Little Wood River Dam was constructed during 1936-1939 and is located 11 miles NW of Carey, Idaho. The rehabilitation work that was done on the dam included raising the dam crest 39 feet, extending the outlet tunnel downstream 150 feet, and relocating the spillway from right to left abutment. The completed structure has a structural height of 129 feet high, with a zoned earthfill embankment containing about 959,000 cubic yards of material. Outlet works reconstruction included enlargement of the entrance channel to the existing intake structure, construction of a gate chamber with a connecting 6-foot-diameter access shaft and shaft house in the existing tunnel approximately 250 feet downstream from the intake structure, extension of the existing tunnel with a 150-foot-long, 6- by 8-foot conduit and a 60-foot-long, 6- by 10-foot chute, and excavation of a stilling basin in the outlet channel. The spillway consists of an inlet channel, a concrete spillway structure with uncontrolled crest, an outlet channel, and a training wall on the right side of the outlet channel beginning approximately 1,000 feet downstream from the spillway crest.
A one-unit, 3,000 kilowatt powerplant was placed in operation in 1985 at the Little Wood River Dam by the Little Wood River Irrigation District pursuant to its license from the Federal Energy Regulatory Commission.
The principal physiographic features of the region are the broad plain of the Snake River and the bordering foothills which merge into the rugged high mountain masses of central Idaho. At lower elevations, vegetative cover is predominately sagebrush and native grasses. At higher elevations trees appear singly or in scattered groups, but not as continuous forest.
Sedimentary rocks, which range in age from very old to moderately young, are found at higher elevations. Younger rocks, volcanic in origin, appear along the border of the mountains and the plain. During geologically recent time, intracanyon basalt flows have covered the valley floor and they are the youngest formations. The present stream has eroded a deep channel through these formations. In the vicinity of the reservoir, the river flows along the contact of the intracanyon basalts and the older volcanic rocks. Downstream from the reservoir the stream has eroded through the basalt flows into the older stream gravel. Little Wood River Reservoir and the stream flows across an alluvial fan onto the Snake River basalts. Project lands occupy this alluvial fan of Little Wood River's which underlies the basalt. The intracanyon basalt flows terminate several miles below the present
The soil throughout the basin is a silty clay loam and appeared to be thin in depth and rocky in nature. The estimated depth of the soil mantel is 6 to 12 inches.
The basin of the Little Wood River is a part of a region of very complex geology. Geological Survey Bulletin 814 "Geology and Ore Deposits of the Wood River Region, Idaho" gives an extensive description of the geology of the area. A synopsis of this information follows:
Sedimentary deposits of sandstone with interbedded conglomerate and limestone (designated the Wood River formation), shale with interbedded blue limestone and quartzite (designated the Milligen formation) and extensive flows of andesite underlie most of the drainage. Metamorphosed limestone, quartzite and schist occupy a small area on the extreme northwest divide and an olivine basalt flow in the vicinity of the Little Wood Reservoir covers a considerable area. The entire region has been subjected to extensive uplift, folding and faulting with considerable igneous intrusion. The headwaters area of the main stem and its minor tributaries have been subjected to some glacial action.