Anderson Ranch Dam


Anderson Ranch Dam and Powerplant is a multiple purpose structure that provides benefits of irrigation, power, and flood and silt control. The dam is 456 feet high and is on the South Fork of the Boise River, 28 miles northeast of Mountain Home. It has a total storage capacity of 474,900 acre-feet (active 413,100 acre-feet) and was the world's highest earthfill dam at the time of its completion in 1950. The power plant had a rated capacity of 27,000 kilowatts with two units installed. These units were up-rated in 1986, increasing the capacity to 20,000 kilowatts each for a total of 40,000 kilowatts.


Anderson Ranch Dam is located near the southwestern edge of Idaho Batholith which is composed of a series of closely related, coarse-grained intrusions. Major normal faulting in the region includes faults in and adjacent to the Batholith, faults bounding the northeast and southwest margin on the adjacent Snake River Plain, and faults forming Camas Prairie Basin east of the dam. These faults show a low to moderate seismic activity level. The dam is located in a narrow steep-sided valley cut through several hundred of igneous extrusive and extrusive rock. In the dam foundation, the rock was found to be intensely fractured and cut by numerous shears and fractures of varying widths. Several dikes were also found normal to the canyon axis. The dikes present a reduction of fractures and shears reducing the foundation permeability and making a tight foundation. Materials within the canyon include talus, colluvium, alluvium, residual soil and intact rock. These materials form the foundation of the dam and have performed well with respect to deformation and seepage.

RegionFacilities in Pacific Northwest | Pacific Northwest Home Page
Dam Type Zoned Earthfill
ReservoirAnderson Ranch
WatercourseSouth Fork of the Boise River
Location28 miles northeast of Mountain Home, Idaho
Net Generation151,014,000 kWh in 2011
Original Construction1941-1950
Modified Construction
National ID NumberID00279
Hydrologic Unit Code

Structural Height456.0 ft
Hydraulic Height (Normal Operating Depth at Dam)330.0 ft
Top Parapet (Elevation)
Spillway Crest Elevation 4174.0 ft
Crest Elevation4206.0 ft
Crest Length1350.0 ft
Crest Width
Base Width
Volume of Dam Construction Materials
Streambed at Dam Axis3866.0 ft
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)4196.0 ft
Top of Active Conservation Pool (Elevation) 4196.0 ft
Top of Inactive Conservation Pool (Elevation)4039.6 ft
Top of Dead Storage Pool (Elevation)3992.0 ft

Hydraulics & Hydrology
Total Water Storage at Elevation 474,900 ac-ft at 4196 ft
Maximum Water Surface Elevation
Normal Water Surface Elevation4196.0 ft
Spillway TypeConcrete-lined chute located on left abutment, controlled by two 25-by-22 foot radial gates
Auxiliary SpillwayNo
Spillway Capacity at Elevation20,000 cfs at 4198 ft
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation10,000 cfs at 4198.0 ft
Diversion Capacity at Elevation
Drainage Area960 sq mi
Hydrometeorological Report (HMR)HMR 57
Probable Maximum Flood (PMF) Report

Contact Information
TitleArea Office Manager
OrganizationSnake River Area Office
Address230 Collins Road
CityBoise, ID 83702-4520
Phone (Fax)208 383-2275
Phone (Office)208 383-2248
TitleArea Office Manager
OrganizationSnake River Area Office
Address230 Collins Road
CityBoise, ID 83702-4520
Phone (Fax)208 383-2275
Phone (Office)208 383-2248

Last updated: Nov 21, 2012