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A geomorphic classification of ephemeral channels in a mountainous, arid region, southwestern Arizona, USA


Despite the global abundance of arid-region ephemeral streams, hydrologic and geomorphic data for these systems are limited compared to their perennial counterparts. High spatial and temporal variability in flow makes hydrologic and geomorphic aspects of dryland ephemeral channels difficult to characterize. Perennial stream classifications have been extended to dryland ephemeral streams but do not adequately describe observed differences in channel geometry and characteristics of ephemeral channels in desert environments. We present a geomorphic classification for ephemeral streams in mountainous regions based on planform, degree of confinement, and composition of confining material. Five stream types were identified in the Sonoran desert of southwestern Arizona: (1) piedmont headwater, (2) bedrock, (3) bedrock with alluvium, (4) incised alluvium, and (5) braided channels. Nonparametric permutational multivariate analysis of variance for 101 surveyed reaches indicated differences (p < 0.001) in channel geometry and hydraulics among the five stream types. Nonmetric multidimensional scaling ordination identified the strongest channel geometry and hydraulic variables capable of distinguishing the five channel types, and a classification tree determined relative importance of these variables in the following order: width-to-depth ratio (W/D), stream gradient (S), stream power (Ω), and shear stress (τ). A classification tree and discriminant analysis used W/D, S, Ω, and τ for 86 study reaches on the U.S. Army Yuma Proving Ground (77% and 77% internal validation hit rate, respectively) to predict stream type of 15 separate study reaches on Barry Goldwater Air Force Range with 67% and 73% external validation hit rates, respectively. Differences in channel geometry among the five stream types reflect likely differences in hydrology, hydraulics, and sediment transport with implications for disturbance regime, channel adjustment to disturbance, and ecological sensitivity.