Extreme rates of channel incision and shape evolution in response to a continuous, rapid base-level fall, the Dead Sea, Israel

In nature, opportunities are limited for recording the response to a rapid ongoing base-level fall and for examining its effects on channel evolution during the process. Following a dramatic base-level fall, the natural laboratory area of the Dead Sea offers such an opportunity. Classical shape evolution models of incised channels describe the response to an "impulse" type base-level drop after it became stabilized. The Dead Sea delineates during its continuous level drop a different mode of adjustment from those previously conceptualized. We have recorded the entrenchment rates of channels and their transient shape evolution "at a station" in real time, i.e., during the ongoing base-level fall. This study represents the first attempt to apply integrated time series of high resolution DEMs, change detection maps, Differential Global Position System measurements, and field methods to monitor vertical and horizontal entrenchment rates, as well as the evolution of the cross-sectional shape of channels in response to a continuous and rapidly lowering base level. Mean vertical entrenchment rates are very high, 0.4-0.8 m/y, and widening rates are even higher (2.1-24 m/y). Elevation-change detection maps demonstrate along banks a patchy pattern of channel entrenchment, indicating the initial widening stage. The incised channels have a trapezoidal, gorge-like, cross-sectional form, i.e., they have developed beyond the initial stage of net vertical incision. However, neither advanced channel widening nor development of an inner floodplain has been observed. We conclude that widening is yet ineffective in damping the vertical adjustment. The continuous and rapid lowering of the Dead Sea level entraps the drainage systems in the vertical incision mode, preventing the evolutionary progression toward a wide, graded, and stable geometry which has been conceptualized in existing models. (C) 2009 Elsevier B.V. All rights reserved.