Late quaternary geomorphology of arroyos in the Mixteca Alta, Oaxaca, Mexico
Holdridge, Genevieve A
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Land degradation in drylands is one of the most critical, ongoing global environmental issues, and gullying and arroyos constitute some of the more serious elements of desertification. The overarching goal was to examine how late Quaternary climate, land use, and environmental change influenced the dryland stream dynamics of the Río Culebra watershed, located in the semi-arid Mixteca Alta in southwestern Mexico. This goal was examined through three separate, but interrelated studies. The first study (Chapter 2) examined the sediment yield and land use of the dryland fluvial system of the Río Culebra watershed. Conservative bedload yield estimates on two tributary arroyos demonstrated that despite headwater conservation, the yield is still relatively high, and suggest the need for larger scale conservation efforts. Present channel bed stratigraphy containing predominantly massive, coarser deposits contrasts the prevalence of finer bedded sediments in the Quaternary stratigraphy, which indicate a different flood regime in the past. The second study (Chapter 3) concerned the uncertainty of the relative contributions of anthropogenic versus climatic drivers on land degradation. The alluvium-paleosol chronology was examined against regional climatic conditions and indicated that greater incision occurred during wetter periods, while alluvial deposition and paleosol formation occurred during transitional wet and dry periods, supporting the “wet-dry-wet” model of arroyo cycles. Comparing the paleoclimatic-paleohydrological relationship of terminal Pleistocene and early Holocene strata versus the late Holocene strata indicated that widepsread agricultural activities greatly impacted sediment rates and influenced the timing and nature of landscape response to climate change. The third study (Chapter 4) addressed the need for better understanding the local paleoenvironment. The local paleoenvironment, as reflected in soil organic matter δ13C values and paleosol data is comparable to the paleoclimatic and paleoenvironmental reconstruction from central and southwestern Mexico. The paleoenvironment of Culebra watershed was impacted by lama-bordo (i.e., check dam) and agricultural terrace constructions and the increased importance of maize cultivation and succulent plant managment during the late Holocene. The results of these studies illustrate the need to understand long-term climatic, hydrologic and land use variability in order to find solutions to dryland desertification in the present.