Anderson Ranch Dam and Powerplant

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

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 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 included refacing the downstream face and spillway channel. After 85 years of service, the Ensign valves had reached the end of their useful life. The lower set of ten 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.

A sedimentation survey completed in 1997 estimated the total capacity of Arrowrock Reservoir at 272,200 acre-feet (active 271,700 acre-feet).

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.

Boise River Diversion Dam and Powerplant

The Boise River Diversion Dam, on the Boise River about 7 miles southeast of Boise, Idaho, is a rubble-concrete, weir-type structure with a hydraulic height of 39 feet. The dam diverts water into the New York Canal which serves distribution laterals and feeds Lake Lowell. A small canal known as the Penitentiary Canal, also originating at the diversion dam, distributes water on the north side of the Boise River to a small area of land east of Boise. The powerplant consists of three 500-kilowatt units that began operation in 1912. Due to the deteriorated condition of the equipment and high operating costs resulting from full-attended operation, the powerplant was placed in ready reserve status in 1982. The plant was reconstructed from 2002 to 2004 and returned to service in June 2004. The nameplate rating for the plant was increased from the original 1500 kW to 3300 kW. The existing double turbine configuration was retained, but refurbished with new materials and technology. Modern generators were installed inside the original generator housings.

Lake Lowell

Lake Lowell, originally known as Deer Flat Reservoir, is an offstream reservoir formed by three earthfill dams enclosing a natural depression southwest of Nampa, Idaho. These three dams are the Upper, Middle (Forest Dam), and Lower Embankments. A fourth embankment, called the East (Roadway Dike) is to protect farmsteads on the eastern end of the reservoir when the reservoir is full. The reservoir is filled primarily during the non-irrigation season by diversions at the Boise River Diversion Dam and conveyance through the New York Canal which discharges into the eastern (upper) end of Lake Lowell.

The Upper Embankment on the north side of the reservoir is 74 feet high and has two outlets; near the right abutment the Deer Flat Nampa Canal (100 cubic feet per second) and near the left abutment the Deer Flat Caldwell Canal (70 cubic feet per second).

The Lower Embankment at the west end of the reservoir is 44 feet high. The Deer Flat Low Line Canal (1,200 cubic feet per second) outlets on the left abutment and the Deer Flat North Canal (70 cubic feet per second) outlets on the right abutment.

The Middle Embankment, which has no outlets, is 16 feet high and helps to close the reservoir near the Lower Embankment. It has been referred to as an emergency spillway; however, no slope protection is provided for this purpose.

Because of safety concerns at the Upper and Lower Embankments, the maximum water level was reduced by 5 feet in 1989 while plans were prepared to address remedial actions. Major modifications were made to the Upper and Lower Embankments from 1991 to 1993. Further, in 1996 an upstream seepage blanket was constructed at the right abutment of the Upper Embankment to address seepage in this area. These activities were accomplished under Reclamation's Safety of Dams Program. A Safety of Dams modification was also performed in 2008-2009. The modifications involved the complete replacement of the Caldwell Canal outlet works structure including the intake tower and access bridge, gate and operator, the entire length of the conduit, and the placement of a circumferential filter around the downstream section of new conduit. In addition, the downstream section of the Nampa Canal outlet works conduit was replaced with a new section of conduit incorporating a downstream circumferential filter.

The original total storage capacity of Lake Lowell was 190,000 acre-feet (active 169,000 acre-feet). A sedimentation survey conducted in 1994, estimated the reservoir's total storage capacity at 173,000 acre-feet (active 159,400 acre-feet).

Arrowrock Diversion Canal System

The New York Canal is about 40 miles long and has a diversion capacity of 2,800 cubic feet per second. It consists of the enlarged old New York Canal, a section of new canal, and a part of the channel of Indian Creek. Diversions are made from the New York Canal into various canals such as the Mora Canal and the Deer Flat High Line Canal and into numerous distribution systems. Water delivered by the New York Canal to Lake Lowell is diverted into the four canals outletting the reservoir.


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Boise Project Board of Control  
2465 Overland Rd  
Boise, Idaho 83705-3155  
Tel: 208-344-1141  
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