Gravel‐Bed Rivers (original) (raw)
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Morphological effects of different channel-forming discharges in a gravel-bed river
Earth Surface Processes and Landforms, 2009
The study analyses the morphological response of a gravel-bed river to discharges of different magnitude (from moderate events that occur several times a year to a 12-year flood) and so defines the range of formative discharges for single morphological units (channels, bars, islands) and a range of magnitude of morphological activity from the threshold discharges for gravel transport and minor bar modification up to flows causing major morphological changes.
Evolutionary trajectory of channel morphology and controlling factors in a large gravel-bed river
Geomorphology, 2012
Interpretation of channel changes is not always straightforward since it requires a detailed reconstruction of the evolutionary trajectory of channel morphology, a quantitative analysis of controlling factors and, finally, identification of links between evolutionary trajectories and controlling factors. The aim of this paper is to explain channel adjustments and controlling factors in the Tagliamento River, a large gravel-bed river in northeastern Italy. Traditional methods for studying historical channel changes (i.e., use of aerial photos, topographic data) were employed, but numerical modelling turned out to be very useful for the quantification of bedload transport and for supporting interpretation of past changes and likely future channel evolution. River channel underwent three main phases of adjustment over the last 200 years. The first two phases, from the end of the nineteenth century to the early 1990s, were characterized by narrowing (channel width decreased from 1250 to 540 m) and incision (about 1 m); and the third phase, from the 1990s to present day, by widening (from 540 to 600 m) and slight aggradation (about 0.2 m). Combining evolutionary trajectories of channel morphology and analysis of controlling factors we argue that the long-term channel evolution of the Tagliamento River was driven mainly by human intervention at the reach scale (i.e., sediment mining and channelization). Changes in sediment supply in the catchment area had no, or minor, effects in the study reach. The most recent changes (i.e., widening and slight aggradation) are explained as a response to past disturbances that have produced a remarkable change of channel geometry and an increase of unit stream power in the reach. Magnitude and frequency of formative discharges are a key driving factor of the intensity of recent changes, while this was not the case during the previous phases of adjustment.
Coordination Chemistry Reviews, 2011
Mid-Cretaceous strata within the Tintina Trench, 3 km west of the community of Ross River, contain evidence of deposition in two distinct, alternating, fluvial settings. Coal-bearing, mud-dominated strata are commonly associated with high-constructive sandy channel systems, with extensive overbank, levee and splay deposits. Channels are between 3 and 30 m wide and 0.4e7 m thick. They show repetitive development of side and in-channel bar-forms, as well as up-channel widening of the rivers by selective erosion of associated overbank and levee deposits. Levees extended for several hundred metres away from the channels. In this setting low-angle inclined stratification and epsilon cross stratification may reflect lateral migration of crevasse channels or small streams. The paucity of exposure prevents recognition of the channels as products of multiple channel anastomosed systems or single channel high-constructive systems.
Channel Form and Processes in a Formerly Glaciated Terrain
2016
Despite that many places around the world in general, and North America in particular, were glaciated during the last ice age, relatively little is known about rivers that evolved over glaciated landscapes once they deglaciated. They are commonly categorized as alluvial with a glacial legacy, and often described as simple gravel-bed or sand-bed rivers. Alternatively, they are classified as bedrock rivers because the glacial deposits have been eroded and the underlying rock is exposed. However, the glacial history of these rivers is important and my research shows that classification for them is "semi-alluvial". This work shows that classification is important, not only for scientific accuracy but for river management that must be based on a sound understanding of river form and process. Poor understanding can be costly (i.e. restoration and management that do not achieve their goals; failed infrastructure), leading to wasted resources and inefficient functioning of the riv...
Temporal and spatial variability in thalweg profiles of a gravel‐bed river
Earth Surface Processes and Landforms, 1999
This study used successive longitudinal thalweg profiles in gravel-bed rivers to monitor changes in bed topography following floods and associated large sediment inputs. Variations in channel bed elevations, distributions of residual water depths, percentage of channel length occupied by riffles, and a spatial autocorrelation coefficient (Moran's I) were used to quantify changes in morphological diversity and spatial structure in Redwood Creek basin, northwestern California. Bed topography in Redwood Creek and its major tributaries consists primarily of a series of pools and riffles. The size, frequency and spatial distribution of the pools and riffles have changed significantly during the past 20 years. Following large floods and high sediment input in Redwood Creek and its tributaries in 1975, variation in channel bed elevations was low and the percentage of the channel length occupied by riffles was high. Over the next 20 years, variation in bed elevations increased while the length of channel occupied by riffles decreased. An index [(standard deviation of residual water depth/bankfull depth) Â 100] was developed to compare variations in bed elevation over a range of stream sizes, with a higher index being indicative of greater morphological diversity. Spatial autocorrelation in the bed elevation data was apparent at both fine and coarse scales in many of the thalweg profiles and the observed spatial pattern of bed elevations was found to be related to the dominant channel material and the time since disturbance. River reaches in which forced pools dominated, and in which large woody debris and bed particles could not be easily mobilized, exhibited a random distribution of bed elevations. In contrast, in reaches where alternate bars dominated, and both wood and gravel were readily transported, regularly spaced bed topography developed at a spacing that increased with time since disturbance. This pattern of regularly spaced bed features was reversed following a 12-year flood when bed elevations became more randomly arranged.
Canadian Journal of Earth Sciences, 1977
The Kicking Horse River is a gravelly, braided stream characterized by very low winter discharge, a peak spring flood (70 m 3 s-1 in 1973), and summer diurnal discharge fluctuations (from 18 to 44 m 3 /s-1 in 1973) related to daily melting on the glaciers that supply the river. The reach studied , near Field, B.C. , is characterized by abundant braid bars, and was subdivided into three parts . · The upstream reach is characterized by only one major channel, with few unit bars within it. Clas! movement takes place at peak flood stages, and the clasts move in 'diffuse gravel sheets' on the channel floor. These sheets are only one to two clast diameters in thickness , and when the clasts stop rolling , the sheet becomes a coarse lag . By contrast, in the midstream and downstream reaches, the channel is more anastomosing, and there are many in-channel bars , mostly transverse and diagonal unit bars . The diagonal bars mostly lack foreset slopes at their downstream margin , and it is suggested that massive to crudely horizontally stratified gravels would be deposited. Transverse bars normally have a foreset slope, giving rise to cross-stratified gravels.
Understanding the spatial organization of river systems in light of natural and anthropogenic change is extremely important because it can provide information to assess, manage, and restore them to ameliorate worldwide freshwater fauna declines. For gravel-and cobble-bedded alluvial rivers studies spanning analytical, empirical and numerical domains suggest that at channel-forming flows there is a tendency towards covarying bankfull bed and width undulations amongst morphologic units such as pools and riffles, whereby relatively wide areas have relatively higher minimum bed elevations and relatively narrow areas have relatively lower minimum bed elevations. The goal of this study was to determine whether minimum bed elevation and flow-dependent channel top width are organized in a partially confined, incising gravel–cobbled bed river with multiple spatial scales of anthropogenic and natural landform heterogeneity across a range of discharges. A key result is that the test river exhibited covarying oscillations of minimum bed elevation and channel top width across all flows analyzed. These covarying oscillations were found to be quasiperiodic at channel-forming flows, scaling with the length scales of bars, pools and riffles. Thus, it appears that alluvial rivers organize their topography to have quasiperiodic, shallow and wide or narrow and deep cross section geometry, even despite ongoing, centennial-scale incision. Presumably these covarying oscillations are linked to hydrogeomorphic mechanisms associated with alluvial river channel maintenance. The biggest conclusion from this study is that alluvial rivers are defined more so by variability in topography and flow than mean conditions. Broader impacts of this study are that the methods provide a framework for characterizing longitudinal and flow-dependent variability in rivers for assessing geomorphic structure and aquatic habitat in space, and if repeated, through time.
Geomorphology, 2008
Within-channel alluvial gravel extraction is one of the most important forms of anthropogenically induced morphological change in river channels. In British rivers commercial gravel extraction was widespread between the 1930s and 1960s, and limited gravel extraction operations to reduce flood risk or maintain navigation continue to the present day. Despite this, gravel extraction has received little attention in UK river studies. This paper examines the significance of within-channel gravel extraction, during the period 1945-1960, on the planform of the River Wear in northern England. The study focuses on two 3 km piedmont reaches at Wolsingham and Harperley Park, located at the margin of the upland zone. Examination of detailed archival accounts of the gravel extraction operations, supplemented by the analysis of aerial photographs has enabled the impact of gravel extraction on the channel of the River Wear to be determined. Sediment budget calculations suggest large sediment deficits in both study reaches, however, assessing potential impacts simply in terms of a sediment deficit may be misleading as channel adjustments depend on local factors and a detailed consideration of the reach-scale sediment budget. Differences in the nature of channel adjustments of both reaches were found to be primarily a function of the method of gravel extraction employed. Overall patterns of channel change along the extraction reaches, over the past 150 years, were similar to reaches where gravel extraction was not practiced. This highlights the difficulty of trying to establish the significance of different processes where both local (gravel extraction) and catchment-scale factors (climate and land use) are operating.