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Geomorphological knowledge is critical in understanding watershed scale surface processes, including steep mountainous areas and flat lowlands, particularly if the mid- and downstream areas are densely populated and hazard assessments are highly required. However, our knowledge about such surface processes has relatively been limited in some areas in South Asia due likely to the lack of comprehensive studies of geomorphology and related fields. This article undertakes an overview of the geomorphological processes of the disaster-prone deltaic landscape of the Ganges–Brahmaputra–Meghna (GBM), particularly focusing on fluvial processes. The area locates in the downstream of the watershed system including Himalayan Mountains and highly connected with the upper basin morphodynamics, hydrology, and sediment flux. The previous studies are summarized at different geomorphic settings concerning hillslopes, fluvial plains, and coastal areas to provide clarity about the geomorphic processes linking erosion-prone upstream source areas to deposition-dominated downstream areas. The review found that most of the geomorphic researches in Bangladesh are exploring landslide inventory and susceptibility mapping in hilly areas; river channel or riverbank shifting, riverbank erosion and accretion in fluvial environments; watershed morphometric analysis and geomorphic unit identification in plain land; and coastline shifting or coastal erosion and accretion in coastal environments at a small scale. Then, we discuss the fluvial dynamics and sediment transport of the GBM river system to address the knowledge gap in the context of deltaic plain land in Bangladesh, where upstream fluvial sedimentation processes impact the geomorphic connectivity from Himalayan to the Bay of Bengal. Although some studies on the fluvial dynamics and sediment dispersal in the upstream GBM river basin are present, the fluvial processes in the downstream domain of Bangladesh are not fully understood with a limited number of research with field-based approaches. Some future perspectives of geomorphic research in Bangladesh are then mentioned to understand better the complex geomorphological settings in the entire GBM watershed and to strengthen the existing research capacity. This review will also develop a holistic understanding of fluvial geomorphic processes of the GBM River to the policymakers and may be helpful to improve the transboundary river basin management policies or strategies. [ABSTRACT FROM AUTHOR]

Knickzones, defined here as locally steep reaches including distinct knickpoints, in bedrock river morphology, have often been investigated in relation to local anomalies in lithology, tectonics, hydraulics, climate and associated base-level change, and/or deformation of valley-side slopes. However, exact formative causes of many knickzones in a humid, tectonically active island arc remain unclear. Using databases of geology, streams and knickzones, we examine knickzone distribution across the Japanese Archipelago to evaluate the effects of the stream network structure and rock type boundaries on knickzone formation. Knickzones are frequently found just upstream and downstream of major stream confluences along mainstreams, whereas knickzones are less frequent around major rock type boundaries. While the major confluences do not form hanging valleys due to similar catchment size, this observation suggests that many knickzones have been formed by the long-term effect of flow turbulence scouring bedrock at the confluences. Such a hydraulic control on bedrock erosion in the steep Japanese mountains under humid climate conditions indicates that the formative cause of many knickzones therein can be autogenic by means of stream hydraulics. [ABSTRACT FROM AUTHOR]

Abstract: Although a knickzone, a location at which stream gradient is locally large and intense erosion occurs, has been regarded as an important geomorphic feature in bedrock river morphology, the distribution of knickzones has not been well investigated especially for broad area. This study examines the distribution of fluvial knickzones along mountain rivers for the entire Japanese Archipelago. Whereas conventional manual methods of identifying knickzones based on map readings or field observations tend to be subjective and are impractical for a broad-scale analysis, this study employs a semi-automated method of knickzone extraction using DEMs and GIS. In a recent study by the authors, this method has been shown to enable efficient examination of knickzone distribution over a broad area. Investigations on major mountain rivers revealed that knickzones are generally abundant in upstream steep river reaches, suggesting hydraulic origins for the knickzones. The broad presence of such knickzones in the steep Japanese mountain rivers indicates that rivers subjected to active erosion show complex morphology induced by natural irregularities of water flow hydraulics as well as various environmental perturbations such as climatic changes. There also seems to be a characteristic frequency of knickzone distribution common to moderately steep to very steep bedrock reaches in Japan. Although volcanic products such as lavas and welded pyroclastic-flow deposits in valleys can cause distinct knickzones, substrate geology plays only a limited role in determining the distribution and form of knickzones. [Copyright &y& Elsevier]

Abstract: The rate of recession of Niagara Falls (Horseshoe and American Falls) in northeastern North America has been documented since the 19th century; it shows a decreasing trend from ca. 1 m y−1 a century ago to ca. 0.1 m y−1 at present. Reduction of the flow volume in the Niagara River due to diversion into bypassing hydroelectric schemes has often been taken to be the factor responsible, but other factors such as changes in the waterfall shape could play a role and call for a quantitative study. Here, we examine the effect of physical factors on the historically varying recession rates of Niagara Falls, using an empirical equation which has previously been proposed based on a non-dimensional multiparametric model which incorporates flow volume, waterfall shape and bedrock strength. The changes in recession rates of Niagara Falls in the last century are successfully modeled by this empirical equation; these changes are caused by variations in flow volume and lip length. This result supports the validity of the empirical equation for waterfalls in rivers carrying little transported sediment. Our analysis also suggests that the decrease in the recession rate of Horseshoe Falls is related to both artificial reduction in river discharge and natural increase in waterfall lip length, whereas that of American Falls is solely due to the reduction in flow volume. [Copyright &y& Elsevier]

Post-eruptive fluvial erosion of welded pyroclastic flow deposits often depends on the recession of waterfalls because of their rapid erosion involved. We examine the recession rate of Shomyo Falls, which consists of Pleistocene welded pyroclastic flow deposits in Tateyama, north-central Japan. The mean recession rate of the waterfall obtained from lithological and topographical evidences is 0.08-0.15 m/a for 100000 a. However, the recession rate estimated by means of an empirical equation comprising physical parameters of erosive force and bedrock resistance is 0.006-0.011 m/a with small uncertainties. The discrepancy between the geology-based and equation-based recession rates indicates that some factors, not taken account of in the equation, significantly influence the recession rate. We suggest that a factor in the rapid erosion of the waterfall is a large amount of transported sediment acting as abrasive material, which is supplied from high mountains in the watershed above the waterfall. [ABSTRACT FROM AUTHOR]

Abstract: Stream gradients of mountain bedrock rivers in central Japan were examined using GIS and 50-m digital elevation models to discuss the distribution of knickzones. The gradients are classified into local and trend types based on the measurement lengths. The transition rate from the local to trend gradients, i.e. the decreasing rate of gradient with increasing measurement length, is then obtained as the indicator of relative steepness of a river segment, which permits the objective identification of fluvial knickzones. The identified knickzones occur widely in the study area, and indicate that the knickzone is one of the common landforms in Japanese mountain rivers. The knickzones are more abundant in steep upstream reaches of the rivers subjected to active erosion and rapid rock uplift. This suggests a hydraulic origin of the knickzones, which is similar to step-pool morphology but on a much larger scale. Some knickzones in piedmont areas can be, on the other hand, consistent with tectonic faulting at the mountain foot. Although some influences of volcanic activities on the formation of knickzones are observed, regional lithology seems to exert a relatively weak influence on the abundance of knickzones. Knickzones are more consistent with other local mechanisms of fluvial erosion, including reach-scale processes. [Copyright &y& Elsevier]

We conducted topographic measurements using unmanned aerial vehicles and by terrestrial laser scanning at inaccessible sea cliffs with an overhanging complex shape in eastern Japan. From the repeated measurements, we constructed multitemporal three-dimensional point clouds of the sea cliffs to quantitatively evaluate the volumetric changes in the cliffs with a high spatial resolution (in centimeter). As potential main triggers of rockfalls and slope failures, high sea waves and strong earthquakes were examined with the time series of eroded volumes. It is suggested that sea waves have a greater impact than earthquakes on sea-cliff erosion. The approaches provided in this study have great potential for a wide range of applications including high spatial monitoring of other inaccessible, complex-shaped sea cliffs with high data accuracy and low acquisition and operational costs. [ABSTRACT FROM AUTHOR]

Abstract: Channel initiation, which is a key factor in the evolution of mountain landforms, is caused by a combination of various hydrogeomorphic processes. We modeled the channel initiation in steep mountains on the basis of the physical mechanism for sediment transport by surface and subsurface flows. Field investigations and Geographic Information Systems (GIS) analysis in the Higashi-gouchi catchment of central Japan showed that our model can well explain the area–slope relationship in steep and highly incised subcatchments, in which surface flow and shallow underground water would be the dominant flow components. In contrast, the area–slope relationship is not clear in gentler subcatchments, in which the contribution of deeper flow components (i.e., deep underground water) on the entire runoff is not negligible. Thus, the contribution of each runoff component to the total runoff is an important factor affecting the location of the channel head. Most channel heads in the deeply incised subcatchments in the Higashi-gouchi catchment have been formed by surface and subsurface flows, although many landslides have also occurred around the channel heads. Compared with the dominant flow components, activity of sediment supply from hillslopes might be a minor factor in determining the area–slope relationship for locating the channel head. [Copyright &y& Elsevier]