PLEISTOCENE CHANNELS IN THE CENTRAL VALLEY OF CALIFORNIA: POTENTIAL CONTAMINANT PATHWAYS, AND EXPLOITATION FOR GROUNDWATER AND MINERALS (GOLD, AGGREGATES)

Major rivers in the California Central Valley are underlain by multiple, generally gravel-filled buried channels produced by regional climatic change (mainly Sierra Nevada glaciations) and by local tectonics. Buried lower American and Mokelumne river channels are traced to ~35 m below sea level where they graded to glacio-eustatic base levels in the California Delta. Upstream, the channels are surficially expressed by inset terraces; downstream they are traced mainly in water-well logs, and are relatively dated to at least ~600 ka bp by soil profile development and by association with the late Quaternary isotope stage chronology.

Buried channels in the northern Sacramento Valley and the southern San Joaquin Valley similarly reflect Pleistocene climatic change, but their depths and downstream trends are largely controlled by local regional subsidence, particularly in the Tulare Basin. Historically, many Pleistocene channels were exploited for gold mostly by dredging and hydraulic mining. The buried channels are also important aquifers for domestic and agricultural water. Where expressed as terraces, the channels have long supplied sand and gravel for aggregate, an increasing valuable commodity for urban development. But the buried channels are also potential pathways for contaminants moving into the subsurface where downstream migration is not readily predictable owing to complex hydraulic connection via local fracture and fault systems.