Flow hydraulics and geomorphic effects of glacial‐lake outburst floods in the Mount Everest region,Nepal

Glacial‐lake outburst floods (GLOFs) on 3 September 1977 and 4 August 1985 dramatically modified channels and valleys in the Mount Everest region of Nepal by eroding, transporting, and depositing large quantities of sediment for tens of kilometres along the flood routes. The GLOF discharges were 7 to 60 times greater than normal floods derived from snowmelt runoff, glacier meltwater, and monsoonal precipitation (referred to as seasonal high flow floods, SHFFs). Specific stream power values ranged from as low as 1900 W m^?2 in wide, low‐gradient valley segments to as high as 51 700 W m^?2 in narrow, high‐gradient valley segments bounded by bedrock. Along the upper 16 km of the GLOF routes, the reach‐averaged specific stream power of the GLOFs was 3·2 to 8·0 times greater than the reach‐averaged specific stream power of the SHFFs. The greatest geomorphic change occurred along the upper 10 to 16 km of the GLOF routes, where the ratio between the GLOF specific stream power and the SHFF specific stream power was the greatest, there was an abundant supply of sediment, and channel/valley boundaries consisted primarily of unconsolidated sediment. Below 11 to 16 km from the source area, the geomorphic effects of the GLOFs were reduced because of the lower specific stream power ratio between the GLOFs and SHFFs, more resistant bedrock flow boundaries, reduced sediment supply, and the occurrence of past GLOFs. Copyright © 2003 John Wiley & Sons, Ltd.     

Predicting fine sediment dynamics along a pool-riffle mountain channel

Fine sediment dynamics in mountain rivers are of concern because of implications for aquatic habitat, channel stability, and downstream sediment yields. Many mountain river systems have episodic fine sediment transport because of infrequent, point-source sediment inputs from landslides; basin instability triggered by land uses such as logging; or infrequent mobilization of the coarse surface layer in channels. Dam removal, which is now more likely along mountain rivers, may also provide a substantial fine sediment input to downstream channel reaches.Fine sediment storage in the interstices of spawning gravels and within pools along mountain rivers is of particular interest because of impacts to aquatic organisms. In this study we focus on sediment dynamics within pools of the North Fork Poudre River in Colorado as an example of the processes controlling fine sediment deposition, storage, and transport within laterally constricted pools. The 1996 release of 7000 m³ of silt-to gravel-sized sediment from a reservoir on the North Fork provided an opportunity to develop a field data set of fine sediment dynamics and to test the predictions of three different one- or two-dimensional sediment transport and hydraulic models against the field observations.The models were calibrated against quantitative measurements of pool scour and fill. One-dimensional HEC-6 results indicate that robust simulations yield the greatest agreement between predicted and measured pool bed elevation change. Model calibration on two pools and validation on one pool indicate that at least 58% of observed bed changes after the sediment release were predicted by HEC-6. Modeling accuracy using quasi-two-dimensional GSTARS 2.0 was considerably more variable, and no pool-wide trends were obtained. The two-dimensional model RMA2 substantially improved the representation of eddy pool hydraulics within a compound pool of the North Fork. Results from the hydraulic modeling, coupled with bed load and total load computations, delineate areas of scour and deposition which are consistent with observations in the field.A conceptual model of sediment delivery and storage for laterally confined pools suggests that persistent deposition of fine sediment within eddies distal from the sediment source may result from sediment releases. The original loss of channel capacity facilitated additional deposition within eddies as sediment within upstream proximal pools became mobilized. At high discharges, the development of a strong shear zone prevents degradation of sediment deposits within the eddy. Central portions of these proximal pools may clear according to existing models, whereas deposition within recirculating zones may be long-term. Water managers could use these models to estimate minimum pool volume for overwinter habitat and residence time of pool sediment.     

Introduction to the themed issue: Wildfire and Geomorphic Systems

Wildfires alter fluxes of water, sediment, solutes, and organic matter in ways that can be transient or persistent. Alterations in material flux then impact geomorphic processes and landforms in a manner that can also be transient or persistent and that can involve complex response. This themed issue includes papers that document distinctive geomorphic responses in the upland and channel components of forested landscapes in southern Europe, northern Australia, and the western United States. Among the themes that emerge from the collected papers are: (i) the importance of ongoing technological developments, including real‐time instrumentation, ground‐based and aerial remote sensing, isotopic ratios, and numerical models of landscape processes, for documenting and predicting fire‐related geomorphic processes; and (ii) the great uncertainties about future landscape change in a global environment of rapidly changing climate and growing human populations that encroach onto remaining wildlands in fire‐susceptible regions. Copyright © 2017 John Wiley & Sons, Ltd.     

Historical land use as a driver of alternative states for stream form and function in forested mountain watersheds of the Southern Rocky Mountains

We demonstrate how land use can drive mountain streams in the Southern Rockies across a threshold to induce an alternative state of significantly reduced physical complexity of form and reduced ecological function. We evaluate field data from 28 stream reaches in relatively laterally unconfined valleys and unmanaged forest that is either old‐growth forest or naturally disturbed younger forest, and 19 stream reaches in managed forest with past land use. We evaluate potential differences in stream form, as reflected in channel planform, cross‐sectional geometry, and in‐stream wood loads, and stream function, as reflected in pool volume and storage of organic carbon. Field data indicate a threshold of differences in stream form and function between unmanaged and managed stream reaches, regardless of forest stand age, supporting our hypothesis that the legacy effects of past land use result in an alternative state of streams. Because physical complexity that increases stream retentiveness and habitat can maintain aquatic‐riparian ecosystem functions, the alternative physical state of streams in managed watersheds creates a physical template for an alternative ecological state with reduced pool volume, organic carbon storage, and ecosystem productivity. We recommend maintaining riparian forests that can supply large wood to streams as a stream restoration technique in historically forested stream segments. Copyright © 2017 John Wiley & Sons, Ltd.     

How does anisotropy in bedrock river granitic outcrops influence pothole genesis and development?

Pothole formation and development may be influenced by joint sets and other heterogeneities within bedrock, as well as by hydraulics. Previous research indicates that most potholes found in rivers of the mountainous Spanish Central System exhibit preferred orientations associated with dominant joints and correlate more strongly with variations in substrate resistance than with hydraulics. Weathering and erosion weaken rock surfaces, which leads to decreased mechanical resistance. We start from the hypothesis that different mechanisms of pothole formation may create around the pothole a distinctive signature in terms of ultrasound pulse velocity and surface hardness. We develop a conceptual model and test it using potholes for which we know the mechanism of formation, demonstrating that the spatial and statistical distributions of dynamical mechanical properties and surface hardness of a pothole may provide insight into its genesis. Copyright © 2016 John Wiley & Sons, Ltd.     

Trends of grain sizes on gravel bars in the Rio Chagres, Panama

We examined the trends of grain sizes along the upper 414 km² of the mountainous Rio Chagres drainage basin in Panama. Gravel bars were sampled along 40 km of the Rio Chagres and five major tributary streams using a transect pebble count of median diameter, lithology, and clast rounding. Although previous investigators have found that downstream fining can be obscured by inputs of colluvial sediment and other local controls in mountain drainages, we decided to examine the trends of grain sizes along a tropical mountain river where rapid weathering and high capability of transport might be capable of overriding the input effects of colluvium. Specifically, we tested the hypotheses that downstream fining would be present as a result of selective sorting, and that weak felsic particles would decrease in size preferentially to strong mafic particles because of abrasion. Statistical analyses reveal a weak downstream decrease of sediment size on gravel bars along the study reach of the Rio Chagres, with a Sternberg diminution coefficient (α) for felsic and mafic grains of − 0.013 and − 0.017, respectively. Felsic clasts have thicker weathering rinds and become rounded downstream faster than mafic particles, but tumbling-mill tests of abrasion show no significant differences in rate of mass loss in relation to lithology, and downstream decreases in grain size are similar between lithologies. Dividing the study reach into six sub-reaches bounded by major tributary junctions, we further tested the hypothesis that downstream trends in fining might be obscured at the basin scale by sediment input from tributaries, but that trends in grain sizes might be more visible at the reach scale between tributaries. We did not find any consistent trends in grain size between tributaries. Stream width appears to assert a local control on grain size; coarse particles are associated with narrow channel reaches, whereas smaller particles are associated with wide channel reaches.     

Remote sensing of large wood in high-resolution satellite imagery: Design of an automated classification work-flow for multiple wood deposit types

Wood researchers increasingly rely on remote-sensing products to augment field information about wood deposits in river corridors. The availability of very high-resolution (<1 m) satellite imagery makes capturing wood over greater spatial extents possible, but previous studies have found difficulty in automatically extracting wood deposits due to the challenge in distinguishing wood from spectrally similar corridor features such as sand. We also lack knowledge on the spectral properties of different wood deposit types in multiple depositional environments. In this work, we explore image classification work-flows for four wood deposit types in three North American environments: in-channel jams deposited in the Tatshenshini River in Alaska, USA; a wood raft on the Slave River in Northwest Territories, Canada; and wood deposited along a lakeshore and coastal embayment in the Mackenzie River Delta in Northwest Territories, Canada. We compare classification results of object-based and pixel-based image analysis with supervised [support vector machine (SVM)] and unsupervised (ISO clustering) classifiers. We evaluate several accuracy assessment parameters and achieve overall classification accuracies of 65–99%, showing automated image classification is a possible approach for analysing wood across larger areas. We also find that wood sensitivity in the classification ranged from 0 to 95%, indicating that some techniques are better suited to wood capture than others. We find that supervised classification produced more accurate wood maps, though there is large variation in classification outcomes across environments related to spatial arrangement of wood in the landscape. We discuss the influence of depositional environment on classification and provide recommendations for designing a wood classification work-flow.     

Wood distribution in neotropical forested headwater streams of La Selva,Costa Rica

Surveys of wood along 30 forested headwater stream reaches in La Selva Biological Station in north‐eastern Costa Rica represent the first systematic data reported on wood loads in neotropical streams. For streams with drainage areas of 0·1–8·5 km² and gradients of 0·2–8%, wood load ranged from 3 to 34·7 m³ wood/100 m channel and 41–612 m³ wood/ha channel. These values are within the range reported for temperate streams. The variables wood diameter/flow depth, stream power, the presence of backflooding, and channel width/depth are consistently selected as significant predictors by statistical models for wood load. These variables explain half to two‐thirds of the variability in wood load. These results, along with the spatial distribution of wood with respect to the thalweg, suggest that transport processes exert a greater influence on wood loads than recruitment processes. Wood appears to be more geomorphically effective in altering bed elevations in gravel‐bed reaches than in reaches with coarser or finer substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

Large in‐stream wood studies: a call for common metrics

During the past decade, research on large in‐stream wood has expanded beyond North America's Pacific Northwest to diverse environments and has shifted toward increasingly holistic perspectives that incorporate processes of wood recruitment, retention, and loss at scales from channel segments to entire watersheds. Syntheses of this rapidly expanding literature can be facilitated by agreement on primary variables and methods of measurement. In this paper we address these issues by listing the variables that we consider fundamental to studies of in‐stream wood, discussing the sources of variability in their measurement, and suggesting more consistency in future studies. We recommend 23 variables for all studies of in‐stream wood, as well as another 12 variables that we suggest for studies with more specific objectives. Each of these variables relates either to the size and characteristics of in‐stream wood, to the geomorphic features of the channel and valley, or to the ecological characteristics of the riparian zone adjacent to the study reach. The variables were derived from an overview of those cited in the literature and from our collective field experiences. Copyright © 2010 John Wiley & Sons, Ltd.