Ancient gold-bearing channels have long been exploited in the Sierra Nevada foothills and in the eastern Great Valley of California. Many channels have been hydraulically mined, whereas others have been dredged. Yet there remains an extensive buried channel system, particularly associated with ancient courses of the lower American and Mokelumne Rivers. The channels are identified in water-well logs, in bridge borings, and locally in quarry exposures. Near the foothills, many channels are expressed geomorphologically by fluvial-filled terraces, many of which have been dredged, and some are even now exploited for gold and aggregate.
At least five now-buried channels were cut and filled by the lower American River. These channels emerge from the foothills near Folsom and are traced in the subsurface to depths of 115 ft (35 m) below present sea level. Some channels are over 25 mi (40 km) long, and 20 to 33 ft (6 to 10 m) thick. The channels are named Older Fair Oaks (oldest), Younger Fair Oaks, Older Riverbank, Younger Riverbank, and Modesto, respectively, after the formations in which they occur.
Five buried channels underlying the lower Mokelumne River are best defined between Clements on the east and Lodi on the west. The oldest channel (Laguna age) is traced to almost 330 ft (100 m) below King Island in the western Sacramento-San Joaquin delta; the youngest channel (Modesto age) is about -33 ft (-10 m) at Lodi.
Because of their depth, these ancient channels are not presently exploitable. They are, however, a gold-bearing repository, and with newer technology may be potentially tappable in the not too distant future.