Trends in River Flow Affecting the Northeastern Reach of the San Francisco Bay Estuary over the Past 7000 Years

Abstract

A variety of stratigraphic analyses (particle grain size, iron concentration, loss on ignition, and macrofossils) from sediments obtained from two marsh sites are used to reconstruct a middle to late Holocene record of stream flow into San Francisco Bay. Browns Island, a freshwater/brackish site, is located at the confluence of the Sacramento and San Joaquin rivers and is dominated by stands of Scirpus americanus. Peyton Hill is a brackish site located near Carquinez Straits and is dominated by stands of Scirpus robustus. Twenty-five AMS 14C dates provide chronostratigraphic control. During the Holocene, discharge from the Sacramento and San Joaquin rivers was broadly comparable to modern flows; however, an extended period of higher flow began 3800 cal yr B.P. and continued for almost two millennia. At this time Browns Island supported Phragmites communis, a freshwater species, and Peyton Hill supported S. americanus. At least two floods, recognized by discrete increases in sand and silt, occurred at 3600 and 530 cal yr B.P.