Overview

Whiskeytown Dam is located in the Clear Creek drainage basin within the extreme southeastern foothills of the Klamath Mountains physiographic province. The dam is about 3.5 miles westward from the Sacramento Valley. The drainage basin is bordered on the east by the low foothill Mule Mountain range at an elevation of 2,300 feet, beyond which lies the northern part of the Sacramento Valley. South and west of the area, the high peak and range of Shasta Valley rises to elevation 6,209 and spreads north to become contiguous with the Trinity Mountains which form the divide between Trinity River and Clear Creek Basins.

A simplified geologic column representative of the Whiskeytown Dam project area is given below.

All the rock formations have been metamorphosed to varying degrees of intensity. Dynamic and thermal forces have physically and chemically altered the original rock.

The Balaklala metarhyolite and the Mule Mountain albite granite stock form the damsitz foundation and the eastern side of the reservoir. This granite is complexly intrusive into the rhyolite and is everywhere faulted along this contact.

Scattered patches of terrace gravel and small deposits of recent alluvium occur in the reservoir and tributary streams. Extensive deposits of terrace gravel and of recent alluvium occur in the French Gulch area several miles upstream from the reservoir. At the dam, Clear Creek flows through a narrow, sharply incised canyon which traverses a strongly dissected, hilly upland with local relief of approximately 300 feet. The more subdued topography upstream from the dam suggests that the ancestral stream flowed in a broader valley through the reservoir area.Two geologic formations underlie the dam. To the east, providing the foundation for the main dam, is a sequence of rhyolitic lava flows which have been metamorphosed and tilted steeply eastward. These flows are part of the Balaklala Metarhyolite Formation. The foundation for the two dikes across the saddles at the right (west) end of the dam is composed chiefly of intrusive granite which is part of the Mule Mountain stock. The contact between the metarhyolite and the granite trends roughly parallel to Clear Creek and crosses the dam axis at station 22+00. Both the metarhyolite and the granite are cut by dikes of metadiorite at several places.

The foundation of the main dam consists wholly of metarhyolite porphyry and associated dikes. The unweathered rock is hard but brittle. Throughout the dam area, this rock has a distinctive platyness or foliation consisting of subparallel irregular parting surfaces spaced 0.25 to I inch apart and apparently parallel to the original flow banding. The weathered rock has a bleached to soft, chalky appearance; it is iron stained along the fractures and has a definite looseness along the foliation.

The granite forming the foundation of the two dikes is medium to coarse grained and variably altered in character. Although the rock is hard when fresh, it is generally brittle due both to its siliceous composition and an incipient foliation or internal shearing. This rock commonly has a splintery fracture. The weathered granite is appreciably soft and has a widespread development of clay minerals and iron oxide. A thick 15- to 52-foot mantle of buff, siltygravelly sand (SM) (decomposed granite) covered the sound granite in the vicinity of the dikes.

At several places in the dam foundation, the granite and the metarhyolite have been intruded by dikes of metadiorite, the largest of which is about 20 feet wide and at least 1,500 feet long. At several of these dikes the rock is deeply weathered to a thick mantle of reddish-brown clayey residual soil. The fresh rock is hard.

The most distinctive and readily apparent geologic structure in the dam foundation is the foliation or platyness exhibited by the metarhyolite. This platyness generally strikes N. 10 degrees to 20 degrees W. and dips 65 degrees to 75 degrees eastward. It occurs uniformly throughout the metarhyolite as wavy partings spaced 0.25 to I.0 inch apart, causing the rock to break into slabby or lenticular fragments. The rock tends to split readily along the foliation and thus allow downward percolating water to weather, soften, and iron stain the rock.

A steeply dipping, incipient foliation or shearing trend occurs in the granite. Shearing of variable intensity is present throughout the area. The splintery break and the numerous wavy slip surfaces have weakened the rock sufficiently in places to cause overbreak during excavation.

Faulting is widespread and complex in the metarhyolite and granite. A wide zone of faulting accompanies the metarhyolite-granite contact, with segments of rhyolite displaying considerable drag. In the metarhyolite under the main dam, more than 175 faults were mapped. Zones of clayey gouge and fault breccia ranged in width from a fraction of an inch to more than 1.5 feet. Offsets along the faults generally are small, but a displacement of many feet was found at some of the intrusive dikes.

General

Dimensions

Hydraulics & Hydrology