Geologic reconnaissance of the West Creek damsite near Skagway, Alaska

Abstract

The proposed West Creek damsite is about 6 air miles northwest of Skagway on the southernmost large tributary of the Taiya River from the west. The damsite lies at a sharp bend in the valley of West Creek, a typical glacial trough, and the site is well situated topographically for a concrete arch or gravity dam. The left abutment is formed by a low bedrock ridge extending into the center of the valley from the north wall, and the right abutment is essentially part of the south valley wall. The stream has cut a relatively deep and narrow gorge into the bedrock. Bedrock at the damsite consists of a strong massive homogeneous granodiorite capable of supporting a darn of any practical size. Three sets of joints cut the granodiorite, two of which strike northeast with steep southerly to vertical dips. The other set trends northwestward with consistently vertical dips. The northeastward-striking joints intersect the axis of the dam at high angles and also trend through the left abutment. A persistent lineation, possibly a fault related to the active Denali fault system, crosses West Creek about three-fourths of a mile downstream from the damsite. Considering this possible fault and the recorded earthquake epicenters near the damsite, the dam should be designed to withstand earthquakes of maximum severity. The reservoir is underlain by unconsolidated sand and gravel deposits of glaciofluvial origin, lacustrine silts, and probably some till. Alluvial fans extend into the valley from both walls. Because the reservoir area is surrounded by high mountains composed of impermeable and relatively undisturbed granitic rock, leakage from the reservoir is unlikely. A tunnel through the granodiorite southeast of the right abutment would probably be the most practical means of diversion from the reservoir area to a powerhouse in the Taiya River valley. Such a tunnel could be unlined except where it crosses the possible fault and some of the more persistent joints. Construction materials available in the reservoir area include gravel aggregate containing varied sizes probably interbedded with lacustrine silts that are suitable for an impervious core in an earth-fill dam. Large angular blocks of granitic rock suitable for riprap might be obtained without quarrying from the alluvial-fan deposits.