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 underlie 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 sma,ll 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.

 | Operator:  Little Wood River Irrigation District,
                       PO Box 355, Carey, ID  83320-0355, (208) 823-4014