Coarse sediment supply sets the slope of bedrock channels in rapidly uplifting terrain: Field and topographic evidence from eastern Taiwan

The incision rate and steepness of bedrock channels depend on water discharge, uplift rate, substrate lithology, sediment flux, and bedload size. However, the relative role of these factors and the sensitivity of channel steepness to rapid (>1 mm yr⁻¹) uplift rates remain unclear. We conducted field and topographic analyses of fluvial bedrock channels with varying channel bed lithology and sediment source rock along the Coastal Range in eastern Taiwan, where uplift rates vary from 1.8 to 11.8 mm yr⁻¹ and precipitation is relatively consistent (1.5–2.7 m yr⁻¹), to evaluate the controls on bedrock channel steepness. We find that channel steepness is independent of rock uplift rate and annual precipitation but increases monotonically with sediment size and substrate strength. Furthermore, in reaches with uniform substrate lithology (mudstone and flysch), channel steepness systematically varies with sediment source rock but not with channel width. When applied to our data, a mechanistic incision model (saltation-abrasion model) suggests that the steepness of Coastal Range channels is set primarily by coarse-sediment supply. We also observe that larger particles are mainly composed of resistant lithologies derived from volcanic rocks and conglomerates. This result implies that hillslope bedrock properties in the source area exert a dominant control on the steepness of proximal channels through coarse-sediment production in this setting. We propose that channel steepness may be insensitive to uplift rate and flow discharge in fast-uplifting landscapes where incision processes are set by coarse sediment size and supply. Models assuming a proportionality between incision rate and basal shear stress (stream power) may not fully capture controls on fluvial channel profiles in landslide-dominated landscapes. Processes other than channel steepening, such as enhanced bedload impacts and debris-flow scour, may be required to balance rock uplift and incision in these transport-limited systems.     

Reach-scale changes in channel geometry and dynamics due to the coastal backwater effect: the lower Trinity River,Texas

In this paper we use multiple field surveys spanning several decades to systematically evaluate the geomorphic consequences of a change in flow hydraulics from uniform flow to backwater flow for the lower Trinity River in east Texas, USA. Spatial changes in lateral migration rate, channel geometry, and point bar size correspond to two distinct geomorphic zones. Within the upstream uniform flow reach, the river channel is defined by fully developed point bars and a high rate of lateral channel migration. This zone transitions where the median channel bottom elevation drops below sea level. At this point flow is affected by the backwater influence of the Trinity Bay water surface elevation, as opposed to being bed slope control dominated. The change in hydraulics within the backwater zone is reflected in the channel morphology, which is characterized by smaller point bars, narrower and more symmetrical cross-sectional channel geometry, lower channel migration rates, and little to no bend deformation or cutoffs. Studying the connection between channel geometry, river bend kinematics, sediment transport, and fluid mechanics in each zone provides a deeper understanding of the relationship between channel shape and river mechanics. © 2019 John Wiley & Sons, Ltd.     

Field evidence for the influence of weathering on rock erodibility and channel form in bedrock rivers

Erosion processes in bedrock‐floored rivers shape channel cross‐sectional geometry and the broader landscape. However, the influence of weathering on channel slope and geometry is not well understood. Weathering can produce variation in rock erodibility within channel cross‐sections. Recent numerical modeling results suggest that weathering may preferentially weaken rock on channel banks relative to the thalweg, strongly influencing channel form. Here, we present the first quantitative field study of differential weathering across channel cross‐sections. We hypothesize that average cross‐section erosion rate controls the magnitude of this contrast in weathering between the banks and the thalweg. Erosion rate, in turn, is moderated by the extent to which weathering processes increase bedrock erodibility. We test these hypotheses on tributaries to the Potomac River, Virginia, with inferred erosion rates from ~0.1 m/kyr to >0.8 m/kyr, with higher rates in knickpoints spawned by the migratory Great Falls knickzone. We selected nine channel cross‐sections on three tributaries spanning the full range of erosion rates, and at multiple flow heights we measured (1) rock compressive strength using a Schmidt hammer, (2) rock surface roughness using a contour gage combined with automated photograph analysis, and (3) crack density (crack length/area) at three cross‐sections on one channel. All cross‐sections showed significant (p < 0.01 for strength, p < 0.05 for roughness) increases in weathering by at least one metric with height above the thalweg. These results, assuming that the weathered state of rock is a proxy for erodibility, indicate that rock erodibility varies inversely with bedrock inundation frequency. Differences in weathering between the thalweg and the channel margins tend to decrease as inferred erosion rates increase, leading to variations in channel form related to the interplay of weathering and erosion rate. This observation is consistent with numerical modeling that predicts a strong influence of weathering‐related erodibility on channel morphology. Copyright © 2017 John Wiley & Sons, Ltd.     

Abrupt variation in channel width along part of the River Severn,near Shrewsbury,Shropshire, England

The River Severn, near Shrewsbury, Shropshire, England, is formed by the union of the uppermost Severn with the Vyrnwy; both of these major headstreams drain from the Welsh upland. That part of the trunk Severn here under consideration has been under investigation for some years as exemplifying Osage-type underfitness. In the course of the investigation, a census was made of width between banktops on two reaches, and of water surface width on one of these reaches: the difference between the two types of width gives the value of a third type, the width of batter. All three width types produce Poisson distributions of frequency. Width, particularly width between banktops, varies rapidly in the downstream direction. In view of the observed Poisson distributions, transform rather than arithmetic values have been used in difference-of-means tests applied to variation along the channel: cube root transforms are the type selected. Although there is little to choose, on the study reaches, between arithmetic means and Poisson modes, the results obtained do suggest a query about the selection of the most appropriate width values, for instance for the purposes of channel morphology. All types of width are found to vary along the channel far more rapidly than would be expected from mere randomness. Variation in bank strength appears to account for about half the variation in width between banktops. The results obtained for batter width could be applied in the design of a sampling programme designed to identify local difference in bank strength.     

Channel‐planform evolution in four rivers of Olympic National Park,Washington, USA: the roles of physical drivers and trophic cascades

Identifying the relative contributions of physical and ecological processes to channel evolution remains a substantial challenge in fluvial geomorphology. We use a 74‐year aerial photographic record of the Hoh, Queets, Quinault, and Elwha Rivers, Olympic National Park, Washington, USA, to investigate whether physical or trophic‐cascade‐driven ecological factors – excessive elk impacts after wolves were extirpated a century ago – are the dominant drivers of channel planform in these gravel‐bed rivers. We find that channel width and braiding show strong relationships with recent flood history. All four rivers widened significantly after having been relatively narrow in the 1970s, consistent with increased flood activity since then. Channel planform also reflects sediment‐supply changes, evident from landslide response on the Elwha River. We surmise that the Hoh River, which shows a multi‐decadal trend toward greater braiding, is adjusting to increased sediment supply associated with rapid glacial retreat. These rivers demonstrate transmission of climatic signals through relatively short sediment‐routing systems that lack substantial buffering by sediment storage. Legacy effects of anthropogenic modification likely also affect the Quinault River planform. We infer no correspondence between channel evolution and elk abundance, suggesting that trophic‐cascade effects in this setting are subsidiary to physical controls on channel morphology. Our findings differ from previous interpretations of Olympic National Park fluvial dynamics and contrast with the classic example of Yellowstone National Park, where legacy effects of elk overuse are apparent in channel morphology; we attribute these differences to hydrologic regime and large‐wood availability. Published 2016. This article is a U.S. Government work and is in the public domain in the USA     

Effect of storms during drought on post‐wildfire recovery of channel sediment dynamics and habitat in the southern California chaparral,USA

Current global warming projections suggest a possible increase in wildfire and drought, augmenting the need to understand how drought following wildfire affects the recovery of stream channels in relation to sediment dynamics. We investigated post‐wildfire geomorphic responses caused by storms during a prolonged drought following the 2013 Springs Fire in southern California (USA), using multi‐temporal terrestrial laser scanning and detailed field measurements. After the fire, a dry‐season dry‐ravel sediment pulse contributed sand and small gravel to hillslope‐channel margins in Big Sycamore Creek and its tributaries. A small storm in WY 2014 generated sufficient flow to mobilize a portion of the sediment derived from the dry‐ravel pulse and deposited the fine sediment in the channel, totaling ~0.60 m³/m of volume per unit length of channel. The sediment deposit buried step‐pool habitat structure and reduced roughness by over 90%. These changes altered sediment transport characteristics of the bed material present before and after the storm; the ratio of available to critical shear stress (τ_o/τ_c) increased by five times. Storms during WY 2015 contributed additional fine sediment from tributaries and lower hillslopes and hyperconcentrated flow transported and deposited additional sediment in the channel. Together these sources delivered sediment on the order of six times that in 2014, further increasing τ_o/τ_c. These storms during multi‐year drought following wildfire transformed channel dynamics. The increased sediment transport capacity persisted during the drought period characterized by the longer residence time of relatively fine‐grained post‐fire channel sedimentation. This contrasts with wetter years, when post‐fire sediment is transported from the fluvial system during the same season as the post‐fire sediment pulse. Results of this short‐term study highlight the complex and substantial effects of multi‐year drought on geomorphic responses following wildfire. These responses influence pool habitat that is critical to longer‐term post‐wildfire riparian ecosystem recovery. Copyright © 2017 John Wiley & Sons, Ltd.     

The effects of deforestation on slope and channel evolution in the tectonically active Darjeeling Himalaya

The main indicators of Quaternary tectonic uplift are the young mountain slopes of the Darjeeling Himalaya, rising straight above the Ganga–Brahmaputra foredeep, fragments of uplifted river terraces and fresh fault scarps. Evidence for the continuation of the uplift includes downcutting of the Tista and other straight rivers in the bedrock, continuing aggradation in the plains and overriding of the metamorphic rocks on the alluvia. Owing to deforestation and extensive land use, the earlier natural tendency of a dominance of channel incision over slope degradation has changed to prevailing aggradation, even in steep valley reaches, caused by intensive slope mass movements and the overloading of the mountain creeks. Aggradation progresses upstream along the rivers dissecting the mountain front.     

Mapping landscape evolution in 3D: Climate change,natural hazard and human settlements across the 1618 Piuro landslide in the Italian Central Alps

Natural and anthropogenic mountain landscapes coevolve responding on different temporal scales to climate changes and geodynamics by a series of increments that cause the dynamic association of morphological stabilization surfaces, stratigraphic units and landforms. Understanding the incremental history of palimpsest landscapes helps to recognize and forecast the effects of climate change on the sensitive mountain environments, contributes to archaeological and historical reconstruction and supports management strategies for natural risks prevention and mitigation. The Italian Bregaglia Valley provides an excellent site to unravel the recent/historical increments of evolution of landforms and human settlement, permitting to map the paleo-digital terrain models (DTMs) corresponding to the relevant landscape turning points. After the last de-glaciation, two large-scale landslides reshaped the valley floor, both predisposed by deep-seated gravitational slope deformations and one surely triggered by intense rainfalls. The most recent and impacting event buried in 1618 the rich border town of Piuro, the ancient one occurred in the same area at least 1.5 ka before. Combining stratigraphic, geomorphological, topographic, archaeological and historical data, we drew the paleo-DTMs of the pre- and post-1618 settings of Piuro, sketching the landscape evolution. Since two millennia, human settlements took advantage of the decadal to secular most stable surfaces, represented by the inactive lobes of debris-flow fans, the highest trunk river terraces and the top of humps formed by the ancient landslide body in the valley centre. Stratigraphic relationships, archaeological findings and age determinations show that both landslides diverted the trunk river and covered the existing fan lobes. On a secular timescale, fan progradation and trunk river terracing buried and reworked both the landslide bodies. The paleo-DTMs show their original areal extent and permit to compute their volume and to sketch the setting of the buried Piuro settlements, drawing the changes of the Mera trunk river course and the chronology of activity of the lateral debris-flow fan lobes.     

The effect of lithology on valley width,terrace distribution,and bedload provenance in a tectonically stable catchment with flat‐lying stratigraphy

How rock resistance or erodibility affects fluvial landforms and processes is an outstanding question in geomorphology that has recently garnered attention owing to the recognition that the erosion rates of bedrock channels largely set the pace of landscape evolution. In this work, we evaluate valley width, terrace distribution, and bedload provenance in terms of reach scale variation in lithology in the study reach and discuss the implications for landscape evolution in a catchment with relatively flat‐lying stratigraphy and very little uplift. A reach of the Buffalo National River in Arkansas was partitioned into lithologic reaches and the mechanical and chemical resistance of the main lithologies making up the catchment was measured. Valley width and the spatial distribution of terraces were compared among the different lithologic reaches. The surface grain size and provenance of coarse (2–90 mm) sediment of both modern gravel bars and older terrace deposits that make up the former bedload were measured and defined. The results demonstrate a strong impact of lithology upon valley width, terrace distribution, and bedload provenance and therefore, upon landscape evolution processes. Channel down‐cutting through different lithologies creates variable patterns of resistance across catchments and continents. Particularly in post‐tectonic and non‐tectonic landscapes, the variation in resistance that arises from the exhumation of different rocks in channel longitudinal profiles can impact local base levels, initiating responses that can be propagated through channel networks. The rate at which that response is transmitted through channels is potentially amplified and/or mitigated by differences between the resistance of channel beds and bedload sediment loads. In the study reach, variation in lithologic resistance influences the prevalence of lateral and vertical processes, thus producing a spatial pattern of terraces that reflects rock type rather than climate, regional base level change, or hydrologic variability. Copyright © 2017 John Wiley & Sons, Ltd.     

Testing geomorphic signal of active normal faulting: The case of the Cittanova Fault (Calabria,southern Italy)

Extracting tectonic signals from the landscape is an important challenge for constraining the style and rate of deformation associated with active faults, especially where their displacement history cannot be independently determined. Based on previous paleoseismological data coupled with new geomorphological field work and ¹⁴C dating of geomorphic markers, we analysed the geomorphic signal of the along‐strike differential throw of the Cittanova Fault in southern Calabria (Italy), the recent activity of which is already well documented and constrained. Through DTM‐derived stream power law parameters (SL and χ), we provide evidence of drainage network disequilibrium and reorganization in response to fault growth and deformation style. Furthermore, a methodological test of the reliability of the χ metric as a proxy for the differential throw along the strike of active normal faults provided good preliminary results, consistent with a strong inverse linear correlation with fault throw. Copyright © 2018 John Wiley & Sons, Ltd.     

The influence of landscape connectivity and landslide dynamics upon channel adjustments and sediment flux in the Liwu Basin,Taiwan

Catchment‐scale analyzes of spatial and temporal variability in landscape connectivity are critical considerations in appraisals of landscape evolution and disaster mitigation in tectonically active mountain belts such as Taiwan. This study uses historical aerial photographs, flow discharge and seismic data to analyze landslide changes and channel adjustments over a 30 year period in the Liwu Basin. Recurrent earthquakes and typhoon events trigger frequent landslide activity, channel adjustment and sediment reworking in this system. Spatial variability in magnitude–frequency relations of hillslope‐valley floor (lateral) and upstream–downstream (longitudinal) connectivity during the study period are shown to reflect annual reworking in source and accumulation zones, while partly‐confined valleys in the mid‐catchment area trap sediment behind landslide‐induced dams that are formed and breached on an approximately decadal basis. This promotes partial longitudinal connectivity in these areas. Landscape responses to disturbance events were especially pronounced following combinations of seismic and typhoon events prior to the 1998 and 2005 images. Although single high magnitude events and series of moderate events affect patterns of landscape connectivity in the Liwu Basin, residence times for sediment storage are very short in this highly‐connected river system, where confined valley settings extend virtually to the coast. Copyright © 2014 John Wiley & Sons, Ltd.     

Remotely sensed rivers in the Anthropocene: state of the art and prospects

The rivers of the world are undergoing accelerated change in the Anthropocene, and need to be managed at much broader spatial and temporal scales than before. Fluvial remote sensing now offers a technical and methodological framework that can be deployed to monitor the processes at work and to assess the trajectories of rivers in the Anthropocene. In this paper, we review research investigating past, present and future fluvial corridor conditions and processes using remote sensing and we consider emerging challenges facing fluvial and riparian research. We introduce a suite of remote sensing methods designed to diagnose river changes at reach to regional scales. We then focus on identification of channel patterns and acting processes from satellite, airborne or ground acquisitions. These techniques range from grain scales to landform scales, and from real time scales to inter-annual scales. We discuss how remote sensing data can now be coupled to catchment scale models that simulate sediment transfer within connected river networks. We also consider future opportunities in terms of datasets and other resources which are likely to impact river management and monitoring at the global scale. We conclude with a summary of challenges and prospects for remotely sensed rivers in the Anthropocene. © 2019 John Wiley & Sons, Ltd.     

Field evidence for the control of grain size and sediment supply on steady‐state bedrock river channel slopes in a tectonically active setting

We exploit a natural experiment in Boulder Creek, a ~ 30 km² drainage in the Santa Cruz mountains, CA, USA to explore how an abrupt increase in the caliber of bedload sediment along a bedrock channel influences channel morphology in an actively uplifting landscape. Boulder Creek's bedrock channel, which is entirely developed on weak sedimentary rock, has a high flow shear stress that is about 3.5 times greater where it transports coarse (~ 22 cm D₅₀) diorite in the lower reaches in comparison with the upstream section of the creek that transports only relatively finer bedload (~2 cm D₅₀) derived from weak sedimentary rocks. In addition, Boulder Creek's channel abruptly widens and shallows downstream and transitions from partial to nearly continuous alluvial cover where it begins transporting coarse diorite. Boulder Creek's tributary channels are also about three times steeper where they transport diorite bedload, and within the Santa Cruz mountains channels in sedimentary bedrock are systematically steeper when >50% of their catchment area is within crystalline basement rocks. Despite this clear control of coarse sediment size on channel slopes, the threshold of motion stress for bedload, alone, does not appear to control channel profile slopes here. Upper Boulder Creek, which is starved of coarse sediment, maintains high flow shear stresses well in excess of the threshold for motion. In contrast, lower Boulder Creek, with a greater coarse sediment supply, exerts high flow stresses much closer to the threshold for motion. We speculate that upper Boulder Creek has evolved to sustain partial alluvial cover and transfer greater energy to the bed via bedload impacts to compensate for its low coarse sediment supply. Thus bedload supply, bedrock erosion efficiency, and grain size all appear to influence channel slopes here. Copyright © 2017 John Wiley & Sons, Ltd.     

Characteristics,extent and origin of hydrothermal alteration at Mount Rainier Volcano,Cascades Arc,USA: Implications for debris-flow hazards and mineral deposits

Hydrothermal alteration at Mount Rainier waxed and waned over the 500,000-year episodic growth of the edifice. Hydrothermal minerals and their stable-isotope compositions in samples collected from outcrop and as clasts from Holocene debris-flow deposits identify three distinct hypogene argillic/advanced argillic hydrothermal environments: magmatic-hydrothermal, steam-heated, and magmatic steam (fumarolic), with minor superimposed supergene alteration. The 3.8 km³ Osceola Mudflow (5600 y BP) and coeval phreatomagmatic F tephra contain the highest temperature and most deeply formed hydrothermal minerals. Relatively deeply formed magmatic-hydrothermal alteration minerals and associations in clasts include quartz (residual silica), quartz–alunite, quartz–topaz, quartz–pyrophyllite, quartz–dickite/kaolinite, and quartz–illite (all with pyrite). Clasts of smectite–pyrite and steam-heated opal–alunite–kaolinite are also common in the Osceola Mudflow. In contrast, the Paradise lahar, formed by collapse of the summit or near-summit of the edifice at about the same time, contains only smectite–pyrite and near-surface steam-heated and fumarolic alteration minerals. Younger debris-flow deposits on the west side of the volcano (Round Pass and distal Electron Mudflows) contain only low-temperature smectite–pyrite assemblages, whereas the proximal Electron Mudflow and a < 100 y BP rock avalanche on Tahoma Glacier also contain magmatic-hydrothermal alteration minerals that are exposed in the avalanche headwall of Sunset Amphitheater, reflecting progressive incision into deeper near-conduit alteration products that formed at higher temperatures.     

Paleocene deep-water sediments and radiolarian faunas: Implications for evolution of Yarlung-Zangbo foreland basin, southern Tibet

Only a few Paleocene radiolarian assemblages have been reported, while the Early Paleocene zonal schemes remain poorly delineated. The Early Paleocene on-land radiolarians were described in the Hidaka melange belt of Japan and the North Island of New Zeal…     

Mineralogical and geochemical study of a newly discovered mafic granulite, northwest China: Implications for tectonic evolution of the Altay Orogenic Belt

Abstract A mafic granulite body was newly discovered in the Altay Orogenic Belt, northwest China. The rocks comprise a suite of coarse‐grained and fine‐grained granulites. Orthopyroxenes (hypersthenes) in the rocks have high X_Mg and low Al₂O₃ contents, whereas clinopyroxenes have low TiO₂ and Al₂O₃ contents. Amphiboles and biotites have a high Mg/(Mg + Fe²⁺) ratio and low contents of F and Cl. The peak metamorphic pressure–temperature (P–T) conditions are estimated as 750–780°C and 6–7 kbar, and retrograde P–T conditions are in the range of 590–620°C and 2.3–3.7 kbar, indicating significant decompression. Metamorphic reactions and P–T estimates define a clockwise P–T path. Geochemically, the rocks are high in Mg/(Mg + Fe) and Al₂O₃, depleted in U, Th, K and Rb, and characterized by light rare earth element enrichment and a weak positive Eu anomaly. The Altay mafic granulite shows depleted Nb, P and Ti contents in the mid‐oceanic ridge basalt normalized spider diagram. The geochemical characteristics suggest that the protolith of the Altay mafic granulite was calc‐alkaline basalt and andesite with an island‐arc affinity. The rock has a high ¹⁴³Nd/¹⁴⁴Nd ratio with ?Nd(0) > 0, indicating derivation from a mantle‐depleted source. In the present study, a two‐stage model for the evolution of the Altay mafic granulite is proposed: an early stage in which calc‐alkaline basalt and andesite with island‐arc affinity were subducted into a deeper level of the crust and subjected to granulite‐facies metamorphism generating the mafic granulite, followed by the later stage exhumation of the system into the upper crust by the late Paleozoic thrusting.     

Deep-seated gravitational slope deformation (DSGSD) and slow-moving landslides in the southern Tien Shan Mountains: new insights from InSAR,tectonic and geomorphic analysis

We investigated deep-seated gravitational slope deformation (DSGSD) and slow mass movements in the southern Tien Shan Mountains front using synthetic aperture radar (SAR) time-series data obtained by the ALOS/PALSAR satellite. DSGSD evolves with a variety of geomorphological changes (e.g. valley erosion, incision of slope drainage networks) over time that affect earth surfaces and, therefore, often remain unexplored. We analysed 118 interferograms generated from 20 SAR images that covered about 900 km². To understand the spatial pattern of the slope movements and to identify triggering parameters, we correlated surface dynamics with the tectono-geomorphic processes and lithologic conditions of the active front of the Alai Range. We observed spatially continuous, constant hillslope movements with a downslope speed of approximately 71 mm year⁻¹ velocity. Our findings suggest that the lithological and structural framework defined by protracted deformation was the main controlling factor for sustained relief and, consequently, downslope mass movements. The analysed structures revealed integration of a geological/structural setting with the superposition of Cretaceous–Paleogene alternating carbonatic and clastic sedimentary structures as the substratum for younger, less consolidated sediments. This type of structural setting causes the development of large-scale, gravity-driven DSGSD and slow mass movement. Surface deformations with clear scarps and multiple crest lines triggered planes for large-scale deep mass creeps, and these were related directly to active faults and folds in the geologic structures. Our study offers a new combination of InSAR techniques and structural field observations, along with morphometric and seismologic correlations, to identify and quantify slope instability phenomena along a tectonically active mountain front. These results contribute to an improved natural risk assessment in these structures. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd