Arid ephemeral stream classification using channel geometry and basin characteristics

Because understanding of ephemeral stream characteristics is limited and many stream classifications do not adequately describe them, it is necessary to develop a better understanding of these dryland fluvial systems and develop more precise terminology to discuss their physical attributes. In addition to development of a geomorphic classification system, we examine relationships between basin characteristics and channel geometry that will indicate where these ephemeral stream types might occur. Our conceptual model includes five geomorphic ephemeral stream types; 1) braided washes, 2) incised alluvium, 3) bedrock with alluvium, 4) bedrock, and 5) piedmont headwater channels. Preliminary watershed classification and cluster analysis of the U.S. Sonoran Desert was conducted using NHD 10-digit Hydrologic Unit Boundaries, PRISM precipitation data, state geologic survey lithology, and data derived from 30m DEMs. A total of 85 reaches were surveyed on the U.S. Army Yuma Proving Ground in southwestern Arizona representing the five stream types within three watershed categories. Following delineation of small-scale watershed characteristics using 10m DEMs for each reach location, statistical analysis will be performed to examine correlations and significant relationships among stream type, basin and channel characteristics. We hope to identify physical drivers resulting in the development of distinct geomorphic stream types and predict where the relative abundance of those stream types are likely to exist in arid environments of the southwestern U.S. We posit that locations and relative distributions of the five stream types will correlate significantly to local basin characteristics. Initial findings verify that composition of confining material dictates the level of confinement and largely influences occurrence of the five channel types. Additionally, we expect to see significant differences in width/depth ratios, grain size, stream gradient, basin hillslope gradient, and drainage area among stream types. Field observations and preliminary results support a progressive increase in width/depth ratios, drainage area, and distance from the mountain front with a concurrent decrease in grain size, stream gradient and hillslope gradient from bedrock, bedrock with alluvium, incised alluvium, to braided washes, respectively. Piedmont headwater channels are expected to occur with the smallest of drainage areas and the most variability in hillslope and stream gradient, distance from the mountain front, and grain size. The results of this study contribute to the collective understanding of arid ephemeral streams through the development of a geomorphic classification related directly to physical drivers within dryland watersheds.