Search

Rivers disrupted by sediment cutoff often experience degradation, but the migration of the erosion center, defined as the location with the greatest degradation rates, has not been thoroughly understood. This paper focuses on the streamwise migration of the erosion center along the ∼400‐km‐long Yichang to Chenglingji reach (YCR) downstream of the Three Gorges Dam (TGD), China. We analyzed channel morphological adjustment based on water, sediment and channel geometry data collected during 2002–2020. Based on the location and time for the occurrence of relatively large channel degradation, a clustering algorithm was used to identify the location of the erosion center. Characteristics and morphodynamic controls of the erosion centers were studied based on the migration of incisional and coarsening waves simulated by a one‐dimensional morphodynamic model for nonuniform sediment. Results show that the erosion center migrated downstream along the Yichang‐Zhicheng reach with gravel‐sand bed during 2002–2012, the migration rate was rapid after the dam closure then decreased with time. After ∼2012, large cascade dams started to operate along the upper Yangtze River, sediment load further decreased and degradation accelerated at the YCR. Correspondingly, the erosion center migrated to the sand‐bedded upper Jingjiang reach with faster rates. The erosion center migrated for a total of over 200 km with an average rate of ∼14 km/yr during 2002–2020. The underlying gravel layer was exposed due to degradation, which enhanced bed coarsening and resulted in the propagation of the erosion center downstream of the TGD. Key Points: Erosion center, area with the greatest degradation rates, moved downstream below the damBed coarsening impacting the streamwise migration of the erosion centerExposed underlying gravel layer enhanced armoring and triggered migration of the erosion center [ABSTRACT FROM AUTHOR]

The sediment supplied to a stream channel impacts the morphological trends experienced by that channel, and the long‐term trends are important to understand for many riverine applications. We introduce the term 'net morphological displacement' (NMD) to denote the net channel change revealed by the morphological method over multiple sediment transport events to make the concept more explicit for river management and use it to determine equilibrium or disequilibrium states. This study explores the morphological response of East Creek, a small threshold gravel‐bed channel in the Coast Mountains of British Columbia, Canada, to variations in flow and sediment supply at multiple spatial and temporal scales over 15 years. High‐resolution topographic data (HRTD) of the bed were combined with cross‐sectional surveys of the banks to determine the sediment supply that the channel responded to annually. The level of detection used to remove noise associated with HRTD was calibrated using an independent tracer stone dataset. The net effects from multiple floods caused distributions of bed elevation change to generally follow the log‐normal distribution, and mean depths of erosion and deposition were predominantly similar between morphological units. At the reach scale, the various reaches of East Creek responded differently to the same hydrological events due to the impacts from the varying supply conditions on the NMD. Shorter measurement periods would have resulted in inconclusive information that does not show the long‐term morphological trends of the channel. Determination of these trends can take years or decades, depending on the time and space scales of change, but there is generally a lack of long‐term channel monitoring programmes, notably after river restoration. More long‐term channel monitoring programmes are required to assess restoration projects and ensure their long‐term sustainability. [ABSTRACT FROM AUTHOR]

The influence of glaciation‐induced watershed configuration on channel response to flooding remains a topic of comparatively little research despite the prevalence of paraglacial landscapes. This study examines the response of Guichon Creek, a 1,200 km2 catchment in the southern interior of British Columbia, to major flood events during consecutive freshets with return periods of 20–40 years. The glacial history of the Guichon Creek landscape has influenced the spatial pattern of sediment availability to the channel, with erosion of thick glaciofluvial deposits located adjacent to the mainstem channel leading to extensive destabilization of downstream reaches. Over the two flood events, Guichon Creek widened by a factor of eight, but only downstream of sediment sources. Where sediment was not available to enter the channel, major changes in width were not observed. At least 163,000 ± 83,000 m3 and 73,890 ± 11,752 m3 of sediment entered the stream channel during the 2017 and 2018 freshets, respectively, representing an input more than 200 times the mean annual transport capacity. Between 40% and 60% of this sediment was of a sufficient caliber to be transported as bedload. While aggradation and degradation exceeding several meters in depth was observed at some sites, net increase in storage was small, indicating substantial sediment export. Specific patterns of channel change downstream of sediment input sites correspond to gradients in stream power. We present a conceptual model contrasting profile form, sediment availability, and transport capacity between a previously glaciated and a non‐glaciated basin of similar size and relief. These findings illustrate the broad implications of glacial landscape history on channel response to flooding, and have direct implications for river management, particularly as it pertains to channel stability and aquatic habitat. [ABSTRACT FROM AUTHOR]

Understanding suspended sediment dynamics and their driving factors is essential for effective watershed management. In this study, the spatiotemporal patterns of suspended sediment were identified for 11 sub‐watersheds of the Goodwin Creek, Mississippi, USA, for the period 1982–2002 using wavelet transforms. The continuous wavelet transform (CWT) results of streamflow time series showed that a near‐continuous annual periodicity influenced all stations over the study period, yet an overall decline occurred in wavelet spectral power of sediment load during the 1990s. This decline in sediment load is a result of the indirect effect of decreased cultivation on reducing the rate of peak flow and thus fluvial erosion processes. The CWTs of fine sediment (clays and silts) concentration (FSC) time series showed that factors other than land‐use (e.g., ponds, ephemeral gullies, and bed composition) better explain the variations in FSC. A comparison of CWTs of FSC at two stations with different bed composition in their channels showed that sandy channels are less responsive to erosion control measures compared to gravely channels indicating that they are major sources of sediment. A change in phase relationship between flow and FSC for the gravel‐dominated channel was associated with the transition in upland land‐use indicating a change in sediment supply mechanisms. The CWTs of sand concentrations reflected the spatiotemporal impact of bank materials, the growth cycle of riparian vegetation, and in‐channel sand availability on sand dynamics. While CWT‐based analysis is sensitive to input conditions, the method is able to provide valuable outcomes for distinguishing patterns in complex temporal datasets. Key Points: An overall decline in fine sediment load occurred across all time scales during the period of land‐use change and in‐channel stabilizationStreambed composition and the locations of ponds and upland sediment sources explain the spatiotemporal patterns of suspended finesThe spatiotemporal patterns of sand dynamics reflect both the state of channel stability and the availability of in‐channel sand [ABSTRACT FROM AUTHOR]

Large wood (LW) recruitment, transport, and storage were evaluated over a century in Gregory and Riley creeks (Haida Gwaii, British Columbia) by modeling a reach-scale LW budget using two frameworks for output: LW loss through decay and downstream transport, and loss through depletion. At reach and at watershed scales, mass movement and bank erosion dominated inputs, and fluvial transport was an important flux term in several reaches. Large wood recruitment by mortality was relatively minor in comparison. Large proportions of the in-channel LW were stored in jams with a mean age of 40–50 years. Overall, both modeling approaches yielded reasonable stored LW predictions in the study creeks, with the omission/inclusion of transport responsible for the largest differences between models. Modeled storage generally was within 30% of that measured in the field, and our results illustrate the large temporal variation in storage resulting from episodic inputs of LW from hillslopes. [ABSTRACT FROM AUTHOR]

The role played by the texture of the sediment supply on channel bed adjustments in gravel-bed rivers is poorly understood. To address this issue, an experimental campaign has been designed. Flume experiments lasting 96 h in a 9 m long, 0.60 m wide have been performed with different sand-gravel mixtures as feed textures. The response of the surface texture has been found to be highly dependent on the grain size distribution of the feed. When the feed texture included gravel, the finest fractions of the sediment supply infiltrate beneath the surface. Conversely, sand remains on the surface when the feed texture lacks gravel. This different textural response becomes obscured when water discharge increases. Further, the sediment transport rate approaches the feed rate differently depending on the content of gravel in the feed texture. When a small proportion of gravel is part of the feed texture, bed load transport rate asymptotically approaches the feed rate. However, when a significant fraction of gravel is part of the feed grain size distribution, bed load transport rate approaches the feed rate by following an oscillatory path. These findings have been verified in terms of a one-dimensional numerical model. This modeling reveals that the higher the differences in mobility among the grain sizes contained in the feed texture, the more evident is the nonasymptotic transient trend toward equilibrium. [ABSTRACT FROM AUTHOR]

Erosion from logging road surfaces, cut slopes, banks, and ditches represents a chronic source of sediment input to streams that can degrade aquatic habitats. Road surface erosion is of particular concern because the magnitude of sediment generation when traffic levels are high can be large. Current models for predicting sediment production from roads require information on area-specific sediment delivery, which is not often available. Here, we developed a model to quantify suspended sediment concentrations (SSC) generated by forest roads surfaces under different conditions of use and density. This model is designed for a typical medium-size coastal watershed of British Columbia or the American Pacific Northwest, and was applied to the Chilliwack River watershed as a case study. The results illustrate that intensive use of forest roads combined with high road density can increase the number of extreme sedimentation events over a predetermined threshold. A comparison of the effects of road density and the level of road use suggests that the level of road use is more important than the road density for the generation of fine sediment from road surfaces. However, the model omits the impact of roads on mass movements in a watershed, which represent a major source of sediment in steep watersheds, so the effect of road density is likely more substantial than the model predicts. The model is an attempt to overcome field data limitations by using an empirical relation between SSC and traffic variables, and presents a starting point for more intensive field studies that could be used to validate it. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The article evaluates the influence of in-channel sediment supplies and bed composition on suspended sediment concentrations (SSC). Using the suspended sediment and discharge measurements on two reaches of an agricultural watershed from 1982-2001, results show that in-channel supplies of sediment has an important control on SSC dynamics during low flows associated with storm events. Also, the results show that suspended sediment is both more available and more easily transported from sand-dominated streambeds, particularly during low flows.

This paper reviews sediment transport and channel morphology in small, forested streams in the Pacific Northwest region of North America to assess current knowledge of channel stability and morphology relevant to riparian management practices around small streams. Small channels are defined as ones in which morphology and hydraulics may be significantly influenced by individual clasts or wood materials in the channel. Such channels are headwater channels in close proximity to sediment sources, so they reflect a mix of hillslope and channel processes. Sediment inputs are derived directly from adjacent hillslopes and from the channel banks. Morphologically significant sediments move mainly as bed load, mainly at low intensity, and there is no standard method for measurement. The larger clastic and woody elements in the channel form persistent structures that trap significant volumes of sediment, reducing sediment transport in the short term and substantially increasing channel stability. The presence of such structures makes modeling of sediment flux in these channels – a potential substitute for measurement – difficult. Channel morphology is discussed, with some emphasis on wood related features. The problem of classifying small channels is reviewed, and it is recognized that useful classifications are purpose oriented. Reach scale and channel unit scale morphologies are categorized. A "disturbance cascade" is introduced to focus attention on sediment transfers through the slope channel system and to identify management practices that affect sediment dynamics and consequent channel morphology. Gaps in knowledge, errors, and uncertainties have been identified for future research. [ABSTRACT FROM AUTHOR]

Magnetically tagged particles were used to investigate the effects of sockeye salmon (Oncorhynchus nerka) and floods on the dispersion of coarse bed material in the Stuart-Takla region, British Columbia, Canada. The dominant annual sediment transporting event in the channels is the snowmelt flood events, with lesser activity usually accomplished during summer floods. Annually in August, the channel bed material is reworked by the Early Stuart sockeye salmon spawning, as the fish excavate the streambed to deposit and bury their eggs. These nesting cavities are called redds. Results from 67 tracer recovery experiments over five years were highly variable, subject to the magnitude of floods and the returning population of salmon. Overall, the depositional pattern from nival flood events usually demonstrated a high degree of clast mobilization, long travel distances (up to 150 m), and mean depths of burial up to 18 cm. Summer flood events showed somewhat lower rates of mobilization, distances of travel, and depths of burial. Although the fish did not move the tracers very far, their effect on the bed was generally quite pervasive -- up to 100 percent of the clasts were mobilized, and the depth of burial was considerable (mean burial depths up to 14 cm). The amount of vertical mixing of sediment by salmon was often on the same order of magnitude as flood events. The significant vertical mixing of sediments by the fish has important implications for the mobility of sediment in the stream. Since any armoring layer formed during high flows throughout the year is subject to the bioturbation of salmon, this suggests that the transport threshold in the creeks remains relatively low. Salmon likely play an integral role in the sediment transport dynamics and annual sediment budget of the lower reaches of these creeks. [ABSTRACT FROM AUTHOR]