Fluvio-pedogenic processes in an ephemeral stream channel, Nahal Yael, Southern Negev, Israel

Abstract

A detailed stratigraphic survey conducted in the alluvial fill of the channel of Nahal Yael—a small catchment in the Southern Negev Desert—indicates the existence of a continuous, compacted, red-colored unit at an average depth of 50 cm beneath the surficial grey non-cohesive alluvium. Granulometric distinction between the grey and the red alluvium is evident only within the fine (<0.063 mm) fractions; the clay content in the red alluvium, though less than 5%, is more than double in the grey alluvium. Cementation by calcium carbonate causes the hardening of the `red' unit. Reddening is due to small amounts of iron oxide coatings. While the upper contact of the `red' unit is abrupt, the compaction and coloring diminish gradually downward, forming a diffuse lower boundary. The structure controlled infiltration rates into the upper grey alluvium exceed by an order of magnitude those into the `red' unit, but are two orders of magnitude less than the velocity of the flood front, indicating that the flood fronts progress over unsaturated alluvium. The proposed model is one of a dual component flow within the alluvial fill: a longitudinal flow within the upper active layer, proceeding mainly along structure determined paths, and a vertical flow which affects the whole alluvial fill. The formation of the `red' unit is tied to the cumulative influence of persistent differences in water availability to various parts of the channel before and during flood events. The `red' unit forms at the lower limit of the contemporary scour and fill processes, and therefore, its depth reflects the contemporary hydroclimatic regime. Its identification as a partial buffer to floodwater infiltration offers new insights into the problem of the generation of desert floods and the mechanism of transmission losses in ephemeral streams.