ARROYO FORMATION AND FILLING IN THE SOUTHWESTERN UNITED STATES: A CONCEPTUAL MODEL

The causes of arroyo downcutting in Utah and Arizona, which occurred periodically during the late Holocene, is the most-studied geomorphic question of this region. Downcutting has been attributed to a variety of climatic, geomorphic, and land-use changes, but historic downcutting, while loosely associated with the introduction of livestock in the 19th century, occurred in the course of large floods during exceptionally wet periods or extreme regional storms between about 1862 and 1909. Although downcutting is a relatively well-known phenomenon, arroyo filling has received little study, despite its relevance to alluvial stratigraphy, hillslope sediment production, and fluvial processes. A five-stage conceptual model may explain processes driving past and present cycles of arroyo downcutting and filling, both the result of climatic variability. The initial stage of rapid downcutting and coalescence of discontinuously entrenched mainstem channel segments occurred during either a single event or a series of floods associated with regional storms or extreme events, resulting in deep, narrow channels. The early 20th century pluvial, which spanned the period of about 1905 through about 1940, generally widened channels, damaging infrastructure and agricultural lands. The mid-century drought, from about 1947 through 1976, was a period of floodplain aggradation and establishment of riparian vegetation in the presence of relatively high ground-water levels. Regional flooding caused reversion to channel downcutting and widening from the late 1970s through the mid-1990s; these effects were greater in southern Arizona than either northern Arizona or southern Utah. Prolonged drought from about 1996 to the present has encouraged channel aggradation, particularly on floodplains colonized by riparian vegetation, where ground water remained high. Arroyo filling, driven by fewer regional floods, relatively high ground-water levels, and riparian vegetation that traps sediment transported during summer floods, is presently occurring. Although the latest arroyo filling may be impeded by excessive ground-water withdrawals, which decreases riparian vegetation and channel roughness, arroyo filling may eventually compromise fixed cross section flood control structures by decreasing channel conveyance.