Draft EIS_072312.pdf - Middle Fork American River Project ...

20120723-4002 FERC PDF (Unofficial) 07/23/2012
accreted terranes, separated by faults, occurs within the Foothills Suture Zone in the
lower watershed. Magmatic intrusion of metamorphic rocks and accreted terranes
created gold-bearing veins that were mined in the region beginning in the mid-1800s.
Volcanism buried much of the topography on the western slope of the northern Sierra
Nevada with mudflows (lahars) of the Mehrten Formation. This volcanic activity was
followed by a long period of erosion that eroded the overlying volcanics, forced
development of a new drainage network, exposed the underlying granitics, and
deposited placer gold in stream channels. Several million years ago, the Sierra Nevada
Range was uplifted on its eastern margin and tilted westward, resulting in steep and
deeply incised canyons on the western slope. The Sierra Nevada Range was
subsequently glaciated several times.
Seismic Hazards
The Alquist-Priolo Act of 1972 12 requires identification of earthquake fault zones
encompassing all “potentially and recently active” traces that show evidence of surface
displacement during Quaternary time (last 1.6 million years). No active fault hazard
zones (i.e., surface displacement within the last 11,000 years) have been identified in
the project vicinity. The lower watershed within the project vicinity is situated in the
Foothills Suture Zone that includes the Melones Fault Zone and the Bear Mountain
Fault Zone. An area of faulting on the North Fork American River in the Melones Fault
Zone shows Quaternary displacement, and several areas of the Bear Mountain Fault
Zone exhibit late Quaternary displacement.
Mineral Resources
Gold, silver, chromium, tungsten, and aggregate are the principal mineral
resources in the project vicinity. Most of the mineral resources mined within the
watershed are associated with the Melones Fault Zone and the accreted terranes of the
Foothills Suture Zone. Historical mining activity is concentrated around the area to the
west and north of Ralston afterbay. Gold was produced from veins in the Western
Metamorphic Belt and from placer deposits. Little mining has been conducted along the
Rubicon River, and studies in the Rubicon Roadless Area indicate a low potential for
mineral resources.
Soils
Soils surrounding the project facilities, reservoirs, and bypassed reaches generally
consist of well-drained, sandy to silty loams forming steep slopes. Much of the area is rock
12 The Alquist-Priolo Earthquake Fault Zoning Act (California Geological
Survey – Public Resources Code Division 2, Chapter 7.5) was signed into California
law on December 22, 1972, to mitigate the hazard of surface faulting and prevent the
construction of buildings used for human occupancy on the surface trace of active
faults.
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