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 286,600 acre-feet (active 286,600 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 sedimentation survey completed in 1997 estimated the total capacity of Arrowrock Reservoir at 272,200 acre-feet (active 271,700 acre-feet). Geology The formation is a hard, sound but seamy, broad zone of granite porphyry, capped in left abutment with 50-foot basaltic lava. |
