PRE-HISPANIC ANTHROPOGENIC LANDSCAPE CHANGE IN THE RÍO VERDE DRAINAGE, OAXACA, MEXICO: THE INTERACTION OF UPPER AND LOWER REGIONS OF A DRAINAGE BASIN

Extensive investigation of arroyo exposures in the Oaxaca Highlands revealed the ubiquitous presence of buried paleosols. These paleosols represent stable landsurfaces separated by sediments from erosion of adjacent hillslopes. Paleosols occur in vertically multiple sequences and are laterally extensive. Radiocarbon dating of 32 samples indicates that most of the hillslope erosion and valley alluviation occurred prior to Spanish Conquest. Radiocarbon dates of paleosols show several periods of sedimentation. Mid-Holocene dates are probably associated with climate change; more recent periods of erosion are correlated with major changes in prehispanic landuse. Highland erosion led to profound geomorphic and archaeological changes along the lower Río Verde near the Pacific Ocean. A network of abandoned stream channels, with little surface expression, was revealed during extensive soil and sediment auguring. Radiocarbon dating of organic-rich channel sediments associated with archaeological sites provided the means to develop a model of changes in floodplain geomorphology. Channels changed both position and form as the sediment load increased due to changing land use in the highland portion of the drainage basin. Increased sediment load also formed bay barrier islands and enclosed previously open bays, creating new food-rich environments. These environmental changes then influenced demography and settlement patterns along the Lower Río Verde and adjacent coastal areas.