Stream channel convexity induced by continuous base level lowering, the Dead Sea, Israel

Abstract

The Dead Sea level has dropped from − 388.4 m in 1896 to − 417.2 m in 2004 and continues to fall rapidly, reaching a record of 1 m/y drop. Such a dramatic base level fall, on time scales of several tens of years, makes the Dead Sea area a unique field laboratory. The aim of this study is to examine the effect of the lowering Dead Sea, as a base level, on the longitudinal profiles of channels, on the formation of knick points, and on the sinuosity. Field studies were made along the lower courses of creeks which drain lakeward. Precise longitudinal profile measurements and their fitting to linear and logarithmic curves, show they are planar to slightly convex. The short convexity, the absence of hydrological or lithological control, and former experimental results of profile response to base level fall, suggest that the convexity results from the recent continuous rapid drop of the Dead Sea. The convexity is also an inheritance of the paleo morphology, i.e., the original convex front of the fan deltas and coastal scarps following channel incision, slope displacement and diffusion. The erodible substrate and the high stream power prevented preservation of nickpoints along the main channels. Sinuosity of the meandering thalwegs remained low 1.1–1.2, indicating no response to the base level lowering along the steep slopes. Although the stream channels are incised in soft sediments, the base level-induced entrenchment dies out within 1000 m or less from the receding shoreline. The channel profiles demonstrate different phases of adjustment to the lowering base level. The uniqueness of the Dead Sea case is that, contrary to former field and experimental studies, the system is in a continuous relaxation period without reaching a final adjustment. The channels are “trapped” in their convex form following a rapid on going base level fall upon exposed paleo fan deltas.