ANTHROPOGENIC INFLUENCED GEOMORPHIC EVOLUTION OF STREAM CHANNELS AND COASTAL ENVIRONMENTS ADJACENT TO IMPORTANT ARCHAEOLOGICAL SITES IN THE LOWER RIO VERDE VALLEY, OAXACA, MEXICO

The Lower Río Verde Valley (LRVV) is the depositional region for one of the largest rivers on Mexico’s west coast. Contained within the LRVV are extensive and very important archaeological sites (e.g. Río Viejo) located in a variety of geomorphic positions. Previous geoarchaeological research showed that the river has shifted course, changed its form, and altered the floodplain environment over the last few millennia. This was due to an influx of sediment from the highlands during periods of ancient urbanization, environmental disturbance for agriculture and subsequent hill slope erosion. The Río Verde has shifted, from the west, to its present location but does not appear to have ever migrated to the east. A recent rise in the watertable caused by an irrigation project revealed, on Google Earth imagery, many buried meanders east of the river. The question then arose as to the origin of these meanders. Do they represent former positions of the Río Verde or are they formed by smaller, local streams?

Mineralogical sourcing of the origin of sediments from these abandoned meanders provides insights into the geomorphic evolution of the area and possible effects on regional archaeology. Sand samples were collected from both the abandoned meanders and from modern depositional environments. Observation and measurement of sand mineralogy provides answers to the origin of the meanders. Sediment mineralogy suggests that the sediments within the meanders do not represent local stream loads. This finding raises additional questions. Do the sediments reflect direct deposition by the Río Verde or were they deposited in a coastal environment from alongshore processes?

Previous research by the authors showed that as the Río Verde changed position and form, the coastal zone also experienced significant morphological change. Sediments and biological specimens indicate that the coast changed from an open bay to an enclosed lagoon with the increase in sediments brought to the coast. These changes were in place by ~2300 cal yr B.P. The formation of an enclosed lagoon better explains the meander pattern of the buried channels in the eastern coastal zone as their tight meanders are more representative of tidal channels. From the archaeological perspective these tidal channels may have provided easy access to marine and estuarine food sources.