Waterfall Retreat Rates along the Dead Sea Western Tectonic Escarpment

Waterfalls are abundant along steep bedrock channels and may play a key role in their evolution. Vertical waterfalls with dolomite caprock and marly-limestone footrock reaching ~100 m in height prevail along the Dead Sea western tectonic escarpment. Upstream migration rates of 6 waterfalls during the last several kyr was evaluated through concentration analysis of the cosmogenic isotope Cl-36 in samples collected from the vertical footrock of each waterfall. Shielding geometry was used to determine the cosmic ray flux arriving to each sampled outcrop as well as to calculate site-specific attenuation depths. The results vary from 15 to 90 cm/kyr and record an increase in lateral erosion rate with both drainage area (10-107 km2) and the annual rain volume over each basin (1-200 106 m3 yr-1). An additional sample was picked from a waterfall whose entire face consists of durable dolomite. This waterfall was recently deprived of its erodible footrock as it retreated across a fault which down-faulted the upstream block. The sample yielded a lateral erosion rate 3-times lower than the rate derived for a waterfall with similar flow characteristics whose footrock consists of marly-limestone. The low lateral erosion rate is manifested in the morphology of the waterfall -- the upper face is no longer vertical and has started to rotate and "diffuse". This sample illustrates that exhumation of erodible formation can increase waterfall retreat rate and the flux of sediment associated with it. Long-term average waterfall retreat rates representing the last 4-8 Myr were measured using the distance of waterfalls from the Dead Sea western tectonic escarpment and the estimated escarpment age. These increase with the basin annual rain volume but are 1-2.3 higher than the short-term lateral erosion rates. This discrepancy could not be attributed to reduction in discharge due to retreat, since all the waterfalls are located close to the outlet of their basins. It may suggest that flow characteristics in the Holocene do not resemble the average long-term flow characteristics. Feedbacks between waterfall retreat rate and downstream incision rates were explored considering: (1) the non-transportable debris supplied to the channel as a waterfall retreats, (2) caprock and footrock critical heights for shear failure and (3) caprock cantilever failure length. Though in some circumstances the rate of lowering of the reach beneath a waterfall may completely dictate the lateral retreat rate of the vertical face, retreat rate can also be decoupled from downstream incision for millions of years. To accurately capture the evolution of a reach with a waterfall it is suggested that landscape evolution models should utilize 3 separate erosion laws: one for lateral erosion of the waterfall vertical face, one for the reach beneath the waterfall and another one for the reach above the waterfall.