DATE OF CHANNEL TRENCHING (ARROYO CUTTING) IN THE ARID SOUTHWEST.
Of the writers known to have considered the problem, Hough alone has attempted to give a date for the whole area. He says that the process began with the active occupancy of the region by white men thirty years before the date of his address, or in 1875. However, the foregoing review of the existing evidence as to the date when trenching began in southwestern United States indicates that these changes were initiated at slightly different times in each stream and occupied a considerable period in their accomplishment. The change from aggradation and the building of flood plains to dissection and the formation of arroyos in many streams of southern Arizona can be confidently placed in the decade 1880 to 1890, although many tributary streams were not affected until the 90's and some are still undissected. The date in southern Utah, northern Arizona and southern Colorado is apparently earlier, and cutting probably began at some time after 1860. The evidence as to the Rio Puerco in north-central New Mexico is conflicting and needs review. The statements of the early explorers indicate that the arroyos were already well formed at the time of the American conquest in 1846 and 47.
Bryan
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