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High-resolution topographic surveys using LiDAR and multibeam sonar can be used to characterize and quantify fluvial change. This study used repeat surveys to explore how topographic change, fluvial processes, sediment budgets, and aggradation and incision rates vary across spatial scales and across two contrasting decadal flow regimes in a regulated gravel/cobble river. A novel method for quantifying digital elevation model uncertainty was developed and applied to a topographic change detection analysis from 2006/2008 to 2014. During this period, which had four modest ~ 3–5 year floods, most sediment was laterally redistributed through bank erosion and channel migration. Erosion primarily occurred in the floodplain (97,000 m 3 ), terraces (80,000 m 3 ), and lateral bars (58,000 m 3 ); while deposition occurred in the adjacent pools (73,000 m 3 ), fast glides (48,000 m 3 ), and runs (36,000 m 3 ). In contrast, significantly higher magnitude and longer duration floods from 1999 to 2006/2008 caused sediment to be displaced longitudinally, with the upstream reaches exporting sediment and the downstream reaches aggrading. The river maintained floodplain connectivity during both periods, despite different processes dominating the type of connectivity. Larger floods promoted overbank scour and avulsion, while smaller floods promoted bank erosion and lateral migration. This study explores and illustrates how the geomorphic response to contrasting flood regimes in a nonuniform river is highly dependent on which landforms are controlling hydraulics. [ABSTRACT FROM AUTHOR]

The supply of sediment from hillslopes to channels is rarely constant, with discrete events (e.g., landslides) known to transfer large volumes of sediment in geologically-short periods of time, especially in tectonically active areas. Understanding the rates and patterns of subsequent sediment evacuation is important for understanding variability of landscape evolution, as well as mitigating the risk of geohazards associated with bed aggradation and the loss of channel capacity to convey flood waters. Here, we performed a series of controlled laboratory flume experiments to explore the controls on sediment transport after a single sediment input in scenarios with (i) different initial input volumes (4–25 kg) under constant flow conditions (40 l/s), (ii) a constant initial input volume (12 kg) under different flow magnitudes (5–60 l/s), and (iii) a repeat of the input sediment volumes in scenario (i) but with the same volume of water delivered using a ramped hydrograph (0–60–0 l/s). We find the presence of sediment piles impacts the flow hydraulics, with a backwater effect developing upstream of the pile that causes a flow acceleration around the location of the pile. For a given discharge, larger sediment piles have a greater impact on the flow hydraulics, which in turn induces higher rates of sediment transport and erosion of the pile. In all cases, sediment remains at the initial pile input location for the duration of the sediment evacuation, acting to protect the bed from erosion. We highlight the role of geomorphic-hydraulic interactions in controlling the sediment evacuation, and suggest there is an optimal combination of pile size and flow conditions (flow magnitude and hydrograph shape) for accelerated rates of sediment transport, which are important for the short-term and long-term channel dynamics and the landscape evolution variability. • Experiments documenting sediment transport processes following landslide inputs. • Identified the co-evolving interactions between hydraulic and geomorphic processes • Backwater effect around sediment pile induces accelerated transport rates. • Highest sediment transport rates associated with larger piles due to backwater effect. [ABSTRACT FROM AUTHOR]

It has been observed that the relation between sediment storage and sediment transport rate in gravel-bed rivers exhibits counter-clockwise hysteresis, or aggradation-degradation cycles. Previous work has attributed these cycles to external changes to the sediment supply rate. We present a set of nine flume experiments that test the hypothesis that aggradation-degradation cycles can occur even under steady sediment supply conditions when the sediment transport rate temporarily nears the sediment supply rate. The experiments are designed to examine the transport-storage relations under a range of conditions with different initial bed condition, sediment supply rate, discharge, and supply material texture. The sediment transport efficiency of the experimental channel was strongly influenced by the existing bed morphology inherited from historical flows. The discharge and grain size distribution of the supply material played a secondary role, with higher discharge conditions and finer sediment feed material producing increased transport efficiencies. The transport-storage relations revealed that the transport rate of the channel changed with the volume of sediment stored and was, therefore, mediated by the channel morphology. Aggradation-degradation cycles were observed even under steady sediment supply and discharge conditions and were mediated by morphologic adjustments within the channel that temporarily led the transport rate to be near equal to the sediment supply rate. In other words, the probability of hysteresis in the transport-storage relation increases when the channel is near a transport equilibrium. The findings demonstrate the complexity associated with developing a unique relation between sediment transport and storage in gravel-bed rivers, and highlights the importance of considering the morphologic evolution and historical bed conditions. • Flume experiments conducted to investigate cycles of aggradation-degradation. • Sediment transport rate variability linked to bed morphology and texture changes. • Transport rate adjusted according to volume of sediment stored. • Aggradation-degradation cycles tend to occur when channel is near equilibrium. [ABSTRACT FROM AUTHOR]

The Jinsha River, the upper reaches of the Yangtze River, winds and incises through the eastern margin of the Tibetan Plateau, where Asian monsoon climate prevails. Along the Jinsha River, extensive and thick Quaternary valley fillings and multiple river terraces have been developed. However, their specific relations to climate change and tectonics remain elusive. In this study, we conducted field investigation, chronological studies and numerical modeling in its lower reaches. Our study indicates that the extensive valley fillings in the Wudongde-Menggu segment were formed in cold-arid climate with Optically Stimulated Luminescence (OSL) ages of ~23–19 ka B.P., coinciding with the timing of the Last Glacial Maximum (LGM). We suggest that the post-LGM warming and humidifying, by restrained sediment supply while enhanced discharge, switched the valley from prevailing aggradation into incision. Multiple subsequent fill-cut terraces, postdating the LGM, were formed and nested in the valley fillings. OSL data indicate that sediments of three terraces (T1–T3) were formed at ~10 ka B.P., ~11.5–12.5 ka B.P. and ~13.5 ka B.P., respectively. Our study suggests that these terraces are sensitive records of millennial and sub-millennial cold-arid climate perturbations superimposed over the post-LGM deglaciation. Our modeling highlights a high sensitivity of valley aggradation and incision to cold-arid and warm-wet climate change, as well as a fast valley landscape response in the Jinsha River characterized by high sediment transport influx. We propose that valley landscape and terraces serve as a timely record for climate change in certain rivers with short response time threshold characterized by high sediment transport influx. • Valley filling was driven by the cold-arid climate in the LGM. • Fill-cut terraces record sub-millennial cold-arid climate perturbations. • Modeling supports a sensitive and fast response of Jinsha River to climate change. • The present valley landscape remains in a transient adjustment since the LGM. [ABSTRACT FROM AUTHOR]

In mountain environments, the coupling of hillslopes processes with the channel network during extreme events is of great importance for rivers dynamics, as debris flows and landslides are among the most important sources of sediments. The Stolla Creek (40 km2 drainage area, South Tyrol, Italy) is a confined and partly confined mountain channel that was affected by an extreme flood in August 2017, followed by a smaller event in August 2020. The geomorphic effects of the two floods were investigated both in the main channel and over the entire basin with the aim to assess the impacts of the lateral sediment connectivity to the channel response and to the event-scale sediment export. An integrated approach was applied, including radar rainfall estimation, hydrologic-hydraulic analysis, analysis of morphological changes and sediment delivery to the stream network. Hillslope and channel processes were mapped and characterized by using geomorphological analysis of multitemporal orthophotos and Digital Terrain Models. Debris-flow connectivity to the main channel was derived by combining field evidence and GIS-based analysis. The 2017 flood was caused by rainfall with a short duration (6 h) and a rainfall intensity exceeding 45 mm/h. More than 600 debris flows were triggered in the Stolla basin, and the main channel experienced widening (width ratio between 1.3 and 4.9) through bank erosion and overbank deposition. Widening was accompanied by aggradation in the river corridor up to 1.2 m or incision down to −2.2 m. The 2020 flood was characterized by lower rainfall intensity (max 17 mm/h) and a longer duration (48 h), and debris flows were not triggered. The moderate magnitude of the 2020 flood peak did not lead to channel widening, but marked bed incision (up to −1.4 m) occurred in the reaches where aggradation took place during the 2017 event. In both flood events, limited volumes of sediments were exported from the catchment outlet. Overall, our results highlight how structural connectivity at the basin scale determines the potential sediment cascades linking hillslopes to channels but time-varying functional connectivity – driven by hydrological drivers as rainfall intensities and durations – eventually control the actual sediment transport effectiveness both on hillslopes and along the channel. • An integrated geomorphological and hydrological approach is applied to two floods • Basin structural connectivity well describes potential sediment delivery to the main channel • Functional connectivity determined by rainfall characteristics controlled actual sediment supply and transport [ABSTRACT FROM AUTHOR]

The dynamics of sediment transport processes over millennial time scales in mountain river valleys remain poorly understood, limiting our overall understanding of sediment routing through river basins. Here, we reconstruct changes of the mainstem riverbed elevation, tributary sediment supply processes, and their inter-relationships since the last interglacial in the upper Ara River valley, central Japan. Aggradation of the mainstem river valley began before 100–95 ka and terminated by 41–40 ka. Then, the elevation of the mainstem floodplain remained stable until ~15 ka. Lateral erosion by the mainstem river, and intermittent periods of incision, continued until 5 ka, after which the mainstem river began actively incising. Aggradation of the mainstem riverbed before 41–40 ka elevated tributary junctions, decreasing the slopes of tributaries. Using reconstructed slope changes, and assuming sediment transport by debris flow in tributaries with slopes steeper than 8% and by traction in shallower tributaries, we classified the tributaries into three types: Type-A, with sediment supply by debris flow since the last interglacial; Type-B, with sediment supply by traction during the last glacial period and afterwards by debris flow; and Type-C, with sediment supply by traction since the last interglacial. Based on the relation between relief ratio and drainage area, we subdivided Type-C tributaries into those that plotted along (Type-C1) or markedly below (Type-C2) the regression line of the mainstem river and Type-A and -B tributaries. During aggradation of the mainstem river, sediments supplied by debris flow from Type-A and -B tributaries accumulated around junctions, interfingering with mainstem deposits. During this period, the junctions of Type-C1 tributaries were elevated almost synchronously with mainstem aggradation, while the junctions of Type-C2 tributaries were blocked by mainstem river deposits, forming blocked-valley lakes. Around 30 ka, Type-A and -B tributaries formed fans around their junctions that were subsequently truncated by lateral erosion by the mainstem river during the late glacial period and early Holocene, forming toe-cut terraces. Tributary deposits that had accumulated around the junctions were then reworked while maintaining a dynamic equilibrium state with the mainstem river. We infer that the responses of tributary landforms and sediment supply processes to the changing elevation of the mainstem riverbed during glacial and interglacial cycles varied depending on the drainage area and relief of each tributary. Considering the river basin as a whole, however, tributary deposits stabilized the elevation of the mainstem floodplain. [ABSTRACT FROM AUTHOR]

Forest management activities have the potential to significantly modify large wood loads in riparian areas, and thus the availability of material for recruitment into the stream channel, hence it is important that forest management practices take account of the role wood supplied from riparian areas play in the development of channel structure and sediment retention capacity. In this study the cellular landscape and river reach process model CAESAR LISFLOOD (C-L) was used to simulate the effect of in-channel obstructions created by large wood on the distribution of erosion and deposition within a small headwater channel and suspended sediment yield at the catchment outlet. The process model was parameterised and calibrated against an observed discharge and suspended sediment record. Following calibration, 12 discharge and large wood obstruction scenarios were carried out to model erosion and suspended sediment transport over a timber harvesting cycle. The inclusion of obstructions into the model runs to simulate in-channel LW pieces substantially reduced suspended sediment yields in all three discharge scenarios and increased sediment storage along the channel. The modelled location of channel bed degradation and aggradation generally coincided with the pattern of sediment storage along the channel bed demonstrating that C_L was predicting the pattern of erosion and deposition in the channel reasonably well. Modelled erosion rates were extremely low for the respective discharge and large wood model scenarios, with the highest rates ranging from 0.02 t ha−1 year−1 to 0.1 t ha−1 year−1 for the no large wood model over the 22-year period. These findings reinforce the importance of maintaining sufficient volume of standing wood in buffer strips alongside small headwater channels to ensure ongoing recruitment of large wood to the channel network in forests managed for timber production. [ABSTRACT FROM AUTHOR]

Many world's large rivers have experienced significant sediment reduction in the past century. However, channel aggradation near the mouth of these rivers may be still occurring or even accelerating, as evidenced by a recent study on the Lowermost Mississippi River. This study aims to investigate whether such a trend exists near the mouth of another large lowland alluvial river, the Atchafalaya River, which enters the Gulf of Mexico via two outlets — the natural mainstem and a man-made straight channel that is shorter and has a much greater slope. The overarching hypothesis is that, despite the reduction in riverine sediment delivery, lowland alluvial rivers such as the Lower Atchafalaya River in their backwater zone aggrade. We collected bathymetric survey records in 1977 and 2006 to analyze riverbed deformation. We utilized stage records at several gauging stations to determine the change in the hydraulic head over time. Geospatial techniques were used to develop channel bed Digital Elevation Models which were used to identify deformation of the 174-km channel bed of the Lower Atchafalaya River. We found a cumulative erosion of 6.34 × 107 m3 and a cumulative deposition of 8.79 × 107 m3, resulting in a net deposition of 2.45 × 107 m3 during the study period. While much of the river including the man-made channel was in a degrading state, the last 35 km of the river's natural mainstem experienced substantial aggradation. On average, riverbed elevation of the Lower Atchafalaya decreased by 1.47 m in the upper 98 km, but increased by 0.96 m in the final 35 km of the river's mainstem to the Gulf of Mexico. The channel erosion in the upstream reach will likely continue with the river discharge projected to increase, and the channel deposition in the downstream mainstem may accelerate because of sea-level rise. • Assessment of channel bed dynamics of a large lowland alluvial river on the Mississippi River Delta • Changes in volume and mass of the channel beds are quantified, and factors affecting them are analyzed. • Channel degradation occurs in the upper reach of the Lower Atchafalaya, mainly affected by flood frequency. • Channel aggradation mainly occurs in the river's final 35 km despite of reduced sediment delivery. [ABSTRACT FROM AUTHOR]