Farouk El-Baz Award for Desert Research: 19TH CENTURY ARROYO CUTTING AND ITS AFTERMATH IN THE TUCSON BASIN

Between 1865 and 1915, arroyos developed in many alluvial valleys of the southwestern United States. The alluvial stratigraphic record is neither systematic nor dated precisely enough to pinpoint comparable episodes of regional downcutting at other times in the Holocene. That continuous gullies formed simultaneously across diverse hydrological, ecological and cultural settings evoke a singular event with a common cause, some unusual climatic episode or historic land use that was equally widespread and synchronized. Empirical evidence for historic arroyo cutting is spotty. The best documented case is the Santa Cruz River near Tucson, AZ, which flows from the Mexican border to a poorly-defined delta near the Gila River. Discontinuous arroyos existed 10 km upstream of Tucson as early as 1849, but elsewhere the floodplain was unentrenched. In the Tucson Basin, perched ground water and springs in two reaches of the Santa Cruz River supported marshes, a dense and tall mesquite forest, and many birds no longer sighted in the area. Perennial flow was impounded to irrigate fertile lands and turn the water wheels at a succession of flour mills. In summer 1890, four major floods spaced three to ten days apart, enlarged the heading of a new intercept ditch and extended the resulting headcut 3 km upstream. Sustained flooding in winter and spring of 1904-1905 and 1914-1915 migrated this headcut another 20 km upstream, putting to ruin wetlands, farmlands and water works. Arroyo cutting paved the way for dramatic ground-water withdrawals and urbanization of the floodplain since the 1940s. Today, despite illusions of restoration, the Santa Cruz River is little more than a stabilized storm drain. During some future flood, the clear and more erosive flows in the stabilized channel are poised to initiate arroyo formation 20 km downstream of Tucson. This future gullying will translate Tucson Basin sediment eroded by the historic arroyo to the featureless desert plains south of Phoenix.