A GEOMORPHIC CLASSIFICATION FOR ARID EPHEMERAL STREAMS

Current stream classifications do not adequately describe arid ephemeral streams because dry-land environments are characterized by high spatial and temporal variability of complex hydrologic interactions. To investigate the influence of channel form on riparian vegetation in the arid southwestern United States, we develop a geomorphic classification for ephemeral streams based on the degree of confinement and the composition of confining material. Our conceptual model includes five stream types: 1) bedrock channels entirely confined by exposed bedrock and void of persistent alluvium; 2) bedrock with alluvium channels at least partially confined by bedrock but containing enough alluvium to create bedforms that persist through time; 3) incised alluvium channels bound only by unconsolidated alluvial material into which they are incised; 4) braided washes that exhibit multi-thread, braided characteristics regardless of the degree and composition of confining material; and 5) piedmont headwater 0-2^nd order streams confined only by unconsolidated alluvium and which initiate as secondary channels on piedmont surfaces. Eighty-six study reaches representing the five stream types were surveyed on the U.S. Army Yuma Proving Ground in southwestern Arizona. Non-parametric multivariate analysis of variance (PERMANOVA) indicates significant differences (P<0.001) between the five stream types using stream gradient, width-to-depth ratio, entrenchment ratio, shear stress, and unit stream power as variable indicators. Following discriminant analysis of physical driving variables, a model that predicts stream type based on channel geometry will be tested on 15 study reaches surveyed on the Barry M. Goldwater Air Force Range in southern Arizona. The resulting classification will provide a basis for examining relationships between hydrology, channel characteristics, riparian vegetation and ecosystem sensitivity of ephemeral streams in arid regions of the American Southwest.