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 penstocks tie into two of the existing 66 outlet conduits. The project also updated the transmission line to the dam.
Geology
The formation is a hard, sound but seamy, broad zone of granite porphyry, capped in left abutment with 50-foot basaltic lava.
General
Project
Boise
Dam Type
Concrete Thick Arch
Location
13 mi E of Boise, ID
Longitude
-115.9230
Latitude
43.5952
Reservoir
Arrowrock
Original Construction
1911-1915
National ID Number
ID00280
Base Width
223 ft
Watercourse
Boise River
Modified Construction
1935-1937
Dimensions
Crest Elevation
3216.0 ft
Structural Height
350.0 ft
Hydraulic Height (Normal Operating Depth at Dam)
257.0 ft
Top Parapet (Elevation)
3220.0 ft
Crest Width
15.5 ft
Volume of Dam Construction Materials
636,000 cu yd
Spillway Crest Elevation
3210.0 ft
Top of Active Conservation Pool (Elevation)
3216.0 ft
Crest Length
1,150.0 ft
Hydraulics & Hydrology
Spillway Capacity at Elevation
40,000 cfs at 3219.75 ft
Hydrometeorological Report (HMR)
HMR 57
Maximum Water Surface Elevation
3224.1 ft
Drainage Area
2,210 sq mi
Total Water Storage at Elevation
271,700 ac-ft
Normal Water Surface Elevation
3216.0 ft
Spillway Type
Side-channel spillway chute located on the right abutment, controlled by six 62-feet-long by 6-feet-high drum gates
Outlet Works Capacity at Elevation
13,000 cfs at 3219.75 ft
Contact
Owner
Title: Area Office Manager Organization: Snake River Area Office Address: 230 Collins Road City: Boise, ID 83702-4520 Fax: 208-383-2275 Phone: 208-383-2248
Operator
Title: Area Office Manager Organization: Snake River Area Office Address: 230 Collins Road City: Boise, ID 83702-4520 Fax: 208-383-2275 Phone: 208-383-2248
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