Arrowrock Dam


Arrowrock Dam, on the Boise River 42 miles downstream of Anderson Ranch Dam and 22 miles upstream from Boise, is a concrete thick-arch structure 350 feet high. When constructed in 1915, the dam was recorded as being the tallest concrete dam in the world. The structure was repaired and raised 5 feet during 1935-1937, increasing its storage by 9,000 acre-feet; total storage capacity 272,200 acre-feet (active 271,700 acre-feet). The original construction involved the use of a rather high proportion of sand cement, and by 1935 the concrete on the downstream face of the structure showed deterioration due to climatic conditions. Repairs made included refacing the downstream face and spillway channel. After 85 years of service, the Ensign Valves had reached the end of their useful life and the lower set of 10 Ensign Valves were removed and replaced with clamshell gates between 2001 and 2004. A contract to permanently plug all five sluice gate outlet conduits was completed in January 2011.

The Boise Project Board of Control completed construction of a 15 MW powerplant at Arrowrock Dam in March 2010. The powerplant penstockes tie into two of the existing 66" outlet conduits. The project also updated the transmission line to the dam.


The formation is a hard, sound but seamy, broad zone of granite porphyry, capped in left abutment with 50-foot basaltic lava.

RegionFacilities in Pacific Northwest | Pacific Northwest Home Page
Dam Type Concrete Thick Arch
WatercourseBoise River
Location13 mi E of Boise, ID
Original Construction1911-1915
Modified Construction1935-1937
National ID NumberID00280
Hydrologic Unit Code

Structural Height350.0 ft
Hydraulic Height (Normal Operating Depth at Dam)257.0 ft
Top Parapet (Elevation)3220.0 ft
Spillway Crest Elevation 3210.0 ft
Crest Elevation3216.0 ft
Crest Length1,150.0 ft
Crest Width15.5 ft
Base Width223 ft
Volume of Dam Construction Materials636,000 cu yd
Streambed at Dam Axis
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)
Top of Active Conservation Pool (Elevation) 3216.0 ft
Top of Inactive Conservation Pool (Elevation)
Top of Dead Storage Pool (Elevation)

Hydraulics & Hydrology
Total Water Storage at Elevation 271,700 ac-ft
Maximum Water Surface Elevation3224.1 ft
Normal Water Surface Elevation 3216.0 ft
Spillway TypeSide-channel spillway chute located on the right abutment, controlled by six 62-feet-long by 6-feet-high drum gates
Auxiliary Spillway
Spillway Capacity at Elevation40,000 cfs at 3219.75 ft
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation 13,000 cfs at 3219.75 ft
Diversion Capacity at Elevation
Drainage Area2,210 sq mi
Hydrometeorological Report (HMR)HMR 57

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: Jan 13, 2015