Alluvial Fill Research Articles

Controls on sediment transfer dynamics at a tributary–trunk transition in the Little Karoo, with implications for interpreting the landscape response to environmental change

ABSTRACT Zones of alluviation at tributary – trunk confluences can act as sediment storage/transfer switches. Evaluating the temporal variation in tributary – trunk connectivity is key to understanding the origin, dynamics and residence time of tributary alluvial fill sequences, and determining relative and interacting effects of different drivers of landscape development. This paper evaluates processes and timescales of tributary (Prins River) – trunk (Touws River) connectivity at a site in the Little Karoo, as context for discussing sediment dispersal dynamics and implications for interpreting the landscape response to environmental change. An alluvial terrace in the tributary valley preserves a chronology (optically stimulated luminescence) of tributary valley alluviation that is regionally synchronous with valley alluviation in the upper Huis River and floodplain alluviation in the lower Touws and Groot rivers. A climatic shift within the Little Karoo at ~1000 years BP from relative aridity to relative humidity (and higher-energy rain-bearing circulation types) may have initiated widespread re-working of alluvial fills and the breaching of geomorphological buffers. Alternatively, there may be an intrinsic limit to sediment preservation potential associated with a regional floodplain cycling time of one to two thousand years. Longer archives are needed to contextualize fluvial responses to climatic variability in the region.

Study on the Deformation Law of the Embankment with Alluvial Fill in the Lower Yellow River

Study on the Deformation Law of the Embankment with Alluvial Fill in the Lower Yellow River

06 Şubat 2023, Pazarcık (Kahramanmaraş, Türkiye) depreminin neden olduğu jeomorfolojik deformasyon örnekleri

The left-laterally strike-slip Pazarcık fault is one of the East Anatolian Fault Zone (EAFZ) segments. On February 6, 2023, the ±85 km long Pazarcık fault generated a highly destructive Mw=7.7 earthquake. This study aims to explain the geomorphological deformations caused by the February 6, 2023, Pazarcık earthquake with typical examples. The surface rupture of the earthquake between Türkoğlu and Gölbaşı was followed precisely, and the changes in the earth's surface due to the left lateral strike-slip were determined, measured, and recorded. A DJI Phantom 4 and a DJI Mini Drone were used for aerial measurements and recordings during the fieldwork. Garmin e-Trex 10 handheld GPS and tape measure were used for terrestrial measurements. During the field studies, the surface rupture of the earthquake was investigated from a geomorphological perspective and mapped by taking location data. It was determined by the measurements that the left lateral offset distances in the surface fracture vary between 4.0-6.5m. One of the geomorphological deformations of the February 6, 2023 earthquake is transpressional ridges and/or transtensional depressions. Transpressional shortening and/or transtensional extension deformations due to a single surface rupture are the natural consequences of the curvilinear slip plane of the left-laterally strike-slip Pazarcık fault. Liquefaction samples with different characteristics were observed in the Sakarkaya alluvial fill area within the Gölbaşı depression. Rockfalls occurred on sandstone, mudstone, and limestone rock slopes weakened by the discontinuity due to the density of cracks outcropping in the valley where the surface rupture passes in the Kartal, Sakarkaya section. During field studies, slides and spreading were also observed. Typical examples of slide occurred on the unconsolidated fill ground on the south coast of Gölbaşı Lake with a slight slope towards the lake as a result of the vibration effect of the earthquake. In addition, the vibration effect of the earthquake caused lateral spreading deformations in the artificial fillings of road and road junction structures.

Macroseismic Study of 4th January 2016, Manipur Earthquake, and its Implications

Abstract An earthquake of magnitude 6.7 M (Latitude 24.834N; Longitude 93.656E) on the Richter scale rocked in the Noney area, Tamenglong district, Manipur on 4th January 2016. The shock was strongly felt in most parts of the Northeastern states of India viz. Manipur, Nagaland, Meghalaya, Assam, and its neighboring countries Myanmar and Bangladesh. Macroseismic study was conducted to assess the intensity of ground motion and related damages caused by the earthquake. The damage incidences were mainly reported in structures constructed over the alluvial fill material. Deformations and development of ground fissures were observed in the epicentral tracts of Manipur and liquefaction phenomenon was observed in the Silchar area Assam. The study revealed that higher damage was confined to the area around Noney, Kabuikhullen, Nungba, Leimakhong, and Imphal valley areas in Manipur and Silchar town areas in Assam. Based on field observation, the degree of damage to building structures, and data gathered from the local population using the standard questionnaire, different intensity values were assigned to each locality. Isoseismal map was prepared by drawing the boundaries between different intensity areas at MSK-64 scale. The result indicates that the intensity was within the range of IV to VII. The maximum intensity was found VII in parts of Imphal, Tamenglong, and Silchar and very close to the epicentral tract area of Noney; and lowest intensity IV was observed in other parts of the northeastern states around Dimapur and Kohima in Nagaland state. The location of the epicenter of main shock (M 6.7) was at the intersection of the two major lineaments trending to ENE-WSW and NW-SE. The trend and shape of isoseismals drawn on seismotectonic/geological maps are elliptical along ENE-WSW direction and are found more closely spaced in the north and southern parts of the affected area. The elliptical nature of the isoseismals indicates that this seismic event was caused by movement along a linear surface. Furthermore, the orientation of elliptical isoseismals with a major axis trend in an ENE-WSW direction is almost parallel to the ENE-WSW trend of major lineament/fault in the area. As a result, the ENE-WSW lineament/fault trend appears to be the causative fault for this quake. In addition, one of the nodal plane (NP2) striking 83°, with dip 81° and slip (rake) 38° obtained from fault plane solution USGS shows a similar trend to the direction of generated elliptical isoseismals pattern confirming the event ocurred along a major tectonic lineament in a strike-slip mechanism.

A technique for quantification of quaternary ground tilting as recorded by alluvial fill terraces: An example from the northern Gulf of Mexico coastal plain

To quantify the degree of Quaternary structural tilting, we examined topographic slopes of treads of alluvial fill terraces. Like strath terrace treads, fill terrace treads can serve as a structural datum if restored to their pre-erosion geometry. Their slope degree and direction can then be compared to the slope of the modern floodplain to assess neotectonic tilting. On the northern Gulf of Mexico coastal plain, a ~45km-wide flight of seven Pleistocene alluvial fill terraces of the Ouachita River records a preferred southwestward migration of the river, and previous authors attribute this lateral migration to Quaternary structural tilting. The original geometries of Ouachita River terrace treads are needed as structural datums, but the treads are dissected by tributary streams. On younger treads, we observe concordant elevations of tributary divides and stronger soil development on divides than on tributary valley flanks. This indicates that divides are minimally eroded and preserve parts of the original treads. In order to restore terrace treads to their pre-dissection geometries, we constructed a topographic envelope model using elevations of tread remnants on tributary divides. This model effectively ‘fills’ the tributary valleys on the treads. We extracted each restored tread from the model and determined the tread's best-fit plane. We used three planes with R2 values >0.9 as datums. Consistent with westward tilting, directions of topographic slope of middle/late Pleistocene treads are rotated 80o toward the south-southwest, and treads have slopes that increase with terrace age from 0.017° to 0.09° (0.3 to 1.6 m/km). This quantitative estimate of late Quaternary westward regional tilting reveals a 10-fold increase in slope relative to the Holocene floodplain (0.14 m/km).

Evolution of the Holocene alluvial fan of the River Stryi in the foreland of the Eastern Carpathians (Western Ukraine)

The research reconstructs the development of the alluvial fan of the River Stryi in the Holocene and dates the phases of increased river activity and their correlation with other valleys in the upper Dniester and Vistula river basins. The age of the palaeochannels and the stratigraphy of alluvial fills are based on radiocarbon dating supported by the results of palynological analyses. The oldest (fossil) palaeochannel of the Stryi was dated to the Younger Dryas. The large-radius meander was abandoned at the end of the Boreal Phase during the period of flood intensification at about 8700 BP. The uneven top of the gravels (cut by palaeotroughs of a depth ranging from 2 to 4 m) indicates that during periods of increased discharges (Younger Dryas, end of the Boreal Phase, late Atlantic and beginning of the Subboreal) the troughs were cut and filled with channel sands. From the beginning of the Subboreal Phase (5400-5300 BP), aggradation of the valley bottom has been predominant, which is manifested not only by the overbuilding of the floodplain but also by the gradual elevation of the channel. The accumulation of sandy-silty overbank sediments dated to about 8300, 5400 and 2800 BP correlates well with the beginning of humid phases and floods in the upper Vistula and upper Dniester basins as well as floods recorded in lakes in the northern foreland of the Alps.

Chronology and dynamics of fluvial style changes in the Younger Dryas and Early Holocene in Central Europe (lower San River, SE Poland)

Reconstruction of fluvial style changes in the San River in the Subcarpathian Basins is based on geomorphological and sedimentological analyses. The time control of alluvial fills and temporal changes in the river channel are derived from radiocarbon and optically stimulated luminescence dating combined with independent pollen-based biochronostratigraphy. The results showed that the alluvial plain of the braided (BR) or braided-meandering (BR-M?) river was abandoned before 12,800 cal BP. Large meanders (LM) were cut off in the older part of the Younger Dryas (YD; ca. 12,600 cal BP), and in the younger part of this period (ca. 12,450 cal BP). The small meanders (SM) developed at the end of the YD and were abandoned at the onset of the Preboreal (PB; ca.11,550 cal BP). The erosion phase at the YD-PB transition, reported from many valleys in Central Europe, was not confirmed in the study area. The full cycle of San River channel transformation (BR (BR-M?) → LM → SM); was estimated to be approximately 1200 years. According to the palynological data, open pine forests with birch that survived from the end of the Allerød dominated the landscape of the river valley during the YD cooling and did not undergo major changes during the warming in the early PB. Therefore, we assume that the influence of vegetation changes in the San River channel pattern transformation was nonsignificant. The location of the studied palaeochannels in the floodbasin filled with silty clayey deposits may have influenced the formation of relatively narrow and deep channels, than that of much the wider and shallower meanders from the YD, situated several kilometres downstream of the surveyed sites.

Sediment Budgets for Small Salinized Agricultural Catchments in Southwest Australia and Implications for Phosphorus Transport

Examples of sediment budgets are needed to document the range of budget types and their controls. Sediment budgets for three small agricultural catchments (7.6 to 15.6 km2) in southwestern Australia are dominated by channel and gully erosion, with sheet and rill erosion playing a subordinate role. Erosion was increased by clearing naturally swampy valley floors and hillslopes for agriculture and grazing, and episodic intense rainstorms. The proportion of sediment from channel and gully erosion in the sediment budget appears to be determined by the depth of alluvial fills. Dryland salinization caused by clearing native vegetation has connected hillslopes to channels across narrow floodplains, increasing the Sediment Delivery Ratio (SDR). Yield and SDR are found to be insensitive to major in-catchment changes of vegetation cover after initial clearing, the ratio of sheet and rill erosion/channel and gully erosion, and sediment storage masses. This supports the idea that yield alone is often a poor indicator of the impact of land use and land management change. Riparian vegetation would reduce sediment yield but not phosphorus yield. This study demonstrates the value of mixed methods where field observations and chemical analysis are combined with information from local people.

The Search for Sikyon’s Sunken Harbor

– Horizontal-to-Vertical Spectral Ratio is used for mapping the thickness of the alluvial fill. – Recording of ambient noise with mini arrays is used to map the shear-wave velocity. – Imaged depression filled with silty material might be connected to former harbor.

Chronology and stratigraphy of the Imlay Channel in Lapeer County, Michigan, USA

The Imlay Channel in Lapeer County, Michigan, was one of two outlets for the glacial Lake Maumee phase of ancestral Lake Erie. Fifteen new radiocarbon and optical ages from within and adjacent to the Imlay Channel constrain sedimentation rates within the channel and the timing of regional deglaciation. For nearly 50 years, the deglaciation of this region of Michigan has been based on a single age from the Weaver Drain site located near the Imlay Channel, and a new radiocarbon age of 16.7–17.0 cal ka BP from 3 km east of the Imlay Channel supports this long-standing deglacial age. On average, there is a 14 m thick sediment fill within the channel. Radiocarbon and optically stimulated luminescence ages reveal that much of the alluvial fill was deposited by 14.9 ka, and alluvial fans building into the channel stabilized in the early Holocene. Cross-sections along and perpendicular to the Imlay Channel, built from geotechnical borings and water-well records, reveal a current-day bedrock sill elevation at 235 m a.s.l. that would have permitted drainage of all stages of glacial Lake Maumee in the past.

Paleoseismologic trenching confirms recent Holocene activity of the major Algeciras fault system in southern Colombia

The Algeciras Fault System (AFS) in Colombia, which is characterized by its predominant dextral strike-slip movement, is part of the major transcurrent fault system known in Ecuador, Colombia and Venezuela as the Eastern Frontal Fault System, that disconnects the North Andean Block (NAB) from the South American plate in a process of tectonic escape. It is responsible for some of the most important seismic events in the seismic history of Colombia that occurred in 1827 with Mw 7.1 that destroyed large part of all settlements in the Huila Department and caused considerable damage and loss of life in the capital of Bogota. More recently another event of Mw 7.0 took place in 1967 that also caused damage in the capital, and in December 2019 an event of Mw 6.0 caused minor damage in various settlements in the Eastern foothills of the Eastern Cordillera. So far, mapping studies based on data obtained from satellite imagery, airphotos and structural field data have outlined the complexity of the AFS and have identified the existence of at least six tectonic basins occurring along the main fault of the system. In 2016, within the framework of activities of the Space Geodesy Research Group of the Geohazards Directorate of the Geological Survey of Colombia (SGC) a program of morphotectonic field studies was carried out in and around the Algeciras pull-apart basin in the Huila Department that led to the opening of two exploratory paleoseismologic trenches. This paper presents the results of the first of these two trenches, located in the center of the Algeciras valley at a place named Lagunillas. It is situated at the rim of a shallow fault-dammed lake formed by an uphill facing scarp generated by the dextral strike-slip displacement of a shortcut fault that traverses the alluvial fill of the basin along its longitudinal axis. The results of the paleoseismologic study prove the occurrence of seven seismic events with magnitudes Mw 7.0 during the Holocene with an average recurrence interval of 1300–1400 years. This constitutes a significant contribution to the study of seismic hazard assessment of the AFS that complements, in a quantitative way, the evidence provided by the data of historic seismicity of this major fault and points out the considerable danger that the fault presents to the country as a whole and in particular to the metropolitan area of Bogota.

Organic sulfur fluxes and geomorphic control of sulfur isotope ratios in rivers

Pyrite oxidation plays a critical role in the relationship between weathering and climate, and its impact on the global carbon cycle has previously been constrained through inversion models utilizing observations of river sulfate (SO42−) and its 34S/32S isotope ratio (δ34SSO4). However, measurements from some rivers have suggested that δ34SSO4 can be substantially impacted by processes such as microbial sulfate reduction and/or sulfur assimilation and cycling, rather than simply reflecting a weighted mixture of lithologic sulfur sources. To study the prevalence and controls on SO42− transformations, we measured dissolved major element concentrations and δ34SSO4 in river water samples from throughout western Iceland. Our analyses focused on samples from a small catchment hosting the Efri Haukadalsá river, a system with relatively uniform and isotopically constrained basaltic bedrock. We also measured sediment δ34S and sulfur speciation using sulfur K-edge X-ray absorption spectroscopy on sediment and vegetation samples from this catchment. Values of dissolved δ34SSO4 in the Efri Haukadalsá ranged from 2.5‰ to 23.7‰ and had a linear relationship with Cl−/SO42− ratios, indicating that SO42− predominantly derived from basalt weathering and meteoric precipitation. The lower δ34SSO4 values were found in fluvial valleys with V-shaped cross sections, while higher values of δ34SSO4 occurred in U-shaped, glacially eroded valleys with thick alluvial fills blanketing the valley floor. Spectroscopic observations identified organic sulfur phases in suspended river sediment, floodplain deposits, and vegetation. Mass balance calculations quantified the organic sulfur flux as less than 10% of SO42− export, and sediment δ34S values were comparable to river δ34SSO4. We interpreted these isotopic and chemical patterns as reflecting differences in the availability of unweathered bedrock across the Efri Haukadalsá catchment, with V-shaped valleys having greater access to fresh sulfide-bearing minerals than alluviated U-shaped valleys; this interpretation is in contrast to one in which the elevated δ34SSO4 values reflect fractionation during sulfur transformations along alluvial reaches. These results validated the application of river inversion models for constraining weathering fluxes and affirmed that pyrite oxidation globally, even in the presence of river sulfur cycling, modulates the abundance of atmospheric carbon dioxide.

Phases of fluvial activity in loess landscapes: Findings from the Sió valley (Transdanubia, central Europe)

Knowledge about the evolution of rivers planforms in loess landscapes of Europe is incomplete. While there are studies on river evolution conducted in loess areas of central and western Europe, the area of Transdanubia (Hungary) remains poorly recognized. Our research work extends previous findings by studying a unique example of alluvial fill maintaining a sedimentary record of river planforms from the Late Pleistocene to the modern times (the Sió Valley, Hungary). The main goal was to determine the main fluvial phases shaping the valley floor. Our study was based on geological, geophysical and remote sensing methods supported by AMS radiocarbon and OSL dating. Three fluvial phases were identified: the formation of large-scale meanders in the Late Peniglacial, transition to an anabranching pattern ~18000 cal. BP, and formation of small-scale meanders ~12700 cal. BP. Their traces are present in the sedimentary record of other rivers evolving in loess landscapes, however, their appearance differs in the timing and type of river planform changes. Phase I was marked by large-scale meanders in the Great Hungarian Plain and Transdanubia, and braided rivers in western Europe. Phase II, marked by transitions to anabranching planforms, or periods of meander cutoffs, appeared first in the Late Pleniglacial in isolated valleys of Transdanubia and northern part of the Pannonian basin. The latest appearance of this phase took place in western Europe owing to a cooler climate and sparse vegetation compared to Transdanubia and Great Hungarian Plain. Phase III, marked by transitions from anabranching rivers to small-scale meanders, started in the Sió valley in conditions of sediment deficit causing river bed incision. Aggradation appeared owing to anthropogenic influence starting ~7000 cal. BP. Its maximum occurred 3000–4000 cal. BP. This triggered phase III in western Europe, and increased deposition of fines, resulting in flow discontinuation in some valleys of Transdanubia.

Evaluating the potential for natural ecosystem recovery in cut-and-fill wetlands: case study of Pietersielieskloof palmiet wetland, South Africa

Cut-and-fill processes, characteristic of many valley-bottom wetlands, present a major challenge to practitioners planning wetland restoration. It is not clear whether the current synchronicity and spatial scale of incision is ‘natural’, and whether these wetlands will naturally recover without intervention during the ‘fill’ phase. The aim of this study, focussed on a Prionium serratum dominated valley-bottom wetland in South Africa, was to evaluate the potential for natural ecosystem recovery in the context of current catchment constraints. Historical rainfall records, aerial photography and a DGPS survey were used to contextualise geomorphic processes. The sedimentology of alluvial fill was investigated by sampling sediment profiles. Samples were analysed for particle size and organic content, five were dated using radiocarbon. Sedimentology and radiocarbon dates were used to ascertain phases of incision and aggradation along wetland reaches. While several phases of incision are represented in the record, none were represented in more than one location, suggesting phases of erosion and subsequent deposition were spatially and temporally segregated. The current synchronous phase of incision across the wetland is therefore unprecedented. Analysis of sedimentation rates indicate that infilling of the gully decreases exponentially over time. Infilling of a 3m deep gully in the Pietersielieskloof wetland under natural catchment conditions is estimated to take ~ 5000 years. Analysis suggests that there has been a reduction in wetland resilience over the last century, and that a change in catchment hydrology and sediment supply has altered this system such that it has crossed a geomorphic threshold.

Erosion and accumulation phases during the last glacial-interglacial cycle: a case study of the terrace system of the Odra and Osobłoga rivers (southern Poland)

In the Odra and Osobloga River valleys in the vicinity of the town of Krapkowice, a system of morphological levels is present which is exceptionally extensive compared to other such systems in southern Poland. The extent, origins and chronology of these levels have been the subject of many controversies. In the light of current research, the terraces, which are situated on average 14.5–13, 12–10, 8–6, 6–4 and 3–1 metres above the river level, were formed in the post-Wartanian interval. The main factor driving their development was erosion, which compensated for the effects of large-scale aggradation that had occurred during the anaglacial phase of the Odranian Glaciation (MIS 8) dated to 261 ±15 ka. The impact of climatic conditions on the trends towards fluvial erosion or accumulation was only of secondary importance. According to OSL dating, successive phases of vertical stabilisation of the valley floor occurred ~118.8 ±8.3, 87.7 ±5.7 and 42.0 ±2.0 ka. In the meantime, erosion intensified, which resulted in the formation of lower terrace levels. During the climatic minimum of the Upper Plenivistulian (21.5 ±1.5 ka), under conditions of the delivery of sheet-wash-generated sediment, the aggradation of distal reaches of the Odra River valley was locally marked. During the Late Vistulian and Holocene, erosive tendencies continued, which were reflected by the fact that the surfaces of subsequent inset alluvial fills were situated ever lower. In the light of dating results, it can be concluded that during the colder periods correlated with the Rederstall Stadial (MIS 5b) and with the Hasselo Stadial (MIS 3), a braided river was present in the river valleys examined, which was most probably accompanied by permafrost. During the Eemian Interglacial (MIS 5e), during the Late Vistulian Interstadials and in the Holocene, it was a meandering river.

Hydrogeochemical and mineralogical factors influencing uranium in background area groundwater wells: Grants, New Mexico

Study regionLower San Mateo Creek Basin (SMCB), New Mexico Study focusGroundwater in the alluvial aquifer of the lower SMCB has a high concentration of total dissolved solids, alkalinity, and dissolved uranium at concentrations above USEPA limits; the source of the water quality was investigated through evaluation of geology, mineralogy, and geochemistry of sediments and groundwater. Surface-expressing bedrock in the larger Basin is dominated by uranium-bearing units such as the Morrison Formation and Dakota Sandstone; erosion of these units contributed sediment to Quaternary alluvium valley fill in the Basin with heterogeneous distribution of uranium. The potential for uranium-bearing alluvial fill to affect the groundwater quality was investigated upgradient of the Grants Reclamation Project (GRP), a former uranium milling facility in the lower SMCB, by hydrogeochemical and geophysical methods. New hydrological insights for the regionUranium is primarily associated with fine-grained materials (clay/silt) and is labile depending upon sediment geochemical conditions. Natural occurrence of uranium minerals in SMCB alluvial sediments here results in concentrations in groundwater greater than the USEPA water quality standards; as such, site-specific standards are established and supported by the findings of this work. Regional groundwater systems, derived from weathered mineralized bedrock, require careful evaluation of water quality to understand background conditions; uranium in groundwater results from hydrogeochemical processes at the interface of fine and coarse grained sediments within the aquifer.

Springwater provenance and flowpath evaluation in Blue Lake, Bonneville basin, Utah

Water in Utah and Nevada is important for agriculture, municipal use, solute transport, and ecosystem preservation. Large spring wetland systems occur on the playa margin of the Bonneville basin, including Blue Lake, 15 km south of the Bonneville Salt Flats on the Utah-Nevada state border. Large spring systems have historically been studied as water budgets don't apparently balance from direct mountain front recharge (Nelson and Mayo, 2014; Gardner and Heilweil, 2014; and others). Blue Lake is no exception; prior studies here have suggested discharge rates 0.04 - 0.05 km3/yr (Louderback and Rhode, 2009), greater than expected for modelled recharge in the surrounding mountain range (0.03 km3/yr). Three hypothesized recharge mechanisms for Blue Lake are tested: mountain-front recharge, interbasinal groundwater flow, and infiltration from historic Lake Bonneville. Remote sensing suggests that a conservative estimate of Blue Lake discharge constitutes 50-64% of modelled mountain-front recharge (Flint et al., 2011). Major ions, δ18O, δ2H, dissolved gases (14C, 3He, 4He, Ne, Ar, Kr, Xe) and trace elements (Sr and 87Sr/86Sr isotopes) comprehensively constrain recharge conditions, water-rock interactions, flowpaths, and groundwater provenance of this large spring system. 14C signatures suggest that Blue Lake discharge has a transit time between 5,600 and 12,200 years, older than that of Fish Springs. Noble gas concentrations in Blue Lake water suggest an elevated recharge temperature greater than 19°C and low salinity, indicating a deep water table and high geothermal gradient in the recharge area. 87Sr/86Sr ratios of playa-margin springs are elevated from that of mid-playa groundwaters, springwaters from the adjacent mountain range, and alluvial fill groundwater from the valley directly south of Blue Lake. Playa-margin spring Sr isotope values (0.713-0.714) are most similar to direct runoff from the Deep Creek Range granodiorite outcrop (>0.713). Interbasinal groundwater flow in combination with mountain-front recharge is best supported by chemical data rather than mountain-front recharge alone or the slow discharge of regional aquifers recharged by lacustrine infiltration from Lake Bonneville.

Clay dispersion: An important factor in channel runoff generation in a semi-arid, loess-covered area with very low rain intensities

Overland flow is usually regarded as an important contributor to storm channel flow. This observation is certainly applicable to dryland areas, where base flow is often irrelevant, particularly in small watersheds. This study examines channel runoff generation in the extensive loess-covered areas that characterize the mildly arid area of western Israel, where the average annual rainfall is 280 mm. Hydrological data point to a peculiar hydrological behavior of the ephemeral streams that experience a high frequency of sporadic channel flow events. Even in extreme rain events, peak discharges are exceptionally low, indicative of a limited contributing area. Hydrographs are characterized by very steep rising and falling limbs, usually representative of saturated areas, located in the vicinity of the runoff recording station. Based on this observation, we advanced the hypothesis that storm runoff originated in the limited area of the active channel, with negligible runoff from the adjoining hillslopes. We argue that a quasi-permanent surface seal, at the top of the alluvial deposit, drastically limits the hydraulic conductivity of the alluvial fill, allowing runoff generation at very low rain intensities. The occurrence of the surface seal is ascribed to the combination of two main factors. A high clay content (~40%), where the dominant clays are smectite and illite, characterized by a laminar structure and a high-water absorption capacity. The swelling of the clay particles considerably reduce the porosity of the alluvial material, allowing runoff generation at very low rain intensities while limiting the depth of water penetration in the channel itself. Data presented fit the concept of “Partial Area Contribution” identified in humid areas. However, the application of this concept to dryland areas is based on completely different reasons.

Spatial and temporal patterns of the upper Pleistocene alluvial fill deposits of the upstream Seine River alluvial plain, la Bassée, France

This study focuses on the contrasted architectural elements of the two late Pleistocene alluvial fill sequences (T1 and T2 attributed to the Late Saalian and Weichselian) of the upstream Seine River alluvial plain, la Bassée, and constitutes the first estimation of the sediment volumes associated to these glacial cycles along this river. Estimations of the incised valley and valley fill spatial distribution are obtained by cokriging of data from 546 wells collected from the French national repository and a digital elevation model (DEM) of the present topography. The well stratigraphic attribution is given by the homogenised geomorphological map of la Bassée.We calculated the sediment volume of eroded material for the T1 sequence to be 1.70 109 m3 and of valley fill deposits to be 1.18 109 m3. This corresponds to a mean thickness of 6.1 m. By contrast, volumes of the T2 sequence are one-third smaller, 1.23 109 m3. The T2 mean thickness is very similar with 5.8 m. The alluvial plain is divided into four reaches representing a typical assemblage of the T1 and T2 incision style and valley fill deposits. Along the downstream reaches (1 - Montereau-Bazoches, 2 - Bazoches-Grisy), the T2 proportion per kilometer are close to the alluvial plain mean value (reach 1: 76%; reach 2: 69%, la Bassée: 71%), the T2 mean incised valley width corresponds to three quarters of the T1 (4 km). In the middle upstream reach (3 - Grisy-Courceroy), the T2 proportion is the largest (92%), the T2 mean incised valley width is the largest and comparable to the T1 (5.5 km). Along the upstream reach (4 - Courceroy-Romilly), the T2 proportion is the lowest (53%) and the T2 incised valley (2 km wide) is enclosed within the T1 incised valley (4 km wide). The rate of exported material, though, is the same for each sequence in any reach (31%).The spatial pattern of the T1-T2 deposits coincides largely with the presence of a knickpoint along the T2 paleothalweg (downstream part of reach 4, slope face 1.5‰), suggesting that the knickpoint had a large influence on the fluvial processes. The toe of the knickpoint (reaches 3 to 1, slope 0.24‰) was the site of the most efficient erosion of the T1 deposits. We propose that the change of slope favored diversion of the channel path. Upstream of the knickpoint face (upstream part of reach 4, thalweg slope 0.34‰), the incision processes were dominated by incision, avulsions or multithreading with limited lateral migration as indicated by the numerous T1 relicts preserved across valley. The sediment export corresponds to a mean incision rate of 40 m/Ma during the last 300 kyr; a fairly low value for the Paris basin, suggesting a low uplift area north of the Massif Central.Such contrast between the fluvial processes during the T1 and T2 sequences is most likely to be looked for in the climatic conditions that existed at the onset of the incision as no local controls for the knickpoint location could be identified such as active faults, substrate heterogeneity, or flow modifications. The lower sediment flux and the knickpoint associated to the T2 sequence suggest a transient river profile and valley incision at the end of the climatic transition associated to the Weichselian (T2).

Late Quaternary Fluvial History of Santa Cruz Island, California, USA

The geologic history of fluvial systems on Santa Cruz Island (SCI) is complex, involving responses to both allogenic and autogenic forcings. During periods of low or lowering sea level, canyons on the island were eroded and sediment was transported off the island onto the exposed marine shelf. When sea level rose, streams aggraded, building a sedimentary wedge that progressed from the shelf upstream into the canyons. This cycle of erosion and aggradation in response to glacial–interglacial sea-level cycles was likely repeated numerous times during the Quaternary, although clear evidence of only the most recent cycle is present in the island’s alluvial sequences. Christy, Sauces, and Pozo Canyons contain thick packages of fine-grained sediments that were deposited as a result of the interaction between autogenic depositional processes and allogenic forcing of continuous base-level rise. Other canyons on the island either have little alluvial fill due to their steep gradients or are filled with coarse-grained, recent (likely late Holocene) alluvium that covers the older alluvial deposits. Differences in the nature and extent of the alluvial exposures on SCI relative to those on neighboring Santa Rosa Island reflect differences in the local topography, geology, and ranching histories of the 2 islands.