Yehouda Enzel

A 5000-year record of extreme floods and climate change in the Southwestern United States

A 5000-year regional paleoflood chronology, based on flood deposits from 19 rivers in Arizona and Utah, reveals that the largest floods in the region cluster into distinct time intervals that coincide with periods of cool, moist climate and frequent El Ni�o events. The floods were most numerous from 4800 to 3600 years before present (B.P.), around 1000 years B.P., and after 500 years B.P., but decreased markedly from 3600 to 2200 and 800 to 600 years B.P. Analogous modern floods are associated with a specific set of anomalous atmospheric circulation conditions that were probably more prevalent during past flood epochs.

Catastrophic arid episodes in the Eastern Mediterranean linked with the North Atlantic Heinrich events

The response of continental climate to the well-documented climate oscillations during the last glacial period has been a subject of intense interest, yet much less is known about the influence on regional continental climates than in the marine or polar realms of Earth. The detailed lake-level history of the closed Lake Lisan (paleo-Dead Sea) in the Middle East has been reconstructed from shoreline indications and high-resolution U-Th and 1 4 C chronologies, thus providing data on the response of the lakes catchment area to the climate changes during the corresponding period. We present a correlation between the newly developed Lake Lisan level curve for the past 55 k.y. and the North Atlantic Heinrich events. The correlation indicates a closely connected climate response between these North Atlantic events and the hydrologic conditions that prevailed in the Eastern Mediterranean. Our findings show that although the generally cooler conditions that prevailed during the last glaciation favored high levels of the lake, catastrophic events in the North Atlantic, which are associated with maximum cooling, have been responsible for droughts in the Eastern Mediterranean. We infer that cold-water input to the Mediterranean originating in the collapse of the North Atlantic Deep Water circulation caused the reduction of evaporation and less precipitation in the Eastern Mediterranean.

Dynamics of Flood Water Infiltration and Ground Water Recharge in Hyperarid Desert

A study on flood water infiltration and ground water recharge of a shallow alluvial aquifer was conducted in the hyperarid section of the Kuiseb River, Namibia. The study site was selected to represent a typical desert ephemeral river. An instrumental setup allowed, for the first time, continuous monitoring of infiltration during a flood event through the channel bed and the entire vadose zone. The monitoring system included flexible time domain reflectometry probes that were designed to measure the temporal variation in vadose zone water content and instruments to concurrently measure the levels of flood and ground water. A sequence of five individual floods was monitored during the rainy season in early summer 2006. These newly generated data served to elucidate the dynamics of flood water infiltration. Each flood initiated an infiltration event which was expressed in wetting of the vadose zone followed by a measurable rise in the water table. The data enabled a direct calculation of the infiltration fluxes by various independent methods. The floods varied in their stages, peaks, and initial water contents. However, all floods produced very similar flux rates, suggesting that the recharge rates are less affected by the flood stages but rather controlled by flow duration and available aquifer storage under it. Large floods flood the stream channel terraces and promote the larger transmission losses. These, however, make only a negligible contribution to the recharge of the ground water. It is the flood duration within the active streambed, which may increase with flood magnitude that is important to the recharge process.

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.

Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution

Tectonism and climate are the primary variables considered in conceptual models of alluvial fan evolution and basin-fill architecture, yet their roles and relative importance are far from resolved. The Madison Range and Madison River valley, southwestern Montana, were affected during late Quaternary time by varying degrees of tectonic activity as well as by climate change through multiple glaciations; we are thus able to evaluate the impact of tectonism and climate on fan evolution. Two primary fan deposits were correlated along the range front. They were deposited as proglacial outwash fans during glacial periods of increased sediment and water discharge. During the present interglacial period, these deposits were entrenched and secondary fans formed on lower fan areas. Temporal and stratigraphic relations indicate that climate was largely responsible for driving both fan aggradation and entrenchment. In contrast, stratigraphic relations between fan deposits and surface faults do not support a direct relation, either spatial or temporal, between local faulting and fan deposition. Because tectonism produces and maintains the relief necessary for fans to form, its primary role is long term, controlling the duration over which fan deposition may occur along a mountain front. If Quaternary alluvial fan evolution is representative of ancient alluvial fan evolution, this study has important implications for interpreting primary controls on deposition of the ancient alluvial fan sequences.

Paleoflood evidence for a natural upper bound to flood magnitudes in the Colorado River Basin

The existence of an upper limit to the magnitude of floods in a region is a long-standing and controversial hypothesis in flood hydrology. Regional envelope curves encompassing maximum flood magnitudes stabilize with progressive increases in the areal coverage and period of observation (Wolman and Costa, 1984). However, the short lengths of conventional gaging records limit substantial advances in testing whether this stabilization is evidence of an upper limit. In the Colorado River basin there are 32,120 station years of gage data, but the average period at a gaging station is only 20 years, with most stations having less than 70 years of observation. Paleoflood magnitudes derived from sediments of large prehistoric floods from 25 sites on rivers in Arizona and Utah provide additional data to extend the records of the largest floods. The paleoflood data identify the maximum flood discharges that have occurred on individual rivers over the last several hundred to several thousand years. Even with this increase in the observational period, the largest paleoflood discharges do not exceed the upper bound of maximum peak discharges delineated by the envelope curve derived from the available gaged and historical records. This result accords with the hypothesis of an upper physical limit for flood magnitudes and suggests that, for the Colorado River basin, the upper limit can be approximated by existing systematic and historical data for large floods. Similar relationships also hold when paleofloods and gaged records are presented for the subregion of southern Arizona.

Translocated Plio-Pleistocene drainage systems along the Arava fault of the Dead Sea transform

Abstract Geomorphic and sedimentologic field studies and analyses of LANDSAT 5 images and topographic maps indicate 15 km of left-lateral displacement of a Pliocene large stream and alluvial fans along the Dead Sea transform in southern Israel and Jordan. In the central Arava valley, a rift valley located along the transform, there is a notable discrepancy between the number and location of the feeding drainage basins within the eastern margins of the Arava valley and those of the alluvial fans and the cross-rift large stream. A few of these large alluvial fans lack any feeding drainage basin. Furthermore, east of the large stream there is no drainage basin that could have fed it. These discrepancies between the physiography, locations, sizes, and lithological compositions of the feeding drainage basins and of the alluvial fans can be explained by 15 km of left-lateral movement since the Late Pliocene or the Early Pleistocene along the Arava-Dead Sea segment of the transform. This is one of the largest displacements of a landform and surficial alluvial deposit in the world. However, the resulting average long-term rate of movement is relatively small (0.3–0.75 cm/year).

Geomorphic and hydrologic aspects of monsoon floods on the Narmada and Tapi Rivers in central India

Abstract High-magnitude floods during the monsoon season are considered to be Indias recurring and leading natural disaster. Such large flood are extremely important events, not only in terms of human impact, but also from the standpoint of geomorphic effectiveness and geomorphic work. The Narmada and Tapi Rivers in central India are characterized by one of the most intense flood regimes in the seasonal tropics. Modern, historical and palaeoflood records indicate that large floods are relatively common hydrologic and geomorphic events on the Narmada and Tapi Rivers. A catastrophic flood on Tapi in July 1991, that occurred from heavy rains and a dam break, represents one of the highest unit discharges yet recorded in any part of central India. Flood competence studies indicate that the flood power of the two rivers is comparable with some of the largest historic and prehistoric floods reported from other parts of the world. The rivers have preserved geomorphic and sedimentologic records of palaeofloods for the last two millennium, and in Choral River a 5000 year old record has been located. Examination of the synoptic conditions associated with the flood generating low-pressure systems reveals that all but a few of them are the result of Bay of Bengal depressions. The flooding in such large rivers profoundly challenges flood-hazard management, because of limited instrumental records and large spatio-temporal variation. It is therefore imperative that the design engineering community in India incorporates the geomorphic, sedimentologic and botanic information in the planning and design of water resource development projects.

Pattern and tempo of great escarpment erosion

Extensive geological, geophysical, structural, and geochronological data suggest that the locations of great escarpments bordering passive margins are exceptionally stable and are probably determined by crustal structure. Together, the data do not support the established paradigm of ongoing, significant, and parallel escarpment retreat. Rather, thermochronologic data and sedimentary sequences in ocean basins suggest that initial, tectonically controlled rift escarpments undergo rapid and significant erosion only during the earliest stages of seafloor spreading. Development of stable passive margin escarpments follows this period of intense erosion. Escarpments increase in sinuosity as embayments retreat more rapidly than interfluves. Measurements of 24 escarpments suggest that sinuosity, and the rate at which it increases, depends upon the location of maximum uplift, the geometry of the preescarpment drainage system, and margin age. All data suggest that the location of passive margin escarpments does not change significantly over time.

Anomalous North Pacific Atmospheric Circulation and Large Winter Floods in the Southwestern United States

Abstract Specific anomalous atmospheric circulation conditions over the North Pacific are conducive to the occurrence of the largest winter floods (≥10-yr return period) on rivers in six hydroclimatic subregions of Arizona and southern Utah, Nevada, and California. Composite maps of anomalies in daily 700-mb heights indicate that floods in all of the subregions are associated with a low pressure anomaly off the California coast and a high-pressure anomaly in the vicinity of either Alaska or the Aleutian Islands. Of these two major circulation features, the presence of the low is the controlling factor in determining whether large floods will occur. Shifts in the locations of the low and high pressure anomalies over the North Pacific appear to control which subregions experience floods, with high-elevation topographic features and proximity to air masses forming a major influence over the specific atmospheric circulation conditions that generate large floods in each hydroclimatic region. Concerning the int...