GEOMORPHIC PROCESSES RESPONSIBLE FOR DECADAL SCALE ARROYO CHANGES, RIO PUERCO, NEW MEXICO

Causes for arroyo aggradation have often been attributed to changes in climate which can affect the magnitude and frequency of streamflows. Here we present evidence for late 20^th century aggradation of the Rio Puerco, NM arroyo (19,036 km²) that is largely a factor of intrinsic processes. The channel and arroyo of the Rio Puerco have continued to evolve since its incision in the late 1800’s. Resurveys of arroyo cross sections and aerial imagery over time indicate that, between the 1970’s and 1990’s, the upstream reaches of the Rio Puerco have continued to construct an inner floodplain through vertical aggradation while downstream reaches continue to show progressive channel width decrease, and channel bed and inner-floodplain aggradation. Trends in rainfall and streamflow alone cannot explain the observed sediment deposition in the Rio Puerco. Streamflow analysis indicates that the majority of runoff (70%) originating in the upper watershed is infiltrated into the streambed and floodplain. With suspended-sediment concentrations reaching 500,000 mg/L, the loss of streamflow through infiltration results in sediment deposition.

Progressive changes in channel morphology, termed arroyo evolution, results in decreasing peak flows, transmission losses, and development of an inner floodplain all of which interact to increase sediment deposition over time. Consequently, increased sediment deposition has resulted in decreased sediment loads and sediment concentrations in downstream reaches. At the oldest surveyed cross section (1936), the channel bed has aggraded 6 meters and, assuming constant aggradation rate, deposition of sediment could fill the entire arroyo cross section in 150 years. Results of this study show that the 20th century aggradation of the Rio Puerco is driven by intrinsic processes that involve a positive-feedback relation among channel morphology, vegetation, climate, streamflow, infiltration, and suspended-sediment loads.