Search

For decades, it has been unclear as to how the world's first cities, in southern Mesopotamia, not only arose in a fluvial environment but also how this environment changed. This paper seeks to understand the long-term fluvial history of the region around Uruk, a major early city, in relation to water-human interactions. This paper applies geomorphological, historical and archaeological approaches and reveals that the Uruk region in southern Mesopotamia had been under the influence of freshwater fluvial environment since the early Holocene. It further demonstrates how canals and long-term human activities since the mid Holocene have been superimposed on the natural river channel patterns. Fieldwork has been conducted to ground-truth features identified applying remote sensing techniques. Five sediment cores were analysed to elucidate palaeoenvironmental changes. Radiocarbon ages for organic samples suggest that the oldest sediment layers, at a depth of 12.5 m, are from the Early Holocene, while results from diatom analyses imply that the whole sediment column was deposited in a freshwater environment. Intensive networks of palaeochannels and archaeological sites within the study area have been reconstructed and these networks have been divided into four different time intervals based on changes in channel courses. The first is from the early 4th to the late 1st millennium BCE; the second is from the late 1st millennium BCE to the middle 2nd millennium CE; the third lasted from after the Islamic period until the 1980s; the fourth is from the 1980s until the present. Key results include evidence for freshwater environments and favourable settlement conditions had already formed by the 8th millennium BCE. The favourable settlement environment resulted in stable (long-lived) canals between the 4th millennium BCE and 1st millennium CE. A significant settlement and irrigation expansion occurred in the early 1st millennium CE. Major abandonment ensued in the late 1st millennium CE and lasted until the mid 2nd millennium CE. [ABSTRACT FROM AUTHOR]

This paper develops a numerical model for the analysis of sediment transport in irrigated channels. The overland flow equation considers infiltration into the subsurface. The model solves Saint Venant and sediment continuity equations to analyze overland flow and sediment transport, while Richard's equation is used to analyze subsurface flow. Saint Venant and sediment continuity equations are solved using MacCormack-scheme-based finite-difference method and Richard's equation is solved using a mass conservative fully implicit finite-difference method. The model is validated by comparing model-predicted aggradation with experimental observations. The model is used to study the effect of infiltration on aggradation of a channel bed during sediment-laden flows. It was observed that infiltration significantly affects and alters the aggradation phenomenon and aggradation considering infiltration is found to be higher than that without considering infiltration. Among the different unsaturated soil parameters, the saturated hydraulic conductivity and initial dryness of the soil were found to be the main factors affecting the sediment transport in irrigated channels. [ABSTRACT FROM AUTHOR]

Typically, the analysis and design of fluvial flood defence schemes is based on a single N year extreme flow event using a single survey of the river channel and flood plains. Adopting this approach assumes that the channel capacity is identical for all subsequent N year events. If one assumes that the typical design life for a flood defence scheme is of the order of 50 years, then such an approach is flawed because river channel morphology, and hence flood conveyance, may change considerably over this time scale. Therefore, to provide a more robust estimate of future flood inundation, a sensitivity analysis of these changes should be undertaken. This paper proposes a modeling methodology that combines a stochastic model, for estimating streamflow throughout the design period, and a 1D sediment transport model (HEC-RAS), to enable this sensitivity to be included in flood inundation modeling and defence scheme design. The methodology is demonstrated through conceptual implementation to evaluate the change in water surface elevation (WSE) along an alluvial river (River Caldew, England) reach after 50 years of sediment transport. Changes in WSE are assessed when the reach is natural (no flood defences) and modified (with idealized flood defences). Results show that the construction of the flood defence scheme does not alter the overall morphological pattern of the reach but can significantly increase (260%) local aggradation. Additionally, 50 years of morphological change have the potential to increase WSE such that high flows, previously confined within the channel, can overtop the banks and become flood events; and that, the standard freeboard levels of the flood defence scheme may be insufficient to prevent overtopping when morphological change is considered. The method can be considered as a semiquantitative modeling methodology to account for the sediment-related sensitivity of flood risk management; and provides valuable insights into the potential magnitude that this has on future flood inundation. [ABSTRACT FROM AUTHOR]

Abstract The arrival of Europeans in the New World, and the rapid deforestation for agriculture and settlement that shortly followed, resulted in large and rapid increases in soil erosion and sediment aggradation. In Australia, as in parts of North America and other New World sites, this event dates to the last several centuries of the Common Era, a period for which we have surprisingly little historical or scientific data capable of informing debates on the impact of European colonisation on soils and erosion. These research themes and debates were central to the work of Karl Butzer, who undertook field and archival research in Australia to test existing ideas of the timing and magnitude of landscape response to colonisation. In this paper, the work and impact of Butzer in Australia is reviewed as part of ongoing scholarship on the impact of colonisation on landscapes to which Butzer contributed (Butzer and Helgren, 2005). Geomorphic records of soil erosion and historical accounts of land use and impact from eastern Australia show that European settlement, land clearing, and agriculture resulted in rapid, and often severe, increases in erosion in the late eighteenth and early nineteenth centuries, mirroring the experience of anthropogenic soil loss experienced in other parts of the world. It is postulated that the geomorphic response to European settlement in Australia is more complex than researchers have previously noted that, in some contexts, continues to influence catchment processes decades and centuries after colonisation. [ABSTRACT FROM AUTHOR]

The quantity of suspended sediment in an estuary is regulated either by transport, where energy or time needed to suspend sediment is limiting, or by supply, where the quantity of erodible sediment is limiting. This paper presents a hypothesis that suspended-sediment concentration (SSC) in estuaries can suddenly decrease when the threshold from transport to supply regulation is crossed as an erodible sediment pool is depleted. This study was motivated by a statistically significant 36% step decrease in SSC in San Francisco Bay from water years 1991-1998 to 1999-2007. A quantitative conceptual model of an estuary with an erodible sediment pool and transport or supply regulation of sediment transport is developed. Model results confirm that, if the regulation threshold was crossed in 1999, SSC would decrease rapidly after water year 1999 as observed. Estuaries with a similar history of a depositional sediment pulse followed by erosion may experience sudden clearing. [ABSTRACT FROM AUTHOR]

The Wairau River is one of the most engineered river systems in the Marlborough region. Historical river control works, particularly the blocking of the Opawa Breach and the subsequent construction of the Wairau Diversion, have had a significant impact on flow and sediment transport in the Lower Wairau. Currently, the Lower Wairau is aggrading as a result of an increase in sediment concentration and a decrease in the duration for which flows exceed the critical threshold to keep sediment entrained. These changes are attributable to the substantial reduction in flow in the Lower Wairau that is a result of over half the total Wairau flow now passing down the Wairau Diversion. During the previous period, following the blocking of the Opawa Breach up until the construction of the Wairau Diversion, the opposite occurred, with significant increases in flow down the Lower Wairau and widespread erosion. This paper reviews the geomorphic setting of the Wairau deposition zone and the impacts of the Opawa Breach and the Wairau Diversion on the Lower Wairau by examining historical cross-sectional data (1927 to present) and assessing the distinct flow and sediment transport regimes driving the aggradation and erosion. The effects on channel slope, width and bed level through the different regimes are also examined, along with their implications for sustainable management decisions. [ABSTRACT FROM AUTHOR]

This paper presents a systematic review and correlation of river terraces identified in 44 river catchments in the North Island, New Zealand, to assess river behaviour over the last c. 30 000 yr. This provides a comprehensive foundation from which to assess controls on river dynamics over this critical period of landscape development from the last glacial through to the present interglacial. From this review, an integrated framework for elucidating fluvial responses to environmental change is proposed. Sensitivity of river systems to climate change is variable over this time-scale. Substantial sediment inputs to valley floors during the last glacial reduced sensitivity to subtle climatic shifts. Sensitivity increased during the Holocene, with terraces forming in response to episodes of storminess. Tectonics and volcanic activity may both override climatic forcing to cause either cutting or filling of the valley floor. [ABSTRACT FROM AUTHOR]

We examined the stratigraphy of alluvial fans formed at the steep range front of the Southern Alps at Te Taho, on the north bank of the Whataroa River in central West Coast, South Island, New Zealand. The range front coincides with the Alpine Fault, an Australian‐Pacific plate boundary fault, which produces regular earthquakes. Our study of range front fans revealed aggradation at 100‐ to 300‐year intervals. Radiocarbon ages and soil residence times (SRTs) estimated by a quantitative profile development index allowed us to elucidate the characteristics of four episodes of aggradation since 1000 CE. We postulate a repeating mode of fan behaviour (fan response cycle [FRC]) linked to earthquake cycles via earthquake‐triggered landslides. FRCs are characterised by short response time (aggradation followed by incision) and a long phase when channels are entrenched and fan surfaces are stable (persistence time). Currently, the Te Taho and Whataroa River fans are in the latter phase. The four episodes of fan building we determined from an OxCal sequence model correlate to Alpine Fault earthquakes (or other subsidiary events) and support prior landscape evolution studies indicating ≥M7.5 earthquakes as the main driver of episodic sedimentation. Our findings are consistent with other historic non‐earthquake events on the West Coast but indicate faster responses than other earthquake sites in New Zealand and elsewhere where rainfall and stream gradients (the basis for stream power) are lower. Judging from the thickness of fan deposits and the short response times, we conclude that pastoral farming (current land‐use) on the fans and probably across much of the Whataroa River fan would be impossible for several decades after a major earthquake. The sustainability of regional tourism and agriculture is at risk, more so because of the vulnerability of the single through road in the region (State Highway 6). [ABSTRACT FROM AUTHOR]

This paper reviews the diversity of relationships between river evolution and karstogenesis. It also underlines the fundamental role of numerical dating methods (e.g. cosmogenic nuclides) applied to sedimentary sequences in tiered cave passages as they have provided new insights into these complex interactions. Although karst terrain is widespread worldwide, we focus on European karst catchments, where the sedimentary records are especially well preserved. We review the recent dating of fluvial sediments and speleothems, to examine the timing of karstification, incision and deposition in cave levels. The most complete alluvial records occur in tectonically uplifted high mountains where some of the oldest sediment fills date to the Miocene. Evidence indicates that not only uplift, but also climatic conditions and fluvial dynamics (e.g. knickpoint retreat, increased channel flow and/or sediment load, and stream piracies) can play a major role in speleogenesis and geomorphological evolution. In evaporite rocks, speleogenesis is characterized by rapid dissolution and subsidence. In European catchments, gypsum cave development largely occurred during cold climate periods, while limestone caves formed during warm interglacial or interstadial phases. Our synthesis is used to propose four models of fluvial and karst evolution, and highlight perspectives for further research. [ABSTRACT FROM AUTHOR]

Ancient Maya wetland agriculture has been studied since the 1960s, but there are still too few multiproxy excavations to adequately portray this phenomenon. This paper builds on multidisciplinary and multiproxy research near the ancient Maya site of Chan Cahal, near Blue Creek, Belize, with two new excavations, four new cores, 22 new AMS dates, and a new review of the site's water chemistry and archeology. A long-term study of the site's water chemistry has shown its pervasive impacts on wetland formation and limits to agricultural uses. We argue that a reservoir in the midst of the wetlands became inundated above a paleosol during the earliest Maya occupation in the Middle Preclassic, was dredged out during a time of population expansion sometime in the Late Preclassic to Early Classic, and was filled again with ‘Maya clay’ during and after the Classic period. Vibracores through two wetland fields and one canal corroborated our earlier model of Preclassic through Classic inundation of a paleosol by peat formation, flood sands, clay deposition, and gypsum precipitation. These cores produced new evidence for field and canal construction starting in the Late Preclassic. A later addition was a platform added at the very end or after the main occupation ended in the Terminal Classic. A new excavation at Sayap Ha on the Coastal Plain wetlands 7 m lower than Chan Cahal experienced a similar history of inundation and aggradation. Here, Maya farmers reclaimed these wetlands with canals and raised fields late in the Late or Terminal Classic and abandoned them soon thereafter. The Sayap Hah fields thus parallel the BOP fields 9 km south since both were built and abandoned at a late stage. As a comparison with other wetland field studies, the stable C isotopes of soil humin through soil profiles showed a strong Classic period to present influence of C 4 species at Sayap Hah and an increase in C 4 inputs in the upper 25 cm at both Sayap Hah and the Chan Cahal platform, perhaps due to tropical pasture grasses prevalent here for the last 50 years. [ABSTRACT FROM AUTHOR]

Using the seismic profiles and analog modeling, this paper addresses the salt structures in the M and B blocks in the Southern Precaspian Basin. The salt structural features, the formation mechanism and the controlling factors of structural deformation are investigated and discussed systematically. The interpretation of the seismic profiles shows that typical salt-related structures include salt wall, (flip-flop) salt diapir, salt roller, salt pillow (dome), salt weld, salt withdrawal minibasin and drag structure (or drape fold). In addition, model results demonstrate that the gravity spreading driven by progradation and aggradation is probably the primary factor in controlling the formation of the salt structures in the research area. Due to the differential loading driven by progradation, passive salt diapir developed near the progradational front followed by the formation of intrasalt withdrawal minibasin bounded by two salt diapirs, and secondary reactive triangle salt diapir or salt pillow might form within the intrasalt withdrawal minibasin. Model results also indicate that the pattern of the subsalt basement has important influence on the formation and evolution of salt structures. Salt diapirs primarily developed along the margin of the subsalt uplift basement, where high shear deformation was induced by differential sedimentary loading between the uplift area and the slope area. [ABSTRACT FROM AUTHOR]

We describe the soils, stratigraphy, distribution and ages of alluvial fans in the North Carolina Piedmont to understand factors contributing to their formation in this low relief, humid‐temperate setting. We find that those contributing factors (i.e., contributing source basin area and rock type; and climate) parallel, but do not precisely mimic those of arid and semi‐arid climate fans. Digital elevation model (DEM) and field observations revealed that about 25% of 209 mapped basin outlets contain alluvial fans. Fans are all < 0.01 km2 in area, with a contributing basin area threshold of ~0.1km2 above which fans are not commonly observed. Fans issuing from basins underlain by more easily weatherable and erodible argillites are generally larger than those that contain more resistant rhyolites. Radiocarbon dates of fan sediments are dominantly late Holocene (13 ages < 3000 ka; two ages between 7000–10,000 ka), with notably no late‐Pleistocene aged fans. We thus infer that fan aggradation is predicated on watershed characteristics – and climates – that favor high sediment supply to discharge ratios. Detailed soil and sediment descriptions from pits located on 15 fans reveal that approximately half exhibit cumulic over‐thickened soil profiles forming dominantly in sheetflood deposits, providing evidence of ongoing, relatively slow deposition. Low iron ratios measured from those fan soils suggests recycling of more developed soils from basin hillslopes. Occasional matrix‐supported debris flow deposits in some fans provide evidence of much less common, rapid (0.5–1 m) fan accumulation. While acknowledging a paucity of detailed climate records in the Piedmont precludes identifying precise mechanistic relationships, our mapping, soils and sedimentologic data suggest that climates favoring modest hillslope erosion without extreme runoff lead to periods of fan aggradation in this transport‐limited landscape. Overall, this work documents significant rock‐type modulated, climate‐driven Holocene landscape evolution in the south‐eastern US Piedmont predating legacy sediment erosion and deposition that has gone largely underappreciated. [ABSTRACT FROM AUTHOR]

The headwaters of fluvial systems on the East European Plain between the boundaries of the Marine Isotope Stage 2 (MIS 2) and MIS 6 glaciations evolved during the last 150,000 years. At least three main events of high surface runoff caused intensive erosion: at the end of MIS 6, at the end of MIS 2 and in the Middle Holocene. Erosion developed in the territory with variable resistance of geological substrate, from hard-to-erode tills to weak sandy deposits. All erosional features in moraines formed in the pre-Holocene time. Even relatively large forms, such as balkas (small dry valleys), have not yet reached concave longitudinal profiles. A general tendency of their development was deepening. Short episodes of incision occurring during climatic events with increased water flow alternated with long periods of stabilization. Sand-covered areas are most favorable for linear erosion. The gullies formed in the Middle Holocene developed concave longitudinal profiles. The diversity of catchment areas, initial slope inclinations and sediment properties causing their resistance to erosion led to greater differences in the relief features and evolution of the upper reaches of the fluvial systems within the MIS 6 glaciation area compared to the more uniform landscape conditions in the extraglacial regions. [ABSTRACT FROM AUTHOR]

Abstract: This paper is focused on the changes over the past 50years in the channel beds of seven major rivers that are part of the Siret drainage basin located in eastern Romania. Each river has been subjected to various degrees of human intervention, assessed based on the morphological quality index (MQI): two rivers (Siret and Bistrita) have undergone complex human interventions on 40 and 60%, respectively, of their channel lengths, two rivers (Suceava and Moldova) are 30% artificial, and the remaining three rivers (Trotus, Putna, and Buzău) have a good or moderate MQI. The changes occurring in channel beds and data on water discharge (Q w) and sediment load (Q s) were assessed simultaneously for the last 50years. The bedload was indirectly evaluated as ranging between 5 and 15% of the total sediment load. It was determined that incision was prevalent among the processes acting on the channel-bed sections under investigation (representing 62% of the altered area and changing bed level by between −0.25 and −2.70m), with aggradation accounting for the remaining 38% (causing changes ranging from +0.15 to +1.25m). The magnitude of the processes (incision or aggradation) for sections with an MQI<0.3 was four times higher than for sections with a moderate or good MQI (>0.3). The pattern of change in the channel-bed processes between 1960 and 2010 for all river categories was as follows: a low rate of incision from 1960 to 1979, followed by a higher rate of incision from 1980 to 1989, and finally a tendency of recovery toward the river's initial state, characterised by a decrease in the incision rate or slight aggradation after 1990. The variable that exhibited the strongest response to climate conditions was water discharge (Q w), whereas the sediment load (Q s) was highly responsive to both climatic signals and anthropogenic factors. The sediment load has been instrumental in the adjustments of the channel beds by maintaining a balance between the two controlling factors, nature and man. [Copyright &y& Elsevier]

Abstract: This paper provides the first detailed, multi-proxy environmental record for the character of Lateglacial conditions in the lowest Pleistocene terrace of the main valley floor of the River Trent at Holme Pierrepont near Nottingham. The analysis of pollen, plant, insect and mollusc remains preserved within organic channels near the base of the terrace, named the Holme Pierrepont Sand and Gravel by the British Geological Survey (historically known as the Floodplain Terrace), provided evidence of cool, though not fully arctic climatic conditions and a largely treeless landscape, roamed by large herbivores. Radiocarbon dating indicates that these sediments were deposited during the Loch Lomond Stadial (Younger Dryas GS-1). Comparison of these dates from Holme Pierrepont with those from morphostratigraphically similar deposits in the wider Trent catchment suggests that the Holme Pierrepont Sand and Gravel may have been laid down in two separate pulses of braidplain aggradation either side of the ‘Last Glacial Maximum’. It has been demonstrated from historical documentation and riverine archaeological evidence that the middle Trent has been particularly sensitive to changing flood frequency and magnitude associated with climatic oscillations during the late Holocene; this study demonstrates that such sensitivity appears to extend back into the late Pleistocene. The timing of fluvial aggradation recorded at Holme Pierrepont agrees broadly with that recorded from other sites across England and north-west Europe. [Copyright &y& Elsevier]

Abstract: This paper begins to differentiate the major drivers and chronology of erosion and aggradation in the fluvial and fluviokarst landscapes of the southern and central Maya Lowlands. We synthesize past research on erosion and aggradation and add new data from water, soils, radiocarbon dating, and archaeology to study the quantity, timing, and causes of aggradation in regional landscape depressions. Geomorphic findings come from many excavations across a landscape gradient from upland valleys, karst sinks, and fans into the coastal plain floodplains and depressions. Findings from water chemistry show that sources in the uplands have low quantities of dissolved ions but water in the coastal plains has high amounts of dissolved ions, often nearly saturated in calcium and sulfate. We found significant geomorphic complexity in the general trends in upland karst sinks. In a few instances, sediments preserve Late Pleistocene paleosols, buried 2–3 m, though many more have distinct middle to late Holocene paleosols, buried 1–2 m, after c. 2300 BP (Maya Early to Late Preclassic). From 2300–1100 BP (Late Preclassic to Classic Periods), the landscape aggraded from five main mechanisms: river flooding, climatic instability, accelerated erosion, ancient Maya landscape manipulation, and gypsum precipitation from a rise in a water table nearly saturated in calcium and sulfate ions. Evidence exists for two or three high magnitude floods, possibly driven by hurricanes. Moreover, lake-core and geophysical studies from the Petén Lakes region have shown high rates of deposition of silicate clays (‘Maya Clays’) starting and peaking during the Maya Preclassic and continuing to be high through the Late Classic. The main driver on upland karst depressions, the Petén lakes, upland valleys, and fans was accelerated soil erosion, but water table rise, probably driven by sea-level rise, was the main driver on the wetlands of the coastal plain because the aggraded sediments here are dominantly composed of gypsum, precipitated from the groundwater. This latter mechanism represents a little recognized mechanism of aggradation over a large region. These large scale environmental changes occurred during periods of intensive ancient Maya land use and climatic instability, both of which may have contributed to erosion by increasing runoff. Despite these geomorphic changes, ancient Maya farmers adapted in several key cases. [Copyright &y& Elsevier]

Rivers flowing from glacier-clad Quaternary volcanoes in southwestern British Columbia have high sediment loads and anabranching and braided planforms. Their floodplains aggrade in response to recurrent large landslides on the volcanoes and to advance of glaciers during periods of climate cooling. In this paper, we document channel instability and aggradation during the last 200 years in lower Cheakamus River valley. Cheakamus River derives much of its flow and nearly all of its sediment from the Mount Garibaldi massif, which includes a number of volcanic centres dominated by Mount Garibaldi volcano. Stratigraphic analysis and radiocarbon and dendrochronological dating of recent floodplain sediments at North Vancouver Outdoor School in Cheakamus Valley show that Cheakamus River aggraded its floodplain about 1–2 m and buried a valley-floor forest in the early or mid 1800s. The aggradation was probably caused by a large (ca. 15–25×106 m3) landslide from the flank of Mount Garibaldi, 15 km north of our study site, in 1855 or 1856. Examination of historical aerial photographs dating back to 1947 indicates that channel instability triggered by this event persisted until the river was dyked in the late 1950s. Our observations are consistent with data from many other mountain areas that suggest rivers with large, but highly variable sediment loads may rapidly aggrade their floodplains following a large spike in sediment supply. Channel instability may persist for decades to centuries after the triggering event. [Copyright &y& Elsevier]

Most discontinuous gullies overlying the almost tabular sedimentary strata of the Moldavian Plateau, Eastern Romania exhibit gullying by gully head advance and aggradation of the gully basin floor. According to Heede [Z. Geomorphol. N. F. 18 (1974) 260] the shape factor of the gullies, relating maximum to minimum depth, expresses both channel shape and hydraulic efficiency of channels.Based on a geomorphic approach, this paper shows that a better understanding of gully hydraulic efficiency may be assessed by another shape factor obtained by relating present to filled gully cross-section. A value of 1.0 for this shape factor, Sp/Sf, represents the threshold of hydraulic efficiency. Strong relationships were established between the shape factor and gully length. In addition, an appropriate substitute expressed as the ratio of gully bottom width (Wb) to gully top width (Wt) was found, which is easy to apply and very fast to determine in the field. [Copyright &y& Elsevier]

We present the results of laboratory experiments specifically designed to quantify the height and migration rate of an aggrading gravel front. The experiments were performed in sediment feed mode with constant water and sediment supply. Particular care was put into the experimental procedure and the methods to determine the quantities of interest to ensure reproducibility of the results. The celerity and height of an aggradation front were modeled as functions of the Froude number of the flow for the initial bed profile and of a load ratio defined as the ratio of the sediment feed rate to the transport capacity of the flow for the initial bed. The two control parameters (Froude number and load ratio) also determined the translational or dispersive nature of a sediment front. Two predictors were provided to estimate the dimensionless height and celerity of an aggradation front. The former was an increasing function of the load ratio and a decreasing function of the Froude number, and the latter was roughly proportional to the squared Froude number and had no evident relationship with the load ratio. The present results are of interest for scholars and practitioners needing to determine the key properties of swift gravel fronts as those developing, for example, during flash floods. [ABSTRACT FROM AUTHOR]

In this study, we captured how a river channel responds to a sediment pulse originating from a dam removal using multiple lines of evidence derived from streamflow gages along the Patapsco River, Maryland, USA. Gages captured characteristics of the sediment pulse, including travel times of its leading edge (~7.8 km yr−1) and peak (~2.6 km yr−1) and suggest both translation and increasing dispersion. The pulse also changed local hydraulics and energy conditions, increasing flow velocities and Froude number, due to bed fining, homogenization and/or slope adjustment. Immediately downstream of the dam, recovery to pre‐pulse conditions occurred within the year, but farther downstream recovery was slower, with the tail of the sediment pulse working through the lower river by the end of the study 7 years later. The patterns and timing of channel change associated with the sediment pulse were not driven by large flow or suspended sediment‐transporting events, with change mostly occurring during lower flows. This suggests pulse mobility was controlled by process‐factors largely independent of high flow. In contrast, persistent changes occurred to out‐of‐channel flooding dynamics. Stage associated with flooding increased during the arrival of the sediment pulse, 1 to 2 years after dam removal, suggesting persistent sediment deposition at the channel margins and nearby floodplain. This resulted in National Weather Service‐indicated flood stages being attained by 3–43% smaller discharges compared to earlier in the study period. This study captured a two‐signal response from the sediment pulse: (1) short‐ to medium‐term (weeks to months) translation and dispersion within the channel, resulting in aggradation and recovery of bed elevations and changing local hydraulics; and (2) dispersion and persistent longer‐term (years) effects of sediment deposition on overbank surfaces. This study further demonstrated the utility of US Geological Survey gage data to quantify geomorphic change, increase temporal resolution, and provide insights into trajectories of change over varying spatial and temporal scales. [ABSTRACT FROM AUTHOR]

The Salt River and Verde River watersheds provide downstream metropolitan Phoenix, Arizona, USA with much of its water supply, and this paper explains how these rivers integrated in an extensional tectonic setting. Near the end of the Pliocene, segments of the proto-Salt and proto-Verde watersheds of central Arizona consisted of local drainage networks supplying water and sediment into internally drained basins, including depressions occupied by late Pliocene natural lakes occupying the Verde Valley and Tonto basins. A key location, the lower Verde River valley (LVRV), is where the modern-day drainages of the Salt and Verde now meet downstream of these Pliocene lakes. At the time of the Nomlaki tuff deposition ~3.3 Ma, a condition of sediment overfill existed in the LVRV, although there was no exoreic drainage and a playa was still maintained. A fanglomerate unit, named here the Rolls formation, spilled over a bedrock sill and an alluvial-fan ramp transported sediment into the Higley Basin that underlies the eastern part of metropolitan Phoenix. Lithologies of preserved remnants of this Pliocene alluvial-fan system match well cuttings of buried sediment in the Higley Basin with a cosmogenic burial isochron age of 3.90 ± 0.70 Ma. Based on cosmogenic burial isochron ages, ancestral Salt River gravels started depositing on top of this fan ramp between 2.8 and 2.2 Ma. Deposition of Salt and Verde river gravels in the Higley Basin continued for ~2 million years and eventually led to an aggradational piracy event that overtopped a bedrock ridge immediately east of Phoenix's Sky Harbor Airport. A cosmogenic burial age of 460 ± 23 ka is a rough maximum age for this river avulsion that relocated the Salt River into the Luke Basin that underlies western metropolitan Phoenix. Available chronometric data are not precise enough to determine whether the Salt River or Gila River integrated first. All of the exoreic rivers of western North America's Basin and Range Province — the lower Colorado, Gila, Rio Grande, Salt, and Verde rivers — employed lake overflow to integrate across half-graben, graben, rift and supradetachment tectonic settings. Unlabelled Image • Endorheic drainage occupied the Salt and Verde river confluence 3.3 Ma • Hypothesized lake overflow processes integrated both drainages from the top down • Drainage integration occurred between 2.8 and 2.2 Ma • Salt River underwent an avulsion <0.46 Ma from aggradational piracy [ABSTRACT FROM AUTHOR]

Low rates of lateral migration (centimetres to decimetres per year) combined with relatively high rates of vertical accretion (millimetres to centimetres per year) recorded in microtidal channels of the Venice Lagoon (Italy) give rise to point‐bar geometries and internal facies arrangements that differ substantially from widely accepted models of point‐bar sedimentary architecture. In this study, field data from the Venice Lagoon are combined with a three‐dimensional forward stratigraphic model, the 'Point‐Bar Sedimentary Architecture Numerical Deduction' (PB‐SAND), to predict the stratal geometries of point bars formed in aggradational settings. The PB‐SAND uses a combined geometric and stochastic modelling approach that can be constrained by field evidence. The model applied determines the geometry of four point bars generated by 9 to 11 m wide channels cutting through salt marshes. An iterative best‐fit modelling approach has been used to obtain multiple simulations for each case study, each of which fits the observations derived from the analysis of time‐series historical aerial photographs and 44 sedimentary cores. Results demonstrate how the geometry of the bars is determined by the development of two key stratal surfaces: the point‐bar brink and channel‐thalweg surfaces. These surfaces are defined by the progressive translation and vertical shift of the point‐bar brink (i.e. break of slope between bar top and bar slope) and the channel thalweg (i.e. deepest part of the channel) during bar evolution. The approach is used to: (i) reconstruct three‐dimensional point‐bar geometries; (ii) propose alternative reconstructions; (iii) provide insight to drive the acquisition of additional data to better constrain the proposed models; and (iv) provide insight into the mechanism of bar growth for slowly migrating channels in settings subject to relatively high rates of aggradation. This study highlights how interaction between styles of planform transformation and latero‐vertical shifts of meandering channels can determine the geometry of related sedimentary bodies. [ABSTRACT FROM AUTHOR]

This paper focuses on two objectives: (i) the presentation of the methodology for using small-sized iron slag (called microslag) as a stratigraphic marker to reconstruct the centennial-scale floodplain evolution and (ii) the study of the geomorphic evolution of the Ardennian rivers situated in the Lienne catchment (southern Belgium) by means of microslag evaluation, 14C dating, topographic surveys and sedimentological analysis. Using these data, the floodplain history since the Late Pleistocene is reconstructed, fine floodplain sedimentation and lateral mobility rates are estimated, and channel incision phases are identified. The first Holocene erosion episode occurred during the Bronze Age. Archaeological data and historical sources indicate Roman and High Middle Age occupations in the area but no evidence of erosion or increased floodplain sedimentation have been observed. Several peat layers have been dated in the Chavanne floodplain to approximately 1000 BP and probably indicate very low anthropogenic activity and a high level of groundwater. Before 1000 CE, the fluvial style of the Ardennian rivers was probably different, characterized by a low-level forested floodplain, which explains the presence of frequent peat layers within the alluvial sequences and abandoned channels. From the eleventh century onwards, a generalized increase in sedimentation rates occurred, coinciding with a higher concentration of charcoal in the alluvium and also the formation of colluvial deposits. Analyses of slag concentrations allowed us to reconstruct the evolution of the floodplain topography. In the Lienne catchment, the mean floodplain aggradation over the last several centuries is between 2.7 and 16 cm/century, depending on the study site. Such differences in floodplain sedimentation rates can be explained by several factors such as the distance to past cultivation areas, the degree of woodland clearance, flow velocities during floods and the incision of the riverbed during recent centuries, which reduces the frequency of inundation and, therefore, sedimentation on floodplains. • A reliable methodology uses microslag as a stratigraphic tracer for identifying alluvium deposited during recent centuries and for quantifying fluvial geomorphic processes. • The floodplain evolution of the Ardennian rivers since the Late Pleistocene is determined by means of 14C dating. • Several periods of increased sediment deposition during the Holocene related to human activities are identified. • River channel migration during recent centuries is characterized. • Channel incision and low terrace formation is estimated. [ABSTRACT FROM AUTHOR]

Aggradation and fluvial incision controlled by downstream base‐level changes at timescales of 10 to 500 kyr is incorporated in classic sequence stratigraphic models. However, upstream climate control on sediment supply and discharge variability causes fluvial incision and aggradation as well. Orbital forcing often regulates climate change at 10 to 500 kyr timescales while tectonic processes such as flexural (un)loading exert a dominant control at timescales longer than 500 kyr. It remains challenging to attribute fluvial incision and aggradation to upstream or downstream processes or disentangle allogenic from autogenic forcing, because time control is mostly limited in fluvial successions. The Palaeocene outcrops of the fluvial Lebo Shale Member in north‐eastern Montana (Williston Basin, USA) constitute an exception. This study uses a distinctive tephra layer and two geomagnetic polarity reversals to create a 15 km long chronostratigraphic framework based on the correlation of twelve sections. Three aggradation–incision sequences are identified with durations of approximately 400 kyr, suggesting a relation with long‐eccentricity. This age control further reveals that incision occurred during the approach of – or during – a 405 kyr long‐eccentricity minimum. A long‐term relaxation of the hydrological cycle related to such an orbital phasing potentially exerts an upstream climate control on river incision. Upstream, an expanding vegetation cover is expected because of an increasingly constant moisture supply to source areas. Entrapping by vegetation led to a significantly reduced sediment supply relative to discharge, especially at times of low evapotranspiration. Hence, high discharges resulted in incision. This study assesses the long‐eccentricity regulated climate control on fluvial aggradation and incision in a new aggradation–incision sequence model. [ABSTRACT FROM AUTHOR]

The links between flood frequency and rates of channel migration are poorly defined in the ephemeral rivers typical of arid regions. Exploring these links in desert fluvial landscapes would augment our understanding of watershed biogeochemistry and river morphogenesis on early Earth (i.e. prior to the greening of landmasses). Accordingly, we analyse the Mojave River (California), one of the largest watercourses in the Great Basin of the western United States. We integrate discharge records with channel‐migration rates derived from dynamic time‐warping analysis and chronologically calibrated subsidence rates, thereby constraining the river's formative conditions. Our results reveal a slight downstream decrease in bankfull discharge on the Mojave River, rather than the downstream increase typically exhibited by perennial streams. Yet, the number of days per year during which the channel experiences bankfull or higher stages is roughly maintained along the river's length. Analysis of historical peak flood records suggests that the incidence of channel‐formative events responds to modulation in watershed runoff due to the precipitation in the river's headwaters over decades to centuries. Our integrated analysis finally suggests that, while maintaining hydraulic geometries that are fully comparable with many other rivers worldwide, ephemeral desert rivers akin to the Mojave are capable of generating a surprisingly wide range of depositional geometries in the stratigraphic record. © 2020 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We draw on published studies of floodplain organic carbon storage, wildfire‐related effects on floodplains in temperate and high latitudes, and case studies to propose a conceptual model of the effects of wildfire on floodplain organic carbon storage in relation to climate and valley geometry. Soil organic carbon typically constitutes the largest carbon stock in floodplains in fire‐prone regions, although downed wood can contain significant organic carbon. We focus on the influence of wildfire on soil organic carbon and downed wood as opposed to standing vegetation to emphasize the geomorphic influences resulting from wildfire on floodplain organic carbon stocks. The net effect of wildfire varies depending on site‐specific characteristics including climate and valley geometry. Wildfire is likely to reduce carbon stock in steep, confined valley segments because increased water and sediment yields following fire create net floodplain erosion. The net effect of fire in partly confined valleys depends on site‐specific interactions among floodplain aggradation and erosion, and, in high‐latitude regions, permafrost degradation. In unconfined valleys in temperate latitudes, wildfire is likely to slightly increase floodplain organic carbon stock as a result of floodplain aggradation and wood deposition. In unconfined valleys in high latitudes underlain by permafrost, wildfire is likely in the short‐term to significantly decrease floodplain organic carbon via permafrost degradation and reduce organic‐layer thickness. Permafrost degradation reduces floodplain erosional resistance, leading to enhanced stream bank erosion and greater carbon fluxes into channels. The implications of warming climate and increased wildfires for floodplain organic carbon stock thus vary. Increasing wildfire extent, frequency, and severity may result in significant redistribution of organic carbon from floodplains to the atmosphere via combustion in all environments examined here, as well as redistribution from upper to lower portions of watersheds in the temperate zone and from floodplains to the oceans via riverine transport in the high‐latitudes. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The present study aims to improve current understanding of the sedimentation of subtidal point bars, analyzing interaction between tidal currents and waves in shaping a submerged meander bend of the microtidal Venice Lagoon (Italy), and it is based on coupling of sedimentological studies, geophysical analyses and numerical modelling. The Venice Lagoon is characterized by an average depth of about 1·5 m over subtidal platforms and a mean tidal range of about 1·0 m. The morphodynamic evolution of the lagoon is strongly affected by intense seasonal windstorms, which promote the formation of wind waves triggering sediment resuspension and bottom erosion. The study channel is 70 to 100 m wide, it has a radius of curvature of about 260 m and cuts through a permanently submerged subtidal platform. Water depth ranges from 1·0 to 5·0 m below mean sea level on the subtidal platform and channel thalweg, respectively. Different from classical architectural models, the study point‐bar beds do not show sigmoidal geometries, but consist of horizontally‐bedded deposits abruptly overlying clinostratified beds. Sedimentation in the study bar is hypothesized to stem from the interaction between the in‐channel secondary helical flow, as for most meander bends, and wave winnowing of the subaqueous overbank areas. Laterally accreting point‐bar deposits point out that the curvature‐induced helical flow redistributed sediment from the channel thalweg to the bar top and contributed to the development of the 'classical' fining‐upward grain size trend. The marked truncation surface, separating clinostratified bar deposits from overlying horizontally‐bedded platform sediments is interpreted here as due to bar top wave‐winnowing, which also possibly promoted bank collapses. In the proposed model, sediments remobilized from bar top and subaqueous overbank areas were transported into the channel, forming peculiar 'apron‐like' accumulations, where sand accumulated through avalanching processes and mud settled down from suspension. [ABSTRACT FROM AUTHOR]