Tropes

ScienceDaily_2014 01620.txt

#Tree rings used to determine history of geological features, arroyosa new GSA Bulletin study uses tree rings to document arroyo evolution along the lower Rio Puerco and Chaco Wash in northern New mexico USA. By determining burial dates in tree rings from salt cedar and willow investigators were able to precisely date arroyo sedimentary beds 30 cm thick or greater. They then combined this data with aerial imagery Lidar longitudinal profiles and repeat surveys to reconstruct the history of these arroyos. Arroyos are oversized deep channels that have vertical or steeply cut walls made up of silt clay or sand. Because of this makeup arroyo systems are inherently unstable shifting at the century to millennial scale between broad floodplains and incised high-walled channels in which floods have a high stream power that causes more rapid erosion. Study authors Jonathan Friedman of the U s. Geological Survey and colleagues note that although the channels of the Rio Puerco and Chaco Wash are narrow like an erosional gully and uniform like a human-made ditch they are in fact natural depositional features. Along both rivers erosion occurred until the 1930s in association with extremely high flows. Subsequent infilling was caused by vegetation growth channel narrowing increased sinuosity and vertical accumulation of sediments. Friedman and colleagues write that strongly depositional sediment transport interacted with floodplain shrubs to produce a characteristic narrow trapezoidal channel. However the 55-km study area along the Rio Puerco demonstrated upstream progression of arroyo widening and filling but not of arroyo incision channel narrowing or floodplain vegetation development. Friedman and colleagues conclude that the occurrence of upstream progression within large basins like the Rio Puerco makes precise synchrony across basins impossible. Arroyo wall retreat is limited now mostly to locations where meanders impinge on the arroyo wall forming hairpin bends for which entry to and exit from the wall are stationary. The team notes that with the Rio Puerco's current rate of sediment deposition and long-term bedrock erosion it would take the arroyo 310 years to completely fill in. Story Source: The above story is provided based on materials by Geological Society of America. Note: Materials may be edited for content and length. Journal Reference e

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