HYDROGEOMORPHIC AND BOTANICAL ASSOCIATIONS OF BAJADA EPHEMERAL DRAINAGES IN THE WHITE TANK MOUNTAINS, SONORAN DESERT

Abstract

Ephemeral drainage plant communities of the Sonoran Desert compose a highly significant yet relatively unexplained portion of the ecosystem. Eighty-one percent of all southwestern and 94% of Arizona drainages are categorized as ephemeral drainages (Levick et al. 2008). Small but significant portions of the bajada environment are also composed of ephemeral drainages. These drainages carry out important landscape scale functions in water movement, groundwater recharge, nutrient movement and cycling, sediment transportation, geomorphology, plant habitat, seed disbursement, as well as wildlife habitat and corridors. In decades past, Sonoran Desert bajada research relating the physical earth sciences to ecology has focused on explaining upland plant community patterns along this landform (Yang and Lowe 1956, Phillips and MacMahan 1978, Key et al. 1984, McAuliffe 1994, Parker 1995, McAuliffe 1999). This body of research, however, has very little information pertaining to ephemeral drainages dissecting the upland bajada environment. The bajada geomorphic environment is a composition of geomorphic surfaces of varying soil development proceeding away from a mountain (Peterson 1981, McAuliffe 1994). Each of these geomorphic surfaces is characterized by a unique lithology, slope, age and degree of argillic and caliche soil horizon development. Generally, geomorphic surfaces containing highly developed argillic or caliche soil horizons are found near the mountain while surfaces of undeveloped soils are furthest away from the mountain. Depending on the bajada, local geomorphic history, however, may result in different landscape scale patterns of geomorphic surfaces and soil development. This physical environment forms the template from which the ephemeral drainage develops its channel morphology, hydrology and botanical associations. It was expected that the various geomorphic surfaces composing the bajada found at the study sites would determine the specific channel morphology, hydrology and plant community associations of the examined ephemeral drainage. The goal of this study was to explain (1) channel morphology, (2) hydrology or ephemeral flow patterns and (3) plant communities found along the ephemeral drainage. Plant communities of drainages were also compared to upland communities. These factors were then utilized to give an overall explanation for the distribution of hydrogeomorphic and botanical associations found along the bajada ephemeral drainage.