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| Arrowrock Dam | |
|---|---|
| Name | Arrowrock Dam |
| Location | Boise County, Idaho, near Idaho City, Idaho |
| Country | United States |
| Status | Operational |
| Construction begin | 1915 |
| Opening | 1915–1918 |
| Owner | United States Bureau of Reclamation |
| Dam type | Concrete thin arch |
| Height | 350ft |
| Length | 660ft |
| Reservoir | Arrowrock Reservoir |
| Capacity total | 180000acre.ft |
| Plant operator | Idaho Power Company |
| Plant capacity | 15 MW |
Arrowrock Dam Arrowrock Dam is a historic concrete thin-arch masonry dam on the Boise River in Boise County, Idaho, United States. Constructed in the 1910s, the structure played a pivotal role in early 20th‑century western water management projects led by the United States Reclamation Service and later overseen by the United States Bureau of Reclamation. The dam forms Arrowrock Reservoir, which supports irrigation, municipal water supply, flood control, and hydroelectric generation for regional beneficiaries including Boise, Nampa, and Caldwell.
Planning for Arrowrock originated amid federal initiatives such as the Reclamation Act of 1902 to open arid western lands and expand irrigation for settlers in the Columbia River Basin drainage region. Early surveys involved engineers from the United States Geological Survey and consultants familiar with projects like Newlands Reclamation Project and Boise Project. Construction began under the United States Reclamation Service in 1915, contemporaneous with projects at Minidoka Dam and Teton Dam planning, and concluded in phases by 1918. Arrowrock’s completion contributed to regional agricultural expansion in the Treasure Valley and later interfaced with water policy developments embodied by the Pick-Sloan Missouri Basin Program and state water rights adjudications.
The dam is a concrete thin-arch design inspired by earlier arch dams such as Hetch Hetchy Dam and innovations from engineers who worked on Hoover Dam and Morrow Point Dam. Its masonry-concrete composition required work by contractors experienced with large-scale river diversions similar to projects at Glen Canyon Dam (later designs) and contemporaneous diversion practices used on the Sacramento River projects. Construction entailed temporary cofferdams, diversion tunnels, and on-site concrete batching; labor forces included workers mobilized during the World War I era, with logistics connected to railheads in Boise and supply lines to Idaho City. Design oversight transitioned to the United States Bureau of Reclamation after agency reorganization, applying standards later codified in federal dam safety programs.
Arrowrock stands approximately 350 feet high and spans roughly 660 feet along its crest, impounding Arrowrock Reservoir with a gross capacity near 180,000 acre-feet. The structure’s thin-arch geometry transfers loads into abutments composed of regional granitic bedrock mapped by the United States Geological Survey. Operational responsibilities involve the United States Bureau of Reclamation for structural integrity and flood control releases, while water deliveries coordinate with irrigation districts such as the New York Canal Company and municipal suppliers including the Boise Project Board of Control. Regulatory interactions include the Federal Power Act provisions for hydroelectric facilities and compliance with standards promulgated by the National Dam Safety Program.
Though primarily built for irrigation and flood control, Arrowrock hosts hydroelectric facilities operated by Idaho Power Company under license from the Federal Energy Regulatory Commission. The plant capacity is modest relative to large western projects, typically in the range of about 15 megawatts, and functions as part of a cascade system that includes downstream facilities like those at Lucky Peak Dam and Anderson Ranch Dam. Generation scheduling integrates seasonal irrigation releases, coordination with regional transmission operators, and market signals from entities in the North American Electric Reliability Corporation footprint. Upgrades over time have addressed turbine efficiency, switchyard equipment, and compliance with environmental flow prescriptions from agencies such as the Idaho Department of Fish and Game.
Arrowrock Reservoir impounds flows of the Boise River and seasonal tributaries mapped by the United States Geological Survey hydrographic surveys. The reservoir’s storage supports late-summer irrigation demands in the Treasure Valley and municipal water supply for Boise and adjacent communities. Hydrologic regime is influenced by snowmelt from the Sawtooth Range and Idaho Batholith drainage patterns, with inflow variability tied to multi‑decadal climate oscillations studied by researchers at institutions including Idaho State University and University of Idaho. Water accounting must coordinate with interstate compacts and state adjudication processes administered through the Idaho Department of Water Resources.
Arrowrock’s impoundment altered native salmonid migrations historically associated with the Columbia River basin and required mitigation measures later mirrored in habitat restoration at projects overseen by the National Oceanic and Atmospheric Administration and U.S. Fish and Wildlife Service. Reservoir creation inundated riparian zones and changed sediment transport dynamics similar to effects documented at Shoshone Falls and other western dams. Fisheries management, invasive species monitoring, and riparian vegetation restoration have involved partnerships among the Idaho Department of Fish and Game, local irrigation districts, and conservation organizations such as the Nature Conservancy.
Arrowrock Reservoir and surrounding lands provide recreation managed by agencies including the United States Bureau of Reclamation and the Idaho Department of Parks and Recreation. Popular activities include boating, fishing for species like rainbow trout and kokanee salmon, camping near access points linked by roads to Idaho City, and wildlife viewing of species cataloged by Audubon Society chapters. Public access is balanced with operational constraints, and visitor amenities are subject to seasonal conditions and infrastructure maintained in coordination with county authorities such as Boise County, Idaho.
Category:Dams in Idaho Category:United States Bureau of Reclamation dams