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, water erosion and glaciation carved deep canyons, depositing massive amounts of sediment to form a coastal plain between the mountains and the Pacific Ocean. About 3 million years ago, multiple terranes were formed and smashed into the North American Plate from the Pacific Plate, causing the uplift of the California Coast Ranges, enclosing the Sacramento Valley and forcing the streams within to flow south instead of west, forming the ancestral Sacramento River. (The Klamath Mountains, which enclose the northwest part of the Sacramento Basin, were formed in the same way but are much older, dating back 7.5 million years.) It is possible that the river once had its outlet in Monterey Bay, and may have played a part in the formation of the Monterey Submarine Canyon when sea levels were lower during the Ice Ages. The Monterey Bay outlet of the Sacramento and San Joaquin rivers was blocked off by uplift 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 north to south and was at least 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 inland Sacramento-San Joaquin River Delta. Since then, this inland sea has periodically reformed during times of intense flooding, the most recent being the Great Flood of 1862. Dams, levees and floodways constructed during the 20th century have thus far prevented this phenomenon from re-occurring. History Native Americans The Sacramento River and its valley were one of the major Native American population centers of California. The river's abundant flow and the valley's fertile soil and mild climate provided enough