Judith Lekach

Sediment yield exceeds sediment production in arid region drainage basins

We use 10 Be and 26 Al to determine long-term sediment generation rates, identify significant sediment sources, and test for landscape steady state in Nahal Yael, an extensively studied, hyperarid drainage basin in southern Israel. Comparing a 33 yr sediment budget with 33 paired 10 Be and 26 Al analyses indicates that short-term sediment yield (113–138 t· km –2 · yr –1 ) exceeds long-term sediment production (74 ± 16 t· km –2 · yr –1 ) by 53%–86%. The difference suggests that the basin is not in steady state, but is currently evacuating sediment accumulated during periods of more rapid sediment generation and lower sediment yield. Nuclide data indicate that (1) sediment leaving the basin is derived primarily from hillslope colluvium, (2) bedrock weathers more rapidly beneath a cover of colluvium than when exposed, and (3) long-term erosion rates of granite, schist, and amphibolite are similar.

AN EVALUATION OF TWO TEN-YEAR SEDIMENT BUDGETS, NAHAL YAEL, ISRAEL

The method used to derive a crude ten-year sediment budget for Nahal Yael—a small hyper-arid catchment in the Southern Negev Desert—is applied to a later ten-year period, for which a much more accurate sediment budget is available. Results indicate that the budget of the first decade has overestimated the suspended sediment output by a factor of four, and underestimated the bed material output by a factor of two. Evaluated against the background of similar sediment budget studies, it is concluded that the main source of potential errors in sediment budgets, especially in those involving considerable amounts of bed material, lies not so much in the volumetric before-after comparisons, based on surveying. Rather, errors are caused mainly by the inadequate procedures presently available for the estimation of sediment input into, and output from, the studied reach.

Evidence for transport of bedload in waves: Analysis of fluvial sediment samples in a small upland stream channel

Abstract Data pertaining to the transport of fluvial sediment sampled in detail during a five hour long event in a small upland stream channel in the extremely arid Southern Negev are presented. Considerations based on comparative evaluations of water discharge, stream power, total and fractional sediment concentration, and total and fractional sediment discharge suggest that bedload transport in channels of even very small upland catchments practically devoid of continuous alluvial cover takes place in waves, which are not a priori related to any water discharge characteristic, but rather reflect some intrinsic feature of watershed dynamics, such as slope-channel topographic contrasts or fluvial bifurcation. The regular spacing of pools and riffles and of gravel bars in the more alluvial reaches downstream may therefore be initiated from upstream.

Fluvio-pedogenic processes in an ephemeral stream channel, Nahal Yael, Southern Negev, Israel

Abstract A detailed stratigraphic survey conducted in the alluvial fill of the channel of Nahal Yael—a small catchment in the Southern Negev Desert—indicates the existence of a continuous, compacted, red-colored unit at an average depth of 50 cm beneath the surficial grey non-cohesive alluvium. Granulometric distinction between the grey and the red alluvium is evident only within the fine (

Late Quaternary weathering, erosion, and deposition in Nahal Yael, Israel: An “impact of climatic change on an arid watershed”?

In their seminal paper in 1979, Bull and Schick proposed a conceptual model for the geomorphic response to Pleistocene to Holocene climate change, based on the hyperarid Nahal Yael watershed in the southern Negev Desert. In this model, the change from semiarid late Pleistocene to hyperarid early Holocene climates reduced vegetation cover, increased the yield of sediment from slopes, and accelerated aggradation of terraces and alluvial fans. The model is now over 30 yr old, and during this time, chronologic, paleoenvironmental, and hydrogeomorphic research has advanced. Here, we reevaluate the model using data acquired in Nahal Yael over the 30yr since the original model was proposed. Recent studies indicate that the late Pleistocene climate was hyperarid, and a transition from semiarid to hyperarid climates did not occur. The revised chronology reveals a major 35–20 ka episode of accelerated late Pleistocene sediment production on slopes (with lower rates probably already at ca. 50ka) due to increased frequency of wetting-drying cycles caused by frequent extreme storms and floods between 35 and 27 ka. Without lag time, these sediments were transported and aggraded in depositional landscape components (fluvial terraces and alluvial fans). This intensified sediment production and delivery phase is unrelated to the Pleistocene-Holocene transition. The depositional landforms were rapidly incised between 20 and 18 ka. Since and/or soon after this Last Glacial Maximum (LGM) incision, most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock. Using these new data, we propose a revision to the Bull and Schick model in this hyperarid environment. Our revision suggests that the model should include the frequent storms and floods responsible for a late Pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans. We also discuss the common use and pitfalls of using the Bull and Schick conceptual model to explain observations in diverse arid environments, usually without sufficient data on basin-specific stratigraphic, chronologic, paleoenvironmental, and paleoclimatic information.

High bedload transport rates in relation to stream power Wadi Mikeimin, Sinai

Summary A reconstruction of a low-frequency high-magnitude flood event, which occurred in January 1971 in the watershed of Wadi Mikeimin in the arid southeastern Sinai mountains, enabled the evaluation of bedload transport rates in relation to flow characteristics. Unit transport rates between 20 and 100 kg/ms, i.e. at least one order of magnitude higher than nearly all data previously reported, are compatible with data on other rivers. The proportionality of bedload transport rate to unit stream power in excess of that necessary for initial motion, raised to the power of 3/2, has been validated for these very high transport rates, which approach debris flows in character.