San Joaquin River - Geology

Geology

In a geologic context, the San Joaquin can be divided into two major segments. The upper 97 mi (156 km) above Friant Dam in the Sierra is characterized as a steep-gradient, rocky mountain stream. Over millions of years, the upper San Joaquin, as well as the upper reaches of most of its tributaries, have eroded enormous amounts of rock and sediment from the mountains. Most of the Sierras are underlain by granitic igneous and metamorphic rock dating back to the Mesozoic Era (250-65.5 MYA); in addition many of the San Joaquin's tributaries flow across a foothills region of metamorphosed volcanic rock more famously known as the Mother Lode Gold Belt.

The 268 mi (431 km) lower part of the river, in sharp contrast, is a meandering stream flowing over Cenozoic alluvial deposits (65.5 MYA-present), which together comprise the flat floor of the Central Valley. The tremendous volume of sediments that underlie the lower San Joaquin River ranges from 6 to 9.5 mi (9.7 to 15.3 km) deep, with distance to bedrock generally increasing in a northerly direction. Prior to the uplift of the California Coast Ranges, more than 20,000 ft (6,100 m) of sediments were deposited at the foot of the Sierras by tidal activity, and the ancestral San Joaquin and its tributaries flowed west over this alluvial plain to the sea, dumping their own sediments onto the marine deposits. Compressional forces along the boundary of the North American and Pacific Plates between 2-4 MYA resulted in the uplift of the Coast Ranges, creating an enclosed basin today known as the Central Valley and resulted in the San Joaquin's present path to the sea.

Because of its highly permeable nature, the San Joaquin River's valley is underlain by one of the largest aquifers in the Western United States. The aquifer underlying the San Joaquin River and Tulare Basin is estimated to hold nearly 686,000,000 acre·ft (846,000,000 dam³) of water, of which about half can be pumped economically or is clean enough for human use. The aquifer receives in excess of 1,600,000 acre·ft (2,000,000 dam³) of inflow per year, mostly from precipitation and irrigation water seepage. Concentration of chloride and other minerals generally increases from east to west across the basin.