Spur Mountain, Tyee, and Eagle Lakes fill glacially scoured bedrock basins in the Coast Range of southeastern Alaska. The bedrock consists of granitic intrusive rocks and high rank metamorphic rocks associated with or resulting from emplacement of the Coast Range batholith.

Spur Mountain damsite is underlain by granodiorite and diorite. The foundation properties of the bedrock are excellent, but the narrowness of the ridge that forms the right abutment and two prominent joint sets that intersect the abutments at high angles may be serious disadvantages. Two possible tunnel routes extend from the upper and lower ends of the lake to the Hulakon River and Unuk River valleys, respectively. They are approximately the same length and both are underlain by intrusive rocks with similar physical properties. Both routes are geologically satisfactory and the choice of one, will probably depend on other factors. The reservoir is underlain and surrounded by impermeable granodiorite, diorite, or related rocks.

The abutments of the Tyee Lake damsite are in massive quartz diorite. The channel section is filled to an undetermined depth with coarse talus which is probably too permeable to grout. If the talus deposit is too deep to be removed economically, it might be possible to develop the site by drawing the lake down. The tunnel and penstock route is underlain by granodiorite, composite gneiss, hornblendite, and quartz diorite which are impermeable except possibly along two zones of close-spaced or open joints. The powerhouse site on Bradfield Canal is underlain by quartz diorite similar to the bedrock at damsite.

The Eagle Lake powersite includes two possible damsites. The Eagle Lake damsize at the outlet of Eagle Lake is underlain by composite gneiss consisting of foliated biotite gneiss interlayered with banded quartz diorite, which is largely concealed with thin deposits of soil and colluvium. The foliation strikes normal to the alignment of the dam, and minor leakage along foliation planes might be expected. The possibility of a deep buried channel or solution cavities in marble underlying the stream bed should be considered. The other damsite is located at the outlet of Little Eagle Lake about 2 1/2 miles below the Eagle Lake damsite. The drainage area and storage capacity above the Little Eagle Lake site would be about 70 percent greater than for the Eagle Lake damsite, but the dam would have to be three to four times larger than the one at Eagle Lake to reach the same water level. This dam may be economically feasible due to large volumes of impervious fill material available for construction of an earthfill dam near Little Eagle Lake. Four saddles, which are probably abandoned stream channels, are in a low divide at the head of Eagle Lake. The depth and permeability of fill in the saddles are unknown factors which should be investigated. The tunnel route extends from the headward part of Eagle River to the head of Bell Arm and is underlain by poorly foliated gneissic quart diorite.