Present-day River Research Articles

Challenges in downstream dam passage and the effect of dam removal on Atlantic salmon (Salmo salar) smolt migrations.

Migration is critical for life-cycle completion in diadromous fish species. River connectivity is vital in facilitating these large-scale movement events, but the extent of present-day river fragmentation can interfere with these migrations. Fish passage solutions (FPSs) are commonly implemented with the aim of improving river connectivity. In our study, we investigated the performance of two types of FPSs, spill regimes and complete dam removal, on Atlantic salmon (Salmo salar) smolt migrations. We used acoustic telemetry to monitor migration behavior and passage success of 120 wild smolts released in three different groups/sites: one group with two dams to pass to reach the river mouth, a second group with one dam to pass, and a control group without any barriers to pass (upstream of a recently removed hydroelectric dam). Smolt passage probabilities were similar for the two studied dams (87% and 86%) but showed variation in path choice, delay times, and loss rates. Passage success was influenced by several factors, such as body size, diel period, and water temperature, but not flow. Cumulative passage success to the river mouth was 61%, with most individuals being lost within lentic river stretches, either in the forebays of hydroelectric power stations or in naturally wide river stretches. Within the recently rehabilitated river sections (post dam removal), passage speeds were significantly faster than all other sections of the river (post-rehabilitation x¯ = 56.1 km/day) with significantly faster speeds compared to pre-rehabilitation (pre-x¯ = 28.0 km/day). Our findings provide valuable information on the benefits of dam removal and highlight the need for further rehabilitation measures in upriver reaches where barriers still affect downstream passage.

FLUVIAL SEDIMENTOLOGY OF THE RIVER ETHIOPE SEDIMENTS, NIGER DELTA, SOUTHERN NIGERIA

Despite modern advances in the study of rivers globally, there remains a plethora of work to be done especially in the area of fluvial sedimentology of some present-day river systems. Previous studies on fluvial sedimentology of the sediments of the river Ethiope (in southern Nigeria) are meagre. Grain size analytical methods are indispensable to infer siliciclastic sediments' hydrodynamic conditions, transportation mode(s), and sedimentary environments. Twenty-eight samples (n=28) of the river Ethiope sediments were selected and studied using granulometric analyses (mechanical sieving and pebble morphometric methods). The granulometric analyses results revealed that the obtained sediments were comprised of 82.75% sand, 9.33% gravel, and 7.92% mud. A ternary diagram of sand-gravel-mud shows the sediments are mainly gravelly sand, with few indicating slightly gravelly sand, gravelly muddy sand, muddy sand, and sandy gravel. The grain size statistical analysis shows that the river Ethiope sediments consist of medium to coarse, poorly-sorted to moderately well-sorted, strongly coarse skewed to strongly fine skewed, and very platykurtic to extremely leptokurtic sands. The pebble morphometric analysis revealed that the pebbles range in shape from bladed (B) 22%, compact-bladed (CB) 17%, compact (C) 16%, compact-platy (CP) 16%, compact-elongated (CE) 12%, platy (P) 5%, to elongated (E) 5%. The integration of bivariate plots, ternary diagrams, and C-M patterns plotted for the sediments of the river Ethiope indicated a fluvial environment with sediments characterised by low to moderately high energy that transport sediments of different sizes and grades through saltation, traction, and suspension modes. This study also confirms that sediment transport modes such as saltation, traction, and suspension typify river environments. In general, the existing sedimentologic models derived from grain size analysis of sediments and pebble morphometric methods obtained from modern-day rivers can be applied to better understand transport modes, sedimentary processes, and palaeoenvironments of their ancient counterparts.

The submerged Palaeo-Ems River in the Quaternary stratigraphic context of the German North Sea

During the Late Pleistocene, the Palaeo-Ems system (PES) was one of the major tributary channels feeding the Elbe Palaeovalley (EPV) and formed an important part of the drainage system of the continental northwestern Europe unglaciated hinterland. In this study, a detailed interpretation of high-resolution 2D sub-bottom profiler and Boomer grids were used to map the overall course of the submerged PES channel, as well as its spatial and stratigraphic relationships with other Quaternary geological units in the German North Sea. Mapping of buried tunnel valleys, a lacustrine-fill unit, straight channel structures was also carried out. The sedimentary record of the PES is represented by a low gradient and meandering channel which branched into two major pathways as it approached the EPV western flank. Its upstream to downstream trends, coupled with seaward decreasing gradient and sinuosity, are similar to those visible in present-day river deltas. Both the PES and EPV subsequently formed a combined depositional system, which was successively drowned due to the fast-rising relative sea level that overwhelmed the adaptation capabilities of the joint drainage system. For the first time, this study sheds light onto the PES/EPV morpho-stratigraphic relationships which played a key role in the coastal landscape architecture of the German North Sea since the Late Pleistocene.

Periodicity of Endogenic Events of West Transbaikalia and North Mongolia (Eastern Segment of the Central Asian Foldbelt): U–Pb Age of Detrital Zircon from the Present-Day River Sediments

Periodicity of Endogenic Events of West Transbaikalia and North Mongolia (Eastern Segment of the Central Asian Foldbelt): U–Pb Age of Detrital Zircon from the Present-Day River Sediments

Historical Loss of Groundwater-Dependent Terrestrial Ecosystems in Undrained and Artificially Drained Landscapes in Denmark

Groundwater-dependent terrestrial ecosystems (GWDTE) have been increasingly under threat due to groundwater depletion globally. Over the past 200 years, there has been severe artificial drainage of low-lying areas in Denmark, leading to a gradual loss of GWDTE nature habitat areas. This study explores the spatial-temporal loss of Danish GWDTE using historical topographic maps. We carry out geographic information systems (GIS) overlap analysis between different historical topographic maps with signatures of GWDTE starting from the 19th century up to a present-day river valley bottom map. We then examine the changes in two protected GWDTE habitat types in different periods and hydrologic spatial locations. Results reveal a decrease in the area of GWDTE over the last 200 years. We attribute this to different human interventions, e.g., drainage, that have impacted the low-lying landscape since the early Middle Ages. We further conclude that downstream parts of the river network have been exposed to less GWDTE habitat loss than upstream ones. This indicates that upstream river valleys are more vulnerable to GWDTE decline. Therefore, as a management measure, caution should be exercised when designing these areas for agriculture activities using artificial drainage and groundwater abstraction, since this may lead to further decline. In contrast, there is a higher potential for establishing constructed wetlands or rewetting peatlands to restore balance.

Delineation of Paleochannel and Groundwater Resources in the Khari River Basin, Kachchh using Transient Electromagnetics

Abstract The paleochannels are riverine geomorphic features, which are either buried or shifted due to tectonic, climate, and anthropogenic activities. In general, these channels serve as potential groundwater repositories, and their identification can help in developing potable groundwater sources. Here, the results of transient electromagnetic (TEM) study carried out near and around the Khari river basin of the tectonically active Kachchh intraplate region, western India are presented. The TEM investigations at 22 sites along three traverses have been carried out in the river basin that cut the present-day river channel and its presumed paleochannel. The geoelectric sections of the three profiles decipher information down to 300-350m and suggest a multilayer aquifer system. The sub-surface model infers a deeper confined aquifer at variable depths from 100-150m associated with the river as well as its paleochannel. The paleochannel aquifer is observed to be buried under 15-20m thick unconsolidated sediments that overlie a Mesozoic sandstone layer of the Bhuj Formation. The resistivity sections across the paleochannel suggest a relatively fresh aquifer layer with higher storage capacity. A vertical bedrock offset of 100-120m and the discontinuity of lithostratigraphic layers observed from the study suggest the dominant tectonic control in the formation of the paleochannel that correlated with the location of the reactivated splay of the Katrol Hill Fault.

The influence of subsurface architecture on scour hole formation in the Rhine–Meuse delta, the Netherlands

Abstract Scour holes are common features in deltaic rivers which can destabilise embankments through oversteepening of the river bed. Their development has been studied extensively from the hydraulic perspective, but another important control is the erodibility of the river bed which varies considerably due to thickening of heterogeneous deltaic substrate towards the coast. Therefore, we assessed the influence of delta-scale geological heterogeneity and local subsurface architecture on scour hole formation in addition to the hydrodynamic controls. We (1) created an inventory of 165 scour hole locations in the Rhine–Meuse delta, (2) assessed the hydrodynamic conditions at the locations, (3) extracted geometric characteristics and (4) determined the subsurface architecture from geological data. Central and lower delta branches have 0.6–0.7 scours per km while upper delta branches have less than 0.2. Downstream, 58% of scour holes were related to architectural elements, notably sand bodies from former Holocene channel belts and Early Holocene cohesive beds. These scours have steeper slopes due to higher proportions of cohesive sediments near the river bed. Furthermore, scours related to channel belt sand bodies are limited in downstream length and depth, up to maximum of approximately two times the water depth. From our results, we provide a delta-scale explanatory framework that relates the position of present-day river channels with respect to Pleistocene river deposits and Holocene fluvio-deltaic deposits to scour hole formation. Upstream rivers are incised in Pleistocene deposits showing less local variation in erodibility. The majority of scour holes here relate to engineering works. In central and lower delta branches, geologically inherited heterogeneity of the Holocene substrate at critical depths near the channel bottom adds to anthropogenic induced scours and results in high abundances. This demonstrates that downstream variation in subsurface architecture should be considered as a key control on scour locations and characteristics for management purposes.

New evidence of Neolithic industries from the West Garo Hills, northeastern India

Abstract This paper is based on the field investigations conducted in the West Garo Hills of northeastern India. The work presents a brief report on the Neolithic manufacturing sites of the West Garo Hills. It addresses the various issues of lithic sequence, typological classifications, lithic technology, settlement pattern and site distribution pattern of the known sites and newly discovered sites in the West Garo Hills. Large clusters of Neolithic manufacturing sites have been located on the hilltops and hillslopes close to present-day rivers, streams, natural springs, water channels and modern Garo community habitats. Dolerite raw materials were mostly preferred for fashioning the artefacts, and manufacturing units are found close to the dyke basalt rock sources. Findings of habitation deposits or pottery are very sparse; however, evidence of plant and animal domestication, burial, bone tools, and metal use pose new hypotheses on their use owing to poor preservation. The Palaeolithic evidence and the region's transition to early farming cultures remain vague.

Longest sediment flows yet measured show how major rivers connect efficiently to deep sea

Here we show how major rivers can efficiently connect to the deep-sea, by analysing the longest runout sediment flows (of any type) yet measured in action on Earth. These seafloor turbidity currents originated from the Congo River-mouth, with one flow travelling >1,130 km whilst accelerating from 5.2 to 8.0 m/s. In one year, these turbidity currents eroded 1,338-2,675 [>535-1,070] Mt of sediment from one submarine canyon, equivalent to 19–37 [>7–15] % of annual suspended sediment flux from present-day rivers. It was known earthquakes trigger canyon-flushing flows. We show river-floods also generate canyon-flushing flows, primed by rapid sediment-accumulation at the river-mouth, and sometimes triggered by spring tides weeks to months post-flood. It is demonstrated that strongly erosional turbidity currents self-accelerate, thereby travelling much further, validating a long-proposed theory. These observations explain highly-efficient organic carbon transfer, and have important implications for hazards to seabed cables, or deep-sea impacts of terrestrial climate change.

Can we incorrectly link armouring to damming? A need to promote hypothesis-driven rather than expert-based approaches in fluvial geomorphology

Present-day river forms and processes are in many cases conditioned by the consequences of anthropogenic modifications such as dams, embankments and gravel mining. Fluvial geomorphologists have typically investigated the effects of these human impacts using a so-called expert-based approach, whereby observed association or synchronicity between geomorphological changes and a given, preidentified impact, are interpreted as evidence of causation. This approach has important limitations when the effects of multiple human interventions interact along the same river corridor or overlap with the legacy of natural changes affecting the sediment - water balance. In such situations, the establishment of causal links between channel morphology and single impacts is not as straightforward as commonly assumed and the conclusions are susceptible to ‘confirmation biases’. In this paper we highlight this risk through an assessment of human impacts on the Rhône River within a multi-driver context. The French Rhône is an excellent example of an Anthropocene river impacted by two main development phases during the twentieth century: embankments (1890s–1930s) followed by a series of multiple dams (1950s–1990s). We began by laying out several geomorphologically consistent hypotheses for the geomorphological trajectory of the Rhône over the twentieth century. Next, we tested these hypotheses against grain size data collected in the field in a structured and hypothesis-oriented way. Using this hypothesis-driven and deductive attribution analysis we identified the relative impacts of the different development phases on the present-day grain size distribution and in particular on armouring in the Rhône River, and proposed a hierarchy of dominant drivers of geomorphological change along the Rhone over the last century and a half. Our results led us to conclude that in the case of the Rhône, the effect of dams on armouring was negligible compared to a legacy of natural heritages and embankments.

Seesaw longitudinal–transverse drainage patterns driven by Middle and Late Pleistocene climate cycles in the foreland basin of the south-eastern European Alps

The multi-proxy sediment provenance analysis of two sedimentary successions (GER1 core, 130 m long, and CB core, 103 m long) provides new data for detecting the temporal and spatial variations in fluvial drainage patterns and landscape evolution of the Venetian Plain (NE Italy) in relation to Middle–Late Pleistocene climate fluctuations. Detailed petrographic analyses, compared to present-day river signatures, highlight compositional variations of the sediments, providing crucial information for detecting the depositional histories of the two sites, which lie about 15 km apart, and considering a source-to-sink multi-source context. The successions span from a cold phase older than MIS 9/11 to MIS 1, and are chronostratigraphically well correlated from the MIS 7.3 marine transgression. Despite belonging to the same alluvial system since the Last Glacial Maximum, the GER1 and CB cores show different depositional histories during previous Marine Isotopic Stages, starting at least from MIS 8 (i.e., ca 300–250 ka). A glacial interval older than MIS 9/11, is encountered only within records of the GER1 core. Due to its composition and the particularly coarse grain size, it represents a key element for detecting the depositional dynamics of megafans during past climate oscillations. Conversely, the presence of sediments not fully ascribable to any of the present-day catchment suggests the existence of a trunk paleo-river, longitudinal to the foreland axis, flowing from west to east along the Po Plain and occasionally draining the Venetian Plain. A causal relationship between prograding mechanisms of transverse alluvial megafans and Pleistocene climate variations is detected, taking into account responses of local alluvial systems to glaciers' development, sediment yield from the orogenic hinterland, and sea-level lowstands and highstands. A focus on glacial termination phases allows identification of two additional incised valleys, established at the end of the pre-MIS 9/11 glaciation and of MIS 6.

Impact of the Southern Ecuadorian Andes on Oxygen and Hydrogen Isotopes in Precipitation

Determining how the elevation of the Northern Andes has evolved over time is of paramount importance for understanding the response of the Northern Andes to deformational and geodynamic processes and its role as an orographic barrier for atmospheric vapor transport over geologic time. However, a fundamental requirement when using stable isotope data for paleoaltimetry reconstructions is knowledge about the present-day changes of δ18O and δD with elevation (isotopic lapse rate). This study defines the present-day river isotopic lapse rate near the Equator (∼3°S) based on analysis of δ18O and δD of surface waters collected from streams across the Western Cordillera and the Inter-Andean depression in Southern Ecuador. The results for the two domains show a decrease of δ18O with elevation which fits a linear regression with a slope of −0.18‰/100 m (R2= 0.73,n= 83). However, we establish a present-day lapse rate of −0.15‰/100 m for δ18O (R2= 0.88,n= 19) and -1.4‰/100 m for δD (R2= 0.93,n= 19) from water samples collected along the west facing slopes of the Western Ecuadorian Cordillera which is mainly subject to moisture transport from the Pacific. We argue that this empirical relationship, consistent with those obtained in different tropical areas of the world, can inform stable isotope paleoaltimetry reconstructions in tropical latitudes.

Post-IR IRSL dating the oldest (?) river terrace sediments in the Polish Outer Carpathians: Insights into the landscape evolution

Post-infrared infrared stimulated luminescence (post-IR IRSL or pIRIR) was applied to determine the age of the highest-elevated alluvial sediments in the Polish Outer Carpathians. These sediments are preserved on the western slope of the Dunajec River valley, and are located 150–155 m above the present-day river channel. So far, they have been interpreted as accumulated during the Praetiglian (2.58–2.40 Ma). The pIRIR ages, however, indicate that the sediments were deposited ~2 Ma later. Such results incorporate an important record of river system evolution which has been developing very dynamically since the time of the sediment accommodation. The determined ages introduce average rate of river incision which can be calculated as 0.48–0.50 mm a−1 since the last ~309 ka. This implies that the river section has been fairly tectonically uplifting and brings attention to the rates of uplift in the rest part of the Polish Outer Carpathians. Also, adopting results from the former dating campaign, it is clear that relics of Pleistocene alluvial sediments are predominantly of cold-climate origin, and a scenario for this style of sediment preservation is proposed.

Calibrating fluvial erosion laws and quantifying river response to faulting in Sardinia, Italy

It is now widely accepted that rivers modify their erosion rates in response to variable rock uplift rates, resulting in changes in channel slope that propagate upstream through time. Therefore, present-day river morphology may contain a record of tectonic history. The simple stream power incision model can, in principle, be used to quantify past uplift rates over a variety of spatial and temporal scales. Nonetheless, the erosional model's exponents of area and slope (m and n respectively) and ‘bedrock erodibility’ (k) remain poorly constrained. In this paper, we will use a geologically and geomorphically well constrained Plio-Pleistocene volcanic landscape in central Sardinia, Italy, to calibrate the stream power erosion equation and to investigate the slip rate of faults that have been seismically quiescent in the historic past. By analysing digital elevation models, geological maps and Landsat imagery, we have identified the geomorphic expression of several volcanic features (eruption centres and basaltic lava flows) and three normal faults with 6 to 8 km fault traces within the outcrop. Downstream, river longitudinal profiles show a similar transient response to relative base level fall, probably as a result of relief inversion at the edge of the volcanic outcrop. From measurements of incision, local slope and upstream catchment area across eight different rivers, we calculate n ≈ 1, m = 0.50 ± 0.02 and, using a landscape age from literature of 2.7 Ma, bedrock erodibility k = 0.10 ± 0.04 m(1−2m) Myr−1. There are also knickpoints on rivers upstream of two normal faults, and we used numerical inverse modelling of the longitudinal profiles to predict the slip rate of these faults since 2.7 Ma. The results from the inverse model show that the erosional parameter values derived in this study can produce theoretical longitudinal profiles that closely resemble observed river profiles upstream of the faults. The lowest misfit theoretical longitudinal profiles were generated by a model of temporally discontinuous footwall uplift with consistently low throw rates (<0.1 mm yr−1). The predicted footwall uplift history is similar for the two faults, both showing periods of fault slip and no fault movement since 2.7 Ma.

Detrital Zircon Fission-Track Thermochronology of the Present-Day River Drainage System in the Mt. Kailas Area, Western Tibet: Implications for Multiple Cooling Stages of the Gangdese Magmatic Arc

It is still controversial how the high elevation of the Tibetan Plateau established after the Indian-Asian collision during the Cenozoic. The timing of Gangdese magmatic arc exhumation and uplift history would provide useful message for this disputation. We present six zircon fission-track (ZFT) data from modern river sand in the western Tibet, around the Mt. Kailas, to decipher the long-term exhumation histories of the Gangdese magmatic arc. The data suggests that all the Gangdese magmatic arc rocks experienced rapid cooling during the Eocene (∼46’35 Ma) and Oligocene (∼31’26 Ma). The movement along the north-south trending extensional fault and dextral strike-slip Karakoram fault induced the adjacent rocks exhumed at the Middle Miocene (∼15’16 Ma) and Late Miocene (∼10’11 Ma), respectively. According to the minimum and central AFT ages for each sample, the fastest exhumation rate is about 0.4 km/Ma, with average long-term exhumation rates on the order of ∼0.3 km/Ma since the Oligocene. This result supports the outward growth model for plateau forming, indicating the southern margin of the Gangdese magmatic arc attained high elevation after the Oligocene.

A multidisciplinary overview of the lower Miño River terrace system (NW Iberian Peninsula)

The Miño River basin represents one of the main fluvial catchments draining the Atlantic side of the Iberian Peninsula. The extensive sedimentary deposits of this basin have been documented since the 19th century, but limited research has been undertaken on these features historically, and the Quaternary record of the basin has remained understudied until recently. Research carried out over the last decade on the Spanish margin of the Miño River has enabled more detailed classification and mapping of the main fluvial landforms, as well as numerical (luminescence, electron spin resonance, cosmogenic) dating of some of the deposits associated with Palaeolithic archaeological sites. Here we synthesise the existing Quaternary fluvial record for the basin, and present new geospatial and chronological analyses for the lower catchment area. Our latest examination has enabled the identification of nine fluvial terrace levels and other regionally significant sedimentary features (e.g., alluvial fans) formed in response to tectonics, eustatic changes and associated global climate changes. The chronological data and calculated incision rates indicate that the various fluvial terraces were formed during the Early to Late Pleistocene. Numerous Palaeolithic sites have been found in association with the middle terrace levels (between +40 and +13 m above present-day river level). Primarily, these archaeological sites preserve assemblages that feature large flake Acheulean (LFA) tools, though a number of Middle Palaeolithic sites have also been documented. Direct dating of these sites, together with morphostratigraphic correlations across the terrace system, suggest that the basin has been extensively occupied by human populations during the last 300 thousand years.

Extreme wave events recorded in sedimentary archives of the Geropotamos River (north-central Crete, Greece)

Recent tsunami events have shown that tsunamis may propagate far inland by entering rivers mouths and may cause massive damage along the river banks. However, so far, only a few studies have been conducted such a search for studying tsunami signals in incised valley systems along the Mediterranean coasts although the tsunami hazard is high. The island of Crete is known to have been affected several times by strong tsunamis, e. g., by the AD 365 and the Late Bronze Age (LBA) Santorini tsunamis. The narrow Geropotamos River valley, distinctly incised into local bedrock and located at the northern coast of Crete and fully exposed to the Cretan Sea, was selected as a promising natural setting to search for palaeotsunami signatures in fluvial sedimentary archives. Based on a multi-electrode geoelectrical survey and a set of sediment cores, we investigated the event-geochronostratigraphic record of both the Geropotamos River mouth area and the river valley ca. 1 km upstream by means of sedimentological, geo-chemical, geochronological, geomorphological, and micropalaeontological methods. The sedimentary environment towards the present-day river mouth is dominated by (fluvio-)lagoonal muds since the mid- Holocene. These lagoonal sediments are intersected by six coarse-grained sand layers each representing an extreme wave event (EWE). EWE layers are up to several decimetres thick and are characterized by an allochthonous foraminiferal assemblage comprising shallow marine to open marine species. Also ca. 1 km further upstream, the sedimentary record revealed grain size and microfossil evidence of two high-energy events showing a clear marine imprint. Based on this, we suggest inundation from the seaside that reached minimum 1 km inland and left EWE signatures in a presently inactive external bank position of the Geropotamos River. Considering the sedimentary characteristics, the local wind and wave climate of the Cretan Sea, and the overall geomorphological setting, we interpret these EWE layers as tsunami- related. A major hiatus identified in the Geropotamos River mouth sediments seems to be related to the LBA Santorini tsunami as can be inferred based on local age-depth relations. The LBA tsunami is known to have severely hit the northern coast of Crete. However, the hiatus may also reflect changes in the subsidence rate and the local accommodation space architecture. The youngest EWE signal in the Geropotamos River archive appears to have been caused by the AD 365 tsunami event. Candidate deposits for both tsunami deposits were identified ca. 1 km further inland. Evidence of EWE impact documents channelling and acceleration effects of intruding water masses caused by the narrow and steeply incised Geropotamos River valley in an upstream direction. Further geochronological studies based on OSL dating are necessary for a reliable age control of these EWE candidate layers.

Specific Features of the Formation of Water Flow and Pollution Export from Agricultural, Forest, and Urbanized Landscapes

The overland flow from different landscapes during flood season and its variations under the effect of agrotechnical activities and climate in the southern Russian Plain have been evaluated based on the analysis of data from water balance plots. The present-day overland flow in forest steppe and steppe regions has been shown to be several times less than that 50–70 years ago. At the same time, the infiltration runoff (perched-water and groundwater runoff) has increased. The structure of the present-day river runoff and the contribution of the overland flow, the runoff from a hydrographic network area, and the runoff of infiltration origin in individual natural zones to the formation of the river runoff have been determined. The export of biogens in the Volga and Don basins, including that in urbanized areas, has been calculated based on the analysis of water samples in different landscape zones. Such export from urbanized landscapes has been shown to contribute little to the total export of biogens in these basins. The volume of biogens of diffuse origin considerably exceeds that of biogens carried out by wastewater. However, this relationship may change for large rivers and reservoirs since river water carrying diffuse biogens mostly moves for a long time before it reaches the main river partially self-purified, whereas wastewater biogens mainly directly flow into large rivers.

Synoptic variations of residual currents in the Huanghe (Yellow River)-derived distal mud patch off the Shandong Peninsula: Implications for long-term sediment transport

Huanghe (Yellow River) is one of the largest contributors of global terrestrial materials to the sea. A large amount of the Huanghe-derived sediment are responsible for forming not only the nearshore delta depositional system, but also a distal mud patch off the Shandong Peninsula, some 350 km east of the present-day river mouth. To better understand the spatial and temporal variations of sediment dynamics in the distal mud area, two field surveys employing the phase-averaging method (PAM) were conducted in September 2017 and January 2018, respectively along a 108-km East-West transect across the mud patch. The residual currents derived from the PAM surveys were featured by the southward current in the nearshore area and the northward current in the offshore region in summer, while a unidirectional current transported from south to north across the entire transit line in winter. These patterns, which deviate from the traditional understandings of circulations in magnitude and even in direction, are believed to result from episodic wind reversals and associated strengthening and weakening sea level pressure gradients. The northerly wind episodes in summer, works hand-in-hand with sea-level tilt by preceding southerly wind, to enhance the water exchange between the Bohai Sea and the Yellow Sea by intensifying both the southward flow along the coast and the northward flow of the South Yellow Sea water further offshore. In winter, episodic southerly wind causes relaxation or even reversal of the prevailing winter northerly wind that created the northward sea-level tilt in the Bohai Sea and the Yellow Sea, effectively enhances the South Yellow Sea water transport to the North Yellow Sea and the Bohai Sea. Consequently, a considerable amount of sediment previously exported out of the Bohai Sea to the mud patch actually return with these backflows. Since the synoptic time scale oscillations in the Bohai and Yellow seas are considerably frequent throughout summer and winter, the cumulative effects on the long-term sediment transport need to be further investigated. The observations in this study provide a base for new understanding of how episodic weather events impact the regional circulation and sediment dynamics on the Huanghe-derived distal mud patch.

Gombore II (Melka Kunture, Ethiopia): A new approach to formation processes and spatial patterns of an Early Pleistocene Acheulean site

To assess the integrity of Pleistocene archaeological sites is crucial in the analysis of human behaviour. Most of the Early Palaeolithic sites are in active fluvial environments where it is necessary to understand the degree of sedimentary disturbance. The analysis of the formation processes through geoarchaeological and geostatistical techniques offers new tools to evaluate if the archaeological assemblage is in autochthonous or allochthonous position. Gombore II, ≈850 Ka, within the archaeological and paleontological complex of Melka Kunture (Ethiopia), extends over estimated 1000 m2 and yielded a large number of Acheulean artefacts, fossil mammal bones and two fossil hominin remains. The geomorphological setting and deposition patterns of high-energy sedimentation in a fluvial channelized environment are similar to those of many other Early Palaeolithic African sites. This is traditionally described as producing a disturbed record, with the fortuitous association of faunal remains and lithic industry driven by fluvial processes. To assess this hypothesis, we analyse here the formation processes and the spatial patterning of the remains. We apply geoarchaeological (orientation and fabric) and spatial tests (density, grouping, k-means) to the mapped archaeological surfaces and to a present-day fluvial surface. We observe substantial differences in geoarchaeological features and spatial patterning between the archaeological record (lithic materials and faunal remains) and the natural clasts of both the archaeological deposit and the bed of the present-day river. This suggests different depositional processes or temporal events. We conclude that the remains produced by the hominins did not haphazardly accumulate after extensive erosion and re-sedimentation. They rather preserve a reasonable degree of taphonomic and spatial integrity and are possibly representative of different activities.