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

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. (C) 1998 Elsevier Science B.V. All rights reserved.