Sacramento River - Geology

Geology

By geologic standards, the Sacramento is a fairly young river; the borders of its watershed began to form only a few million years ago as magma welling up below the Earth's crust pushed up by the Pacific Plate colliding with the North American Plate caused the formation of the Sierra Nevada. Although mountains had existed as early as 100 million years ago in this region (before then the land was probably submerged under the Pacific), they were worn by erosion, and the present-day range only formed about 4 million years ago. The northern part of the Sacramento watershed is more ancient, and was formed by intense volcanic activity over 25 million years ago, resulting in lava flows that covered and created the Modoc Plateau, through which the Pit River flows. Mount Shasta and Lassen Peak are among the numerous Cascade Range volcanoes that still stand in the area.

As the Sierra rose, the ancestors of the Sacramento's east side tributaries and numerous glaciations carved deep canyons in the mountains, depositing massive amounts of silt between the Sierra Nevada and the Pacific, slowly building up the floor of the Sacramento Valley. The Sacramento River did not form until multiple terranes were formed and smashed into the North American Plate from the Pacific Plate about 3 million years ago. The resulting geologic folding pushed up the California Coast Ranges and Klamath Mountains, enclosing the Sacramento Valley and forcing the streams within to flow south instead of west and forming the ancestral Sacramento River. It is believed that the river once had its outlet in Monterey Bay (forming the 300-mile (480 km) Monterey Submarine Canyon when sea levels were lower during the Ice Ages). While the Coast Ranges are young by geologic standards, only a few million years old, the Klamath Mountains reached their present form some 7.5 million years ago.

The Monterey Bay outlet of the Sacramento and San Joaquin rivers was blocked off about 2 million years ago, and runoff from the Sierra began to transform the Central Valley into a gigantic lake, called Lake Clyde. This lake stretched 500 miles (800 km) north to south and was at least 1,000 feet (300 m) deep. About 650,000 years ago the lake catastrophically overflowed, draining into San Francisco Bay and creating the Carquinez Strait, the only major break for hundreds of miles in the Coast Ranges. The narrow outlet trapped some of the sediments of the rivers in the Central Valley, forming the Sacramento-San Joaquin Delta. For thousands of years, this inland sea would periodically reform during times of intense flooding, the most recent being the Great Flood of 1862. Dams and canals that control the river now prevent this phenomenon from occurring in most years.