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Humans have become major geomorphological agents, effecting substantial change in the characteristics of Earth's physical landscapes. The agricultural Midwest of the United States is a region marked by pronounced human influence at the landscape scale. Humans undoubtedly have strongly influenced critical zone processes, including fluvial processes, in intensively managed agricultural landscapes, yet the exact nature of human alteration of these processes is unknown. This study documents historical changes in the extent of the stream channel network and in channel planform within the upper Sangamon River basin — an intensively managed agricultural watershed in Illinois. Results indicate that the modern channel network is nearly three times more extensive than the channel network in the 1820s. Most change in drainage density has occurred in headwater portions of the basin where numerous drainage ditches have been added to the network to drain flat uplands. No detectable change in channel position is evident between 1940 and 2012 along about 60% of the total length of the Sangamon River and its major tributaries. Nearly 30% of the total length exhibits change related to meander dynamics (cutoffs and lateral migration), whereas about 8% has changed as a result of channelization. Channelized sections typically remain straight for decades following human modification, supporting the notion that humans produce long-lasting catastrophic change in channel planform in this region. The findings confirm that humans are effective agents of morphological change in fluvial systems in this intensively managed watershed. Documenting human-induced versus natural changes in fluvial systems is important for evaluating how other critical zone processes in intensively managed landscapes have been affected by these changes. Human-induced changes in channel extent and planform most likely have altered this landscape from one dominated by biogeochemical transformations and storage of water and sediment prior to agricultural development into one now characterized by enhanced fluxes of water, sediment, and nutrients. [ABSTRACT FROM AUTHOR]

Abstract: Fluvial geomorphology has been the largest single subdiscipline within geomorphology for many decades. Fluvial geomorphic expertise is integral to understanding and managing rivers and to developing strategies for sustainable development. This paper provides an overview of some of the significant advances in fluvial geomorphology between 1960 and 2010 with respect to: conceptual models; fluvial features and environments being studied; tools used by fluvial geomorphologists; geomorphic specialty groups within professional societies; journals in which fluvial geomorphic research is published; and textbooks of fluvial geomorphology. During this half century, fluvial geomorphology broadened considerably in scope, from a focus primarily on physical principles underlying process and form in lower gradient channels with limited grain size range, to a more integrative view of rivers as ecosystems with nonlinear behavior and great diversity of gradient, substrate composition, and grain size. The array of tools for making basic observations, analyzing data, and disseminating research results also expanded considerably during this period, as did the diversity of the fluvial geomorphic community. [Copyright &y& Elsevier]

Abstract: Large rivers have anabranching channels with components that may be defined as braided, meandering or straight. This paper shows that application of such holistic terminologies is complicated by recognition of within-type and transitional-type variety, a confusingly varied use of terms, and a coverage of pattern characteristics that for many large rivers is incomplete. In natural states, big rivers can be plural systems in which main, accessory, tributary and floodplain channels and lakes differ functionally and vary in terms of morphological dynamics. A distinction is drawn between the hydrological and geomorphological connectivity of components in big river plural systems. At any one time, even at flood stage, only some channels are geomorphologically active. Six types of geomorphological connectivity are described that range from coupled, through to partially-coupled and decoupled. The interplay between geomorphological and hydrological connectivity in large rivers is shown to determine habitat status and therefore ecological diversity. For improved understanding of the dynamics as well as the forms of these large composite systems, it is helpful to: (1) adopt element-level specification, not only for sediment bodies, but also for functioning channels; (2) track the sediment transfer processes and exchanges that produce channel forms over the highly varied timescales operating within large rivers; and (3) recognise the ways in which partially coupled and connected geomorphological systems produce naturally a composite set of forms at different rates. Such augmenting information will provide an improved platform for both river management and ecological understanding. [Copyright &y& Elsevier]

Abstract: Fluvial erosion of peat deposits occurs in many environmental settings; however, the erosion and transport of large peat blocks by river channels has received relatively little attention. This paper describes the sedimentary significance and potential palaeoenvironmental interpretation of peat blocks in alluvial river systems. Evidence is presented from a number of field studies of upland river systems in northern England that illustrate a range of peat block forms and sedimentary features that are briefly compared with examples of peat blocks preserved in gravel stratigraphy. We show that peat blocks are an important geomorphological and sedimentological component of upland rivers draining eroding peatland catchments. They are of widespread occurrence and contribute significantly to river channel roughness and channel sedimentation. A variety of common sedimentary features can be observed including, shadow, crescent, perched, armoured, drape, embedded (part buried), cluster, and step forms. Peat blocks tend to be deposited on channel margins and bar tops and can be used as “maximum” stage indicator for major floods. The role of peat blocks in controlling sedimentation varies with channel width. In narrow channels where the size of the peat block approximates the channel width, blocks become jammed in the channel and exert a primary control on channel sedimentation. In wider channels, blocks tend to occur in isolation or in small clusters and are of only secondary importance in controlling sedimentation. Residence times of peat blocks varies from short periods of temporary deposition (days to months) to much longer timescales (months to years) with some blocks becoming permanently incorporated into the sedimentary record. The sedimentary characteristics of contemporary buried peat blocks have much in common with blocks preserved in alluvial gravel stratigraphy. This offers the potential for using these features for palaeoenvironmental reconstruction. Peat blocks in many ways are analogous to other low density geological materials (ice blocks, woody debris, and some volcanic sediments), and recognising the special sedimentological characteristics of this suite of materials is important as they are not always adequately characterised using conventional hydraulic relationships. [Copyright &y& Elsevier]

The timescale required for a river channel to reach new equilibrium conditions following a disturbance presumably depends on channel substrate size, and previous field studies have suggested that gravel-bed channels should reach equilibrium more quickly than sand-bed channels. To estimate the relative timescale difference between sand-bed and gravel-bed rivers responding to base-level lowering, we developed a simple scaling approximation. Equilibrium timescales derived from these equations for sand-bed rivers were found to be approximately half of those for gravel-bed rivers. While gravel-bed channels do not need to transport as much sediment to reach equilibrium conditions as sand-bed channels, transport of sediment is more rapid in sand-bed channels leading to quicker equilibration. [Copyright &y& Elsevier]

Along the eastern margin of the Qinghai–Tibet Plateau, there are frequent occurrences of strong tectonic movement and large-scale landslides. River blockages often occur in deep-cut river valleys, forming large-scale dammed lakes. In this study, a ~26.5 m thick lacustrine sedimentary profile that underlying the modern riverbed of the Yalong River is reported. Here, 14C dating results show that lacustrine deposition began at ~25 ka and continued till ~10 ka, mainly during the Last Glacial Maximum. Paleoclimatic indicators, such as particle size, magnetic susceptibility, and spore-pollen combination, were used to reconstruct the evolution process of the paleoenvironment in the study area, revealing that a paleo-flood event occurred at ~19 ka. The Tongzilin dammed lake contains an excellent record of the geomorphic evolution of the lower reaches of the Yalong River during the Last Glacial Maximum. Before 25 ka, the Yalong River experienced strong downcutting to form a deep valley, which created the space conditions for the formation of the dammed lake; from 25 to 10 ka, the dammed lake remained relatively stable, with dominating accumulation; and after 10 ka, the dammed lake broke and the river continued to downcut. • A dammed lake deposition during the LGM was found buried in the modern riverbed of the Yalong River. • Through 14C dating, the chronological framework of this lacustrine deposit was established. • Tongzilin dammed lake is controlled by climate and structure during the LGM. • The evolution of fluvial geomorphology in the lower reaches of Yalong River is well recorded in Tongzilin dammed lake. [ABSTRACT FROM AUTHOR]

Avulsions are rare events that cause major realignments to fluvial systems and hold potential for significant societal consequences. We present a case study of an avulsion in a minimally-aggrading landscape that was influenced by long-term human modifications to a fluvial environment that was initially setup for an avulsion during regional Pleistocene deglaciation. The 2006 Suncook River avulsion in New Hampshire, USA, is a valuable example because minimal environmental management took place for a decade following the event, allowing natural processes to dominate initial fluvial responses. Geospatial data collected during this period allow for the evaluation of the short- and long-term morphological evolution of a river that experienced an avulsion. We review data from technical reports on the event and present a new synthesis of aerial imagery and topographic data highlighting key geomorphic features associated with the avulsion. These features include headcutting and widening associated with a 0.6-km long channel created by the avulsion. Downstream of the avulsion site, extensive channel filling, deposition of meter-scale sand sheets on floodplain surfaces, and development of a cutoff channel were major manifestations of the event. The initial setup for the avulsion can be traced to Pleistocene deglaciation, when glaciofluvial ridges deposited in the valley blocked a more direct route for the river and shifted the channel towards an area of shallow bedrock. Human modifications to the channel—including mill dam construction, floodplain narrowing, road development, and gravel mining—contributed to the avulsion setup by increasing superelevation and undermining the glaciofluvial deposits that had directed the river during the Holocene. The avulsion was triggered by a ~50-yr flood event following a 70-yr interval with no major floods but marked by continued human modification to the landscape. The unusual nature of the event combined with the wealth of available geospatial and hydrologic data make the Suncook River system an attractive target for future studies that can examine avulsion processes in greater detail. • Case study of 2006 Suncook River avulsion in New Hampshire, USA • Avulsion setup through combination of regional deglaciation and human impacts • Triggered by ~50-year flood event that followed several decades of low-intensity floods • Avulsion incised floodplain and glaciogenic sediments to create new channel. • Sediment wave caused downstream channel filling and significant overbank deposition. [ABSTRACT FROM AUTHOR]

The tectonic and fluvial setting of the Rhine-Meuse river system in the Lower Rhine Embayment rift system is exceptionally well known. The 19th century, pre-regulation river courses of three rivers are used to study a postulated sinuosity response to faulting. The fault-perpendicular Meuse River shows patterns of sinuosity changes at different spatial scales. The large-scale (>5 km) sinuosity changes are related mainly to the faulting-induced changes of the subsurface lithology, determining the bed and bank characteristics. However, at a smaller scale, some fault-related channel sinuosity anomalies are observed. The fault-parallel Roer River shows sinuosity changes related to a normal, non-tectonic longitudinal gradient change. Sinuosity patterns of the Rhine River are predominantly related to lithological differences and reduced incision rates. Sinuosity can thus be an indicator of tectonic motions, but gradient, subsurface lithology and river bank composition determine sinuosity as well. Therefore, a sinuosity change is no proof for fault activity. On the other hand, the absence of a sinuosity change does not imply inactivity of a fault at geological time-scales. • Sinuosity patterns in the Lower Rhine Embayment are studied for tectonic forcing. • Morphometric analysis are performed on river channels of the Meuse, Roer and Rhine. • The Meuse River is (indirectly) controlled by tectonics on a block-scale. • The relation between fault motions, river gradient and sinuosity is not straightforward. • A sinuosity change alone is no proof for fault activity. [ABSTRACT FROM AUTHOR]

Identifying and understanding functional process domains (sensu Montgomery, 1999) in rivers is paramount for linking the physical habitat template to ecosystem structure and function. To date, efforts to do this have been rare, especially in large rivers, as they require appropriate tools for quantifying habitat heterogeneity with fine-scale resolution across broad spatial extents. In this study, we used side-scan sonar technology to map riverbed substrate at six sites in the Yellowstone and Missouri rivers. Substrate maps were then analyzed and visualized using geospatial analysis to relate fine-grained spatial substrate patterns to process domain structure. Our findings revealed two distinct nested domains of substrate patchiness, suggesting that different factors are responsible for shaping patterns of substrate at different scales. Although small-scale patchiness in substrate was likely driven by internal, or autogenic, physical processes, patterns at larger segment extents (>3 km) were often driven by abrupt transitions in habitat related to exogenous factors such as lateral erosion of talus, tributary inputs, and bank armoring. Additionally, we found that heterogeneity in benthic substrate increased with spatial extent at all of our study sites; however, this relationship was lower in the Missouri River, which is altered by impoundment. Our study represents one of the first efforts to relate benthic habitat heterogeneity to nested process domain structure in large riverscapes, and offers a unique perspective for linking landscape processes, geomorphological habitat heterogeneity, and biological structure and function in large rivers. [ABSTRACT FROM AUTHOR]

The transport of coarse material strongly controls the stability and evolution of mountain fluvial systems but, despite this, bedload dynamics are not yet fully understood especially in mountain streams. In this sense, particular attention was paid on the bedload magnitude (volume) expressed at event-scale and on the long-term, while few studies were focused on when and how the transport of the coarse material occurs. The aim of this work was to investigate the bedload mobility in the Rio Cordon, a mountain stream characterized by cascade and step-pool morphologies with a rough streambed. Here, the critical conditions for initiation of motion, transport distance and virtual velocity expressed by the coarse streambed material were assessed and their relationships with hydraulic forcing conditions and grain size were analyzed. To this end, a monitoring program based on bedload tracing was maintained over 7 years, allowing to analyze the bedload mobility during persistent high frequency/low magnitude flow conditions. To investigate the bedload mobility, 250 tracers were released between 2011 and 2012 and their propagation along study site was monitored until 2018. Overall, 14 tracer inventories were realized, determining 1697 tracer localizations. During the study period, the bedload dispersion resulted well described by the peak discharge magnitude (Q p , q p and ω p − ω c), while no significant relationships were observed with duration of competent flow (t over) and effective runoff volume (ER). Transport distance and tracer grain size were negatively correlated, whereas virtual velocity increased with increasing particle size. In this sense, the propagation velocity seems to be affected by the high frequency flows that, on the one hand triggered mobilization of the coarser tracers only through limited and impulsive events and, on the other hand favored a slowdown on the finer particles due to bedforms disturbance. Compared to other study sites, the Rio Cordon exhibited accentuated threshold conditions with lower transport distance and virtual velocity, confirming that steep mountain streams are generally influenced by a reduced transport efficiency due to protruding bedforms and macro-roughness that cause a pronounced energy dissipation. Interestingly, such condition seems to have progressively reduced from under- to near-bankfull flows. The results were compared to the bedload mobility observed in the Rio Cordon during 1993–1998, enabling to quantitatively assess how the bedload dispersion varied between a setting of stable armouring layer with protruding bedforms (2012–2018) and a partial alteration of these (1993–1998). An evident difference was observed in terms of transport distance, while critical conditions did not significantly change. Lastly, the long-term bedload tracing investigation highlighted that a certain legacy on the transport efficiency was produced by the persistent high frequency flow conditions. Over the study period, a general decrease of transport distance was observed that could be explained by a progressive stabilization of streambed material. Previous studies suggested that such condition can be interrupted by high magnitude/low frequency floods. • Bedload mobility was investigated over 7 years in a steep mountain stream. • Mobility of the coarse material was well described by peak discharge magnitude. • Influence of armour layer and protruding bedforms on bedload mobility was assessed. • A legacy on the transport efficiency was induced by the high frequency flows. [ABSTRACT FROM AUTHOR]