The width of valleys and channels affects the hydrology, ecology, and geomorphic functionality of drainage networks. Valley and channel widths are often estimated through a power-law scaling between width (W) and drainage 15 area (A), and where lithologic variability or differential uplift rates dominate, width was suggested to scale with both slope (S) and drainage area, through the relation = . However, in fluvial systems that experience drainage reorganization, abrupt changes in drainage area distribution can result in widths that are disproportional to their drainage areas. Consequently, in such cases, the width-area-slope scaling is expected to deviate relative to drainages that did not experience reorganization. 20 To explore the effect of reorganization on width-area-slope scaling, we studied 12 valley sections in the Negev desert, Israel, categorized into undisturbed, beheaded, and reversed valleys. We found that the drainage area exponent, b, differs between valley categories, and that reversed valleys are characterized by a negative b exponent, indicating valley narrowing with increasing drainage area. A detailed study of a reversed valley reveals that unlike the negative b exponent that links drainage area to valley width, the relation between drainage area and channel width is best fitted 25 with a positive b exponent. This difference indicates that the timescale of channel width adjustment to postreorganization drainage area distribution is faster than that of the valley width adjustment. We find that the difference in channel width across the divide causes a step change in unit stream power between the adjusted reserved channel and the unadjusted, beheaded channel. Gradients in width and unit stream power across the divide, lead to a widthfeedback that promotes ongoing divide migration and reorganization. 30 The identified distinct width-area-slope scaling of reorganized valleys could assist in recognizing and constraining the dynamics of landscapes influenced by drainage reorganization and likely has critical implications for the distribution of erosion rates in reorganized landscapes.

Overland flow is usually regarded as an important contributor to channel flow. This approach is certainly valid in arid watersheds, where base flow is irrelevant, especially in small watersheds, where non-climatic factors such as lithology, topography, and local surface properties can exert a strong influence on overland flow generation.  Understanding overland flow generation in dryland areas is therefore especially important, as rainfall thresholds for streamflow generation are highly dependent on the frequency and magnitude of overland flow generation. A relatively large literature is available for semi-arid areas. In some of these studies it is clearly stated that despite the lower average annual rain amount, the frequency and magnitude of channel flow is higher in the drier than in the semi-arid, wetter areas. This is attributed to a more extensive soil and vegetation cover in the wetter areas. Nevertheless, our knowledge on the complex factors that control overland flow generation, the main contributor to channel flow in extremely arid areas, (below 50 mm average annual rainfall) are very rare. The main objective of the present work is to obtain accurate data on runoff generation, in an extremely arid area (~28 mm average annual rainfall), under natural rainfall conditions in a variety of geological formations and surface properties. The study was based on a couple of small runoff plots (~ 3 m2) in each of the following lithologies: Limestone, Granite, Sandstone and Siltstone. One plot represents a rocky surface, and the second a colluvial surface. The study lasted two consecutive years. Data obtained showed that the frequency and magnitude of runoff generation, in each of the lithologies, were always higher over the rocky plots than over the colluvial plots. However, runoff generation was not uniform in the rocky, and in the colluvial plots. This is due to the local surface characteristics of each plot, such as stoniness and surface roughness.  Data obtained bring us back to two important issues in geomorphology. The first is the possibility to extrapolate data from one area to another area with the same rainfall regime. The second issue is the relative geomorphic importance of frequent rain events versus rare geomorphic events.  

Current conditions in the southern Levant are hyperarid, and local communities rely on fossil subsurface water resources. The region provided a pathway for the migration of humans out of Africa and their spread in the Near East and beyond in the Pleistocene. However, times of more favourable wetter periods and also their spatial characteristics are not yet well constrained. To improve our understanding of past climate and environmental conditions in the deserts of the southern Levant, sedimentary sections including artefact-bearing beds from Jurf ed Darawish on the Central Jordanian Plateau were investigated using sedimentological and micropalaeontological analyses and OSL dating. Grain-size analysis and structures of the clayey-silty sediments show that they mainly represent reworked loess deposits. The OSL age data suggest that these fine-grained sediments were accumulated during Marine Isotope Stages (MIS) 5-3. The prevailing aggradational setting was replaced by an erosional setting sometime after 30 ka. Recorded ostracod valves (mostly Potamocypris, Ilyocypris and Pseudocandona), remains of aquatic and terrestrial gastropod shells, and charophyte gyrogonites and stem encrustations indicate that an in-stream wetland existed at the location of Jurf ed Darawish during MIS 5-4 which was replaced by a vegetated alluvial plain in MIS 3. Abundant artefacts, distributed over a vertical range of up to 40 cm in a bed covered by a sedimentary sequence of 12-m thickness, provide evidence for the presence of humans in the region during a relatively long period from ca. 85-65 ka. The reconstruction of an in-stream wetland at Jurf ed Darawish, and the presence of humans at the site and in other desert regions of the Jordanian Plateau, the Wadi Arava/Araba, and the Negev and the Nefud deserts, show that the regional climate in the late MIS 5 and MIS 4 was significantly wetter and provided favourable conditions for humans in the Southern Levant and the northwestern Arabian Peninsula.

Overland flow is usually regarded as an important contributor to channel flow. This approach is certainly valid in arid watersheds, where base flow is irrelevant, especially in small watersheds, where non-climatic factors such as lithology, topography, and local surface properties can exert a strong influence on overland flow generation. Understanding overland flow generation in dry land areas is therefore especially important, as rainfall thresholds for stream flow generation are highly dependent on the frequency and magnitude of overland flow generation. A relatively large literature is available for semi-arid areas. In some of these studies it is clearly stated that despite the lower average annual rain amount, the frequency and magnitude of channel flow is higher in the drier than in the semi-arid, wetter areas. This is attributed to a more extensive soil and vegetation cover in the wetter areas. Nevertheless, our knowledge on the complex factors that control overland flow generation, the main contributor to channel flow in extremely arid areas, (below 50 mm average annual rainfall) are very rare. The main objective of the present work is to obtain accurate data on runoff generation, in an extremely arid area (~28 mm average annual rainfall), under natural rainfall conditions in a variety of geological formations and surface properties. The study was based on a couple of small runoff plots (~3 m2) in each of the following lithology’s: Limestone, Granite, Sandstone and Siltstone. One plot represents a rocky surface and the second a colluvial surface. The study lasted two consecutive years. Data obtained showed that the frequency and magnitude of runoff generation, in each of the lithologies, were always higher over the rocky plots than over the colluvial plots. However, runoff generation was not uniform in the rocky, and in the colluvial plots. This is due to the local surface characteristics of each plot, such as stoniness and surface roughness. Data obtained bring us back to two important issues in geomorphology. The first is the possibility to extrapolate data from one area to another area with the same rainfall regime. The second issue is the relative geomorphic importance of frequent rain events versus rare geomorphic events.

Unique geological and seismotectonic settings may trigger a multi-cascading hazard and should be identified beforehand. Such is the head of the Gulf of Elat–Aqaba (HGEA) at the northeastern end of the Red Sea where its geology, tectonics, bathymetry, and earthquake and tsunami history exhibit clear potential for earthquake and submarine-landslide tsunami generation. We thus investigated the possible tsunamigenic sources in the gulf and evaluated the resulting hazard at the HGEA. First, we assembled a bathymetric grid and adopted GeoClaw software to simulate most of the earthquake-tsunami scenarios. Next, we resolved the scheme of the largest possible tsunamigenic earthquakes along the deep basins of the Gulf of Elat (GEA) and the associated Dead Sea rift valley, as well as the potential tsunamigenic submarine landslides in the HGEA. The use of GeoClaw was verified against the 1995 tsunami generated by the Nuweiba Mw 7.2 earthquake, and then operated to simulate a suite of earthquake scenarios. Results showed that the marginal faults of Elat Basin pose the highest tsunami hazard to the Israeli part of the HGEA. To better assess that hazard, we screened the geology and seismotectonics of the HGEA and found that the Elat normal fault presents the worst-case scenario for Elat city. It is capable of generating a multi-cascading threat of earthquake and submarine-landslide tsunami, local subsidence that can increase inundation, and above all, destructive ground motion. Scenarios of a tsunami caused by the worst-case earthquake on the Elat fault simulated by GeoClaw and Ward’s (2011) approach, and submarine landslide in the HGEA simulated by Wang et al.’s (2015) ‘Tsunami Squares’ approach, demonstrated waves as high as 4 m along these coasts. Accordingly, we constructed a map of the evacuation zone. We also show that strong ground-shaking and retreat of the sea at the HGEA should be considered a tsunami warning, although false alarms are inevitable. Furthermore, tsunami hazard exists all along the gulf and further assessments are needed to quantify this hazard and increase awareness among the area's population.

Abstract. The Negev Desert in southern Israel hosts a number of late Cenozoic lacustrine and palustrine sedimentary sequences that attest for past wetter conditions in what today constitutes one of the driest deserts on Earth. These sequences are of special importance because the Negev Desert forms part of the Levantine Corridor, which was probably the only continental bridge that enabled initial out-of-Africa expansion of our genus Homo.Yet, the paleoclimatic significance of these sequences still remains unknown, mainly due to their uncertain (late Pliocene to early Pleistocene) age. Here we present a combined sedimentologic, paleontologic and magnetostratigraphic study of one of these sedimentary sequences, the so-called Kuntila Lake sediments, which was carried out at the 30 m-thick Kuntila Gate section in the Nahal Paran basin, southern Israel.Sedimentological evidence and ostracod faunas indicate that these sediments accumulated in a long-lasting lacustrine basin that underwent periodic lake-level variations. Magnetostratigraphic measurements enable the recognition of a normal (N1) and a reverse (R1) polarity zones in the lower and upper halves, respectively, of the Kuntila Gate section. Correlation of N1 to the Olduvai Subchron (1.778 and 1.945 Ma) appears as the most likely option in view of previously published 10 Be ages derived for the uppermost part of the Kuntila Member in nearby sections. The large errors associated with these ages, however, suggest that correlation of N1 to Subchron C2An.1n (2.582-3.032 Ma) is also possible. Although our results do not resolve the age of the Arava Formation, they unequivocally relate the Kuntila Lake sediments with a long period of enhanced climatic variability because the tops of both subchrons are associated with 400 kyr eccentricity maxima. The inferred wetter conditions in the Negev Desert concurred, regardless of the age correlation, with periods of lake expansion in East Africa and clusters of short-lived expansions of the savannah throughout much of the Sahara. This would have led to the biogeographic connection between Africa and Eurasia, greening the path for the initial out-of-Africa dispersal of Homo. Further research on the Kuntila Lake sediments will be necessary to better determine the timing, extent and significance of such biogeographic connection.

Drainage reversals, an end-member case of drainage reorganization, often occur toward cliffs. Reversals are commonly identified by the presence of barbed tributaries, with a junction angle >90°, that preserve the antecedent drainage geometry. The processes that form reversed drainages are largely unknown. Particularly, barbed tributaries cannot form through a spatially uniform migration of the cliff and drainage divide, which would be expected to erase the antecedent drainage pattern, and tectonic tilting toward the cliff that could reverse the flow direction is inconsistent with geodynamic models of large-scale escarpment, where many reversals are documented. Here, we propose a new mechanism for drainage reversal, where the slope imbalance across a cliff, together with the high erodibility of sediments that fill cliff-truncated valleys, result in faster divide migration along valleys compared to interfluves. We demonstrate this mechanism along channels that drain toward the escarpment of the Arava Valley in Israel. Reversal is established by observations of barbed tributaries and opposite-grading terraces. We show that drainage reversal occurs when erodible valley fill exists, and that the reversal extent correlates with the thickness of this fill, in agreement with the predictions of the proposed mechanism. This new reversal mechanism demonstrates that valley fill could play an acute role in fluvial reorganization processes, and that reversals could occur independently of tectonic tilting.

The tsunami that followed the 1995 Mw 7.2 Nuweiba earthquake along the Dead Sea Transform in the Gulf of Elat–Aqaba (GOE) surprised the local population, who were unconcerned by seismogenic sea waves happening in a closed gulf, far away from the open ocean. Eyewitness reports, field observations, and a mareogram recorded near Elat demonstrated conclusively that tsunami hazard in the GOE deserves focused attention. Here we take up the challenge, adopting the GeoClaw package and investigating which of the available Nuweiba earthquake models are capable of better replicating the actual findings. In general, the simulated tsunamis that are based on both the seismological and Interferometric Synthetic Aperture Radar (InSAR) Nuweiba earthquake models are in line with the eyewitness descriptions of wave height, slight inundation, and limited damage. In addition, the simulations show a reasonable correlation with the amplitude and wave period derived from the analog mareogram, as expected from a tsunamigenic, mostly strike‐slip component earthquake. The InSAR inverse‐based modeling of the coseismic deformation, however, appears closer to the measured parameters of the recorded mareogram. The exception of 3–4 m high waves reported in the Nuweiba port may be the result of local perturbations in the harbor or the effect of a local tsunamigenic submarine landslide. The four countries of Israel, Jordan, Saudi Arabia, and Egypt that encircle the GOE seek to expand intensively their marine infrastructure, tourism, and population. The present study aims to warn the stakeholders around the GOE about the inevitable tsunamis.

In drylands, microbes that colonize rock surfaces have been linked to erosion because water scarcity excludes traditional weathering mechanisms. We studied the origin and role of rock biofilms in geomorphic processes of hard lime and dolomitic rocks that feature comparable weathering morphologies, although these two rock types originate from arid and hyperarid environments, respectively. We hypothesized that weathering patterns are fashioned by salt erosion and mediated by the rock biofilms that originate from the adjacent soil and dust. We used a combination of microbial and geological techniques to characterize rock morphologies and the origin and diversity of their biofilms. Amplicon sequencing of the SSU rRNA gene suggested that bacterial diversity is low and dominated by Proteobacteria and Actinobacteria. These phyla only formed laminar biofilms on rock surfaces that were exposed to the atmosphere and burrowed up to 6 mm beneath the surface, protected by sedimentary deposits. Unexpectedly, the microbial composition of the biofilms differed between the two rock types and was also distinct from the communities identified in the adjacent soil and settled dust, showing a habitat-specific filtering effect. Moreover, the rock bacterial communities were shown to secrete extracellular polymeric substances (EPSs) that form an evaporation barrier, reducing water loss rates by 65 %–75 %. The reduced water transport rates through the rock also limit salt transport and its crystallization in surface pores, which is thought to be the main force for weathering. Concomitantly, the biofilm layer stabilizes the rock surface via coating and protects the weathered front. Our hypothesis contradicts common models, which typically consider biofilms to be agents that promote weathering. In contrast, we propose that the microbial colonization of mineral surfaces acts to mitigate geomorphic processes in hot, arid environments.

This paper provides an in-depth review of evidence for the presence of twelve waterbodies spanning the Late Pliocene through Late Pleistocene in southern Israel and southern Jordan. A comprehensive description of these waterbodies is presented, combined with new field, paleontological and numerical age data, along with a discussion of their implications for paleohydrology and paleoclimate. The region is currently hyper-arid and there are no permanent rivers, wetlands or lakes in the area. Nevertheless, during the time-frame examined, continuous layers of limestones and mudstones were deposited in wetlands and shallow lakes. According to their location, the waterbodies were classified into either resulting from local tectonic depressions or in wide natural depressions at base levels. Following the types of sediments and fauna associated with these waterbodies, it is suggested that four wetter periods occurred: Late Pliocene/Early Pleistocene, Middle Pleistocene, Middle Late Pleistocene and terminal Late Pleistocene. This resulted in the deposition of limestone, chalk, travertine, calcrete, mudstone, marl, clay, silt and sandstone. For several waterbodies, vertical and lateral transitions between white limestone and fine clastic sediments rich in carbonate, indicate changes in depositional conditions from a shallow lake to a wetland, both associated with wetter hydrological settings compared to current climatic conditions.