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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 ofScirpus americanus.Peyton Hill is a brackish site located near Carquinez Straits and is dominated by stands ofScirpus robustus.Twenty-five AMS14C 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 supportedPhragmites communis,a freshwater species, and Peyton Hill supportedS. americanus.At least two floods, recognized by discrete increases in sand and silt, occurred at 3600 and 530 cal yr B.P.

Humans are constrained by the hyperarid environment of the Peruvian Desert, which they have occupied throughout the Holocene Epoch. Habitats amenable to human occupation are limited to the riparian oases and the high-productivity coastal zone. Dramatic cultural and technological evolution was coincident with landscape evolution that responded to climatic and sea level variability. Occupation sites of hunter-gatherers older than 8,000 years are rarely found, as much of the landscape from this period is drowned and unexplored. Seven-thousand years ago, sea level stabilized and coastal middens of this age attest to exploitation of the now stationary marine resource. In response, rivers backfilled and the population became progressively more dependent upon terrestrial resources. The shift to an agricultural economy resulted in a migration of settlements inland along the river valleys. Extreme events (sea level stabilization, droughts, El Niñ o floods) have likely facilitated periods of rapid technological and cultural innovation. ᭧

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Overbank flood deposits of northern coastal Peru provide the potential for the development of a late Quaternary chronology of El Niño events. Alluvial deposits from the 1982–1983 El Niño event are the basis for establishing a type El Niño deposit. Sedimentary structures suggesting depositional processes range from sheet flows to debris flows, with sheet flood deposits being the most common. The 1982–1983 deposits are characterized by a 50‐ to 100‐cm‐ thick basal gravel, overlain by a 10‐ to 100‐cm‐thick sand bed, grading into a 1‐ to 10‐cm‐thick silty sand bed and capped by a very thin layer of silt or clay. The surface of the deposit commonly displays the original shear flow lines crosscut by postdepositional mud cracks and footprints (human and animal). Stacked sequences of flood deposits are present in Pleistocene and Holocene alluvial fill, suggesting that El Niño type events likely occurred throughout the late Quaternary. A relative chronology of the deposits is developed based on terrace and soil stratigraphy and on the degree of preservation of surficial features. A minimum of 15 El Niño events occurred during the Holocene; a minimum of 21 events occurred during the late Pleistocene. Timing of the Holocene events is bracketed by isochrons derived from the archaeologic stratigraphy. Corrected radiocarbon ages from included detrital wood provide the following absolute dates for El Niño events: 1720 ± 60 A.D., 1460 ± 20 A.D., 1380 ± 140 A.D. (error overlaps with the A.D. 1460 event; these may represent a single event), and 1230 ± 60 B.C.