IMPACT OF LARGE TWENTIETH CENTURY FLOODS ON A HYPERARID EPHEMERAL STREAM, ARAVA VALLEY, ISRAEL

The impact of large twentieth century floods on the wide channel of Nahal Pharan, within the Arava valley, southern Israel was documented to (a) identify major geomorphic changes associated with large floods (b) develop tools in identifying recent large floods and their respective magnitudes in alluvial reaches of ungauged streams in the region. Along most of its course, the Pharan is a coarse-gravel, braided ephemeral stream; when it enters the Arava valley it receives eolian and fluvial fine sand from the east and changes its morphology. The sand supply facilitates the recording of large floods as sand layers accreted in-between eolian sediments in sand bars and flood plains up to 2.5 m above the distinct current channels. The sand enhances the establishment of dense riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum) that interact with floods and affect stream morphology. The remarkable temporal association (to a specific year) between tree-ring ages (n=30) of re-growth of Tamarix trees damaged and fallen by floods ("Sigafoos trees") and the >30 year flood record of the Pharan indicate the utility of this method in developing twentieth century flood chronology in ungaged basins in the region. A series of aerial photographs indicate that the 1%-5% floods can alter the course of channels, while the 20%-10% floods leave minor but noticeable morphological change. Eastward channel widening during the last ~40 years was also observed. Although this is an alluvial reach, the estimated magnitude of the largest flood that has covered the entire channel width, as evident from a sequence of flood deposits, is approximately 1700 cms, larger than the ~1000 cms 1970 flood, but in agreement with the magnitude estimation of the largest paleoflood upstream and the regional envelope curve.