USACE CWMS - Boise River Watershed

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Created: Jun 27, 2018 at 8:15 p.m.
Last updated: Jun 27, 2018 at 8:37 p.m.
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The Corps Water Management System (CWMS) includes four interrelated models to assist with water management for the basin:

- GeoHMS (Geospatial Hydrologic Modeling Extension)
- ResSIM (Reservoir System Simulation)
- RAS (River Analysis System)
- FIA (Flood Impact Analysis)

The watershed is composed largely of precipitous mountains, and as a whole, is characterized by a highly dissected topography with deep V-shaped valleys, steep slopes, and narrow and sharp top ridges. The topography becomes increasingly rough toward the eastern boundary. The basin above Lucky Peak dam ranges in elevation from 3,000 to 10,500 feet. The mean elevation of the basin is 5,800 feet. The mean annual average precipitation for the basin ranges from 8 inches in the lower basin to 55 inches at the highest elevations; the value at Lucky peak dam is 15 inches.

The principal tributaries of the Boise River and their respective drainage areas are the South Fork, 1,314 square miles; Middle Fork and North Fork, 830 square miles; and Mores Creek, 426 square miles. These four tributaries comprise 97 percent of the drainage area above the Lucky Peak dam.

Major floods are typically categorized as “Winter” or “Spring” floods. Both floods usually have rainfall and snowmelt components. Winter floods are typically less duration and volume than spring floods. During winter the reservoirs are drafted down to allow space for flood events and there are no irrigation demands. In late spring the reservoirs are being filled to meet “refill requirements” for irrigation by following rule curves. Rule curves define required system flood control spaces as functions of date and operations runoff volume forecasts. A large rainfall system in conjunction with final filling of the reservoirs causes the largest risk to flood flows downstream of the reservoir system.

There are three reservoirs in the drainage area above Lucky Peak Dam; namely, Little Camas, Anderson Ranch, and Arrowrock. Little Camas Reservoir is located 22 miles northeast of Mountain Home, Idaho, on Camas Creek, a tributary of South Fork Boise River. The reservoir has a usable capacity of about 22,000 acre-feet, which is used exclusively for irrigation. This reservoir has no value for flood control and is not considered for USACE operations. Therefore, Little Camas Dam will not be modeled in HMS or ResSim. Anderson Ranch Reservoir is about 30 miles northeast of Mountain Home, Idaho, on the South Fork Boise River. The reservoir has a usable capacity of 418,000 acre-feet, which is joint use for flood control and irrigation. Arrowrock Reservoir, built by the U.S. Bureau of Reclamation for irrigation purposes, is about 22 miles east of Boise, Idaho, on the Boise River below the confluence of the South Fork and the main Boise River. The reservoir has a usable capacity of 286,000 acre-feet, which is joint use for flood control and irrigation. Lucky Peak Reservoir is about 10 miles east of Boise, Idaho, on the Boise River below the confluence of Mores Creek and the Boise River. The reservoir has a useable capacity of 264,000 acre-feet, which is joint use for flood control and irrigation.

The Lucky Peak Dam, built by the U.S. Army Corps of Engineers, is a multi-purpose dam located 9 miles southeast of Boise, Idaho, on the Boise River. The spillway is located south of the da, it is 600 feet long, has training wall abutments at each end, is made of reinforced concrete, and has an ogee shape with an apron on the downstream side. The crest of the spillway is at elevation 3063.3. Discharge over the spillway is uncontrolled and travels over the unlined open hillside to the river below the project structures. At a maximum design pool elevation of 3075.3, approximately 93,300 cfs would be passed over the spillway. The spillway is for emergency use only and should never be used for normal operations, since use of the spillway is expected to severely erode the unlined hillside below the spillway. To prevent overtopping the spillway, either accidentally or by wave action, the reservoir pool must be maintained at elevations below 3063.3.

Subject Keywords



Coordinate System/Geographic Projection:
WGS 84 EPSG:4326
Coordinate Units:
Decimal degrees
Place/Area Name:
Boise River Watershed
North Latitude
East Longitude
South Latitude
West Longitude

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Myers, J. (2018). USACE CWMS - Boise River Watershed, HydroShare,

This resource is shared under the Creative Commons Attribution CC BY.


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