Abstract

Bars are major sediment storage places within channels, and much is known about their character from proglacial and temperate regions. Little is know, however, about bar characteristics from ephemeral streams. This study describes the morphology and composition of simple and compound gravel bars in Nahal Zin, Israel, an ephemeral desert stream. A hierarchy of microscale, mesoscale and macroscale structures and bed forms was identified in Nahal Zin that is similar to the hierarchy described for perennial streams. While the hierarchy is similar between arid and perennial streams, bar composition patterns are distinct. Compound bars and unit bars in humid areas are known to have armored surfaces composed of structured gravels, whereas the bar surfaces of both unit bars and compound bars in Nahal Zin were found to lack surface structures and to contain a large amount of sand. In ephemeral streams, both accreted and dissected compound bars do not display any downstream fining; in contrast, downstream fining along bars in humid areas is well documented. The flashy nature of floods, high sediment supply, high sediment transport rates, and large amount of sand associated with the bars are likely reasons the surfaces were not armored. In Nahal Zin, erosion and accretion dominate the bar edges, and accretion alone dominates the platform part of the bars. In summary, sediments in ephemeral streams of the Nahal Zin type are well mixed, unsorted, and unlikely to leave a well-defined stratigraphic record. These characteristics are distinct from their counterparts in proglacial and temperate regions and may be useful when investigating paleoenvironmental conditions.