MODELING TWENTIETH CENTURY LONGITUDINAL PROFILES OF INCISING ARID EPHEMERAL TRIBUTARIES IN RESPONSE TO THE FAST FALLING BASE LEVEL OF THE DEAD SEA

The artificial Dead Sea lake level drop during the 20th century provides field-scale laboratory to examine tributaries responses to fast base level fall with no hydroclimatic changes in tributary basins. This is similar to controlled flum studies and different from studies documenting longer duration incision under climate or hydraulic changes. We applied a predictive model for longitudinal profiles of Dead Sea tributaries, which is based on a sediment diffusion equation. The research was centered on the lowermost reaches of 8 ephemeral streams draining to the Dead Sea. Each stream has diverse drainage area, minimal human interference, a few 20th century terraces, different fluvial characteristics and different fluvial-lake interactions. We mapped and surveyed the terrace remnants and determined their ages by the known altitude of their associate shoreline, the known lake levels, and by analyzing aerial photographs. The input parameters are: lake levels curve; shoreline location each year (i.e. shoreline's vertical and lateral locations), and the measured longitudinal profiles. The diffusion coefficients and sediment fluxes were calibrated and then validated by a few sets of longitudinal profiles. We show that diffusion processes describe well stream incision along the Dead Sea margins and this process alone can approximate both aggradation and degradation processes in reaction to base level lowering. Therefore this tool can be used for predicting stream profiles under ongoing and future Dead Sea level drop. The resulted diffusion coefficients of streams flowing into the same base level help assessing the sensitivity of incision to various hydrological parameters. These diffusion coefficients are directly related to drainage basin areas.