THE ROLE OF VEGETATION IN ARROYO FILLING

The geomorphic histories of Chaco Wash and Rio Puerco (of the east), New Mexico, have been determined by study of floodplain chrono-stratigraphy and historical documents. Both streams incised more than a century ago forming arroyos, which were in excess of 10 m deep. For decades after incision, broad streambeds dominated the bottom of the arroyos and the streams did not aggrade. The arroyos widened. Plants sprouted on the sandy streambeds but did not survive because of drought stress or erosion during floods. During the 1930's plants began to survive. These included native willow and exotic tamarisk, which was introduced to control erosion. The presence of woody plant stems imposed drag on stream flows and caused sediment deposition. Incipient floodplains began to develop along the margins of the streambeds. During subsequent decades, the so-called “inner” floodplains aggraded and the channels narrowed. After about 1970 channel narrowing was complete and the channels aggraded (vertically) with the floodplains as rapidly as one meter per decade. The narrow channels have trapezoidal cross sections, vegetated upper-banks and levees, width-to-depth ratios of around 8, and silty banks inclined at the angle of repose. These channels are stable and typically do not migrate laterally. The channels can widen, at least locally, during large floods and later will narrow back to the original shape. The initiation of arroyo filling likely requires three hydrological conditions. First, the arroyo must be wide enough that unit stream power during frequent flows will not remove plants. Second, an absence of large floods for 5 to 10 years is required to allow seedlings to sprout from the streambed and grow large enough to withstand flood scour. Third, during that same time period, there must be sufficient water, from precipitation or low stream flows, to allow the seedlings to survive. Once the woody plants become established their presence will foster the accumulation of sediment within the arroyo. Arroyo filling does not always persist until the arroyo is full of sediment. If the bank vegetation dies because of herbicide treatment or by natural causes, the channel will double or triple in width during large floods, and may incise. These conclusions based on observations of historical arroyo filling apply to prehistorical arroyo filling.