Overview

Trinity Dam

Trinity Dam is located in the southeastern Klamath Mountains physiographic province of northern California. Historically, the area has been characterized by low-level seismic activity. Nevertheless, the dam may be subjected to low to moderate levels of ground shaking from nearby or distant earthquakes, particularly those that might originate in adjacent, more seismically active provinces, such as the Coast Ranges, Cascade Range, Great Valley, or Modoc Plateau.

The most notable early work in the area of Trinity Dam is that of J. S. Diller (1906), who mapped the Redding 30-minute quadrangle, as well as other quadrangles in the Klamath Mountains. A more current review of the general geology of the Klamath Mountains province is Irwin (1966). Large-scale geologic mapping of the area surrounding Trinity Dam and Clair Engle Lake was done by Lydon and Klein (1969), Albers and others (1964), and Irwin (1963). Detailed discussions of the Paleozoic and Mesozoic tectonic evolution of the Klamath Mountains are provided by Irwin (1981) and Burchfiel and Davis (1981). The regional geology of Trinity Dam and Clair Engle Lake is depicted on the Redding and Weed sheets of the Geologic Map of California (Strand, 1962, 1963).

The Quaternary geology of the Trinity Dam area is not well understood, at least in part because of a general lack of studies of the Quaternary geologic history of the area. A notable exception is the study of Quaternary glacial deposits in the Trinity Alps, northwest of Trinity Dam, by Sharp (1960). A brief discussion of Quaternary terrace deposits near Trinity Dam is included in Lydon and Klein (1969).

Site-specific studies of Quaternary faulting in the Klamath Mountains are also lacking. The nearest detailed studies of Quaternary faulting to Trinity Dam are those of Helley and others (1981) and Harlan Miller Tait Associates (1983) on the Battle Creek fault, 55 km south of Trinity Dam (enclosure 1). Regional fault maps show a general absence of Quaternary faults in the general vicinity of Trinity Dam (Jennings, 1975; Howard and others, 1978) with the exception of the Battle Creek fault. Similarly, there appear to be very few historical earthquakes in the area of the dam, especially as compared with adjacent geologic provinces to the east and west (Real and others, 1978; Toppozada and others, 1981). The state of crustal stress in the Klamath Mountains is poorly defined, due to the near absence of recorded seismicity prior to 1983, when the USBR monitoring effort began (Zoback and Zoback, 1980; USBR, 1984a). Zoback and Zoback place the southeastern Klamath Mountains in the Sierra Nevada stress province, which is characterized by a combination of strike-slip and normal faulting. However, most major faults in the Klamath Mountains appear to be reverse faults (Irwin, 1981).The site geology of Trinity Dam is described by Lydon and Klein (1969) and is also summarized in the March 26, 1980 SEED Examination Report. The brief synopsis below is based on these sources and on data provided by Irwin (1966, 1981), Strand (1962, 1963), and Albers and others (1964).

Trinity Dam is located on the eastern Klamath plate, the oldest and easternmost of four eastward-dipping thrust sheets that comprise the Klamath Mountains province. Rocks of the eastern Klamath plate are Devonian to Jurassic metavolcanic and metasedimentary rocks that are underlain by widespread sheets of ultramafic rocks, and have been intruded by Jurassic to Cretaceous granitic stocks and batholiths. Trinity Dam is founded on the uppermost part of the Copley Greenstone, a mildly metamorphosed volcanic formation of probable Devonian age. Most of the reservoir area is underlain by the Mississippian Bragdon Formation, a slatey argillite of marine origin. Other units of relatively minor extent include Paleozoic ultramafic rocks, Mesozoic granitic intrusive rocks, Oligocene nonmarine sedimentary rocks of the Weaverville Formation and Quaternary alluvial and glacial deposits.

The Copley Greenstone is quite competent where it is fresh, but there was originally a thick weathered zone formed on the rocks in the abutments. This zone was thickest near several fault zones encountered during construction. Available information contained in the SEED Examination Report, dated March 26, 1980, indicates that potentially unstable material in the abutments was removed or adequately treated prior to placement of the embankment. The remaining weathered and unconsolidated materials in the abutments consist of residual soil and weathered bedrock that appear to be stable and well-drained, according to the March 26, 1980 SEED Examination Report. Construction geology records also indicate that unconsolidated alluvial deposits in the channel section were completely removed down to fresh rock before fill placement.

The Bragdon Formation is relatively weak and unstable, especially on steep slopes where bedding is roughly parallel to the slope. Several landslides exist where these conditions are present, most notably on the downstream left abutment, above the spillway chute. Further assessment of the potential for earthquake-induced failure of this and other unstable masses around the reservoir is needed.

Several faults were mapped in the Paleozoic rocks in the foundation of Trinity Dam during construction (Lydon and Klein, 1969). All of these faults appear to be long inactive, and displacements have been relatively minor. The March 26, 1980 SEED Examination Report states that one fault in the left abutment was overlain by 12 to 18 m of weathered rock, which is suggestive of a considerable amount of time since the fault was last active, perhaps on the order of millions of years.

General

Dimensions

Hydraulics & Hydrology