Holocene intertidal deposits of Sacramento-San Joaquin Delta, California: Abstract

Rivers draining nearly one third of California reach sea level in the Sacramento-San Joaquin delta. Though tapered toward a constricted outlet and separated from the sea by a chain of estuarine bays and straits, the delta resembles deltas built into open-marine environments in that (1) its principal landforms--tidal wetlands and natural levees--are typical features of such delta, and (2) the fundamental process of deltaic deposition--discharge into standing water--affected the delta during historic floods. Tidal wetlands (marshes and swamps), generally served by fresh water, covered most of the delta (about 1,400 sq km) before agricultural reclamation. Rising sea levels began to create such wetlands at the site of the delta around 7,000 years ago. Continued sea level rise caused the wetlands to spread across flood plains of the Sacramento and San Joaquin Rivers, alluvial fans of lessor tributaries, and fields of eolian dunes. Once established, these wetlands built upward, apparently by accumulation of roots and rhizomes in growth position, to keep pace with sea level. Inorganic sediment largely bypassed the delta and settled instead in the brackish-and salt-water bays to the west. Accordingly, the typical vertical sequence of facies in the delta consists of intertidal peat overlying alluvium or dune sand. Principal exceptions are the sands and muds of channels, the silts of natural levees along distributaries of the Sacramento River, and the clays of tidal wetlands near these distributaries.

Major high stands of the sea during Pleistocene time should have produced intertidal peat at the site of the delta. The scarcity or absence of Pleistocene peat implies removal by streams, wind, exudation, and/or anaerobic decomposition. The latter three agents have removed Holocene peat from farmed, reclaimed wetlands of the delta as rapidly as 7 cm/year.