2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 25
Presentation Time: 8:00 AM-12:00 PM


HAVIV, Itai1, ENZEL, Yehouda1, WHIPPLE, Kelin2, ZILBERMAN, Ezra3, STONE, John4, MATMON, Ari5 and FIFIELD, L. Keith6, (1)Institute of Earth Sciences, The Hebrew Univ of Jerusalem, Givat Ram, Jerusalem, 91904, Israel, (2)Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, (3)Geol Survey of Israel, 30 Malkhe Israel, Jerusalem, 95501, Israel, (4)Quaternary Research Center, Univ of Washington, 19 Johnson Hall, University of Washington Box 351360, Seattle, WA 98195, (5)U.S. Geol Survey, 345 Middlefield Rd, Menlo Park, CA 94025, (6)Department of Nuclear Physics, Australian National Univ, Canberra, ACT-0200, yenzel@vms.huji.ac.il

Knickpoint retreat processes may govern the pattern and rate of bedrock channel evolution. As such they play a major role in setting the boundary conditions for hillslope development, conveying tectonic and climatic signals, and dictating sediment flux. Despite their importance they are still poorly understood.

Lithologically-controlled, sheer bedrock knickpoints, reaching up to 300 m in height, prevail along the Dead Sea arid escarpments. Although field observations suggest that rockfall rather than direct fluvial abrasion is the dominant retreat process, drainage area strongly controls retreat velocity. This points to a linkage between the frequency of mass wasting events and drainage area. Since no buttressing of the knickpoints faces or plunge pool erosion was observed another process should account for the observed dependency. A probable candidate is groundwater sapping whose intensity relies on infiltration of flood water through joints and potholes.

Cosmogenically-derived (36Cl) short-term retreat rates calculated in two channels with an order of magnitude difference in drainage area (17 km2 vs. 173 km2) were found to be 0.45 m/ka and 1.2 m/ka. These rates are remarkably similar to long-term retreat rates (0.25-0.5 m/ka vs. 1-2 m/ka) calculated using the estimated age of the escarpment (4-8 Ma) and the distance of the knickpoints from its base.

A free-fall of flowing water across a sheer knickpoint is expected to increase the shear stress above the free-fall lip and thus to accelerate incision and create an oversteepened drawdown reach (Gardner, 1983). Relying on hydraulic engineering equations (Rouse, 1936; Hager, 1983; Stein and Julien, 1993) we derive an expression predicting the increase in erosion rate (E) due to a free-fall effect: Efreefall/Enormal=(1+0.4/Fr2)3n, where Fr is the Froude number and n ranges between 0.6-2. This relation suggests that erosion rates above the lip can be more than 5 times higher than the expected erosion rates away from the knickpoint. Initial results of a numerical incision model comply well with measured profiles of several Dead Sea drawdown reaches. Though the uniqueness of the results, as well as complete sensitivity analysis, is yet to be tested, we suggest that landscape evolution models should explicitly account for drawdown effects.