INCISION OF THE HYPERARID, LOWER NAHAL (WADI) ZIN, ARAVA VALLEY, ISRAEL IN RESPONSE TO LATE PLEISTOCENE LAKE LISAN LEVEL FALL

Since the late Pleistocene Nahal Zin, the largest ephemeral stream in the southwestern margin of the tectonically active Dead Sea basin, has incised ~50 m into the thick and relatively unconsolidated deposits of Lake Lisan and earlier basin fill. This incision was forced by the continuous regression of Lake Lisan from its highest stand at ~25 ka and the exposure of the lake-floor sediments to fluvial and coastal processes. Alluvial cut-terraces of the incising channel are well preserved along the 17.5 km of the lowermost reach of Nahal Zin. At the outlet to the Dead Sea basin, the Holocene alluvial fan of Nahal Zin is deposited at the base of a 10-80 meters high escarpment that is associated with the Amatzyahu fault. This fault is the southern boundary of the Dead Sea basin. Geomorphic mapping, OSL ages and soil stratigraphy allowed correlation and reconstruction of several past longitudinal profiles of Nahal Zin, the incision history and modeling of profile evolution. The OSL ages of the terraces fit well the known lake level chronology. Between 24-17 ka Lake Lisan level dropped by about 100 m, exposing the surface of the lake bottom. The northward migration of the lake's shoreline was followed by the lengthening of Nahal Zin channel by >10 km, but due to the low gradient of the lake bottom (~0.01) only minimal incision occurred. Wide and shallow channels advancing with the retreating shore deposited a thin cover of alluvium on top of the lacustrine sediments. Later, between 17-14 ka, lake level dropped an additional 100 m, the submerged Amatzyahu fault scarp was exposed, and an over-steepened reach in the stream was formed. Fast incision (~40 m) at the stream mouth occurred in a relatively short time interval. The last 14 ka are characterized by slow and continuous incision, mainly in the upper part of the reach, causing a decrease in channel gradient. This stage was largely influenced by sediment supply and a hard bedrock knickpoint at the reach upstream end. Our observations and results indicate that the timing of fast stream incision was significantly controlled by the geometry of the exposed lake floor and not only by the amplitude of base level fall.