report

Contact Information
TitleArea Manager
OrganizationMontana Area Office
AddressP.O. Box 30137
CityBillings, MT 59107-0137
Phone (Fax)406 247-7338
Phone (Office)406 247-7300
TitleTiber Dam Field Office
OrganizationMontana Area Office
AddressP.O. Box 220
CityChester, Montana 59522
Phone (Fax)406 456-3226
Phone (Office)406 456-3238

Last updated: Apr 02, 2013