| 1. |
- Abbühl, Luca, et al.
(författare)
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Erosion rates and mechanisms of knickzone retreat inferred from Be-10 measured across strong climate gradients on the northern and central Andes Western Escarpment
- 2011
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Ingår i: Earth Surface Processes and Landforms. - : John Wiley & Sons. - 0197-9337 .- 1096-9837. ; 36, s. 1464-1473
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Tidskriftsartikel (refereegranskat)abstract
- escarpment edge, deep gorges and distinct knickzones in river profiles characterize the landscape on the Western Escarpment of the Andes between ~5°S and ~18°S (northern Peru to northern Chile). The Western Escarpment straddles strong north-south and east-west precipitation gradients, which we exploit in order to determine how climate affects denudation rates in three river basins spanning an otherwise relatively uniform geologic and geomorphologic setting. The Western Escarpment reflects an ongoing transient response to major tectonic perturbation in the Late Miocene when surface uplift created a steep ramp separating drainage systems above the growing escarpment from those below. Upstream of this ramp, and now forming the uplifted Meseta/Altiplano (~3000 m a.s.l.), streams remain graded to the Late Miocene base level constituted by a series of Tertiary volcanic-volcanoclastic rocks, Streams below the ramp have responded to the Late Miocene surface uplift by incising deeply into fractured Mesozoic rocks via a series of steep, headward retreating knickzones that grade to the present-day base level defined by the Pacific Ocean. We find that the Tertiary units function as cap-rocks, which aid in the parallel retreat of the sharp escarpment edge and upper knickzone tips. Upstream of these knickzones, on the Meseta/Altiplano, 10Be-derived catchment denudation rates of the Rio Piura (5°S), Rio Pisco (13°S) and Rio Lluta (18°S) average ~10 mm ky–1 irrespective of precipitation rates; whereas, downstream of the escarpment edge, denudation rates range from 10 mm ky-1 to 250 mm ky-1 and correlate positively with precipitation rates, but show no significant correlation with hillslope angles or channel steepness. These relationships are explained by the presence of the cap-rock and climate-driven fluvial incision that steepens hillslopes to near-threshold conditions. In addition, valley width and the extent of dissection both appear to increase with increasing precipitation to the north, consistent with climate-forcing of landscape morphology in the deeply incised terrain beneath the escarpment edge.Since escarpment retreat and the precipitation pattern were established at least in the Miocene, we speculate that the present-day distribution of morphology and denudation rates has probably remained largely unchanged during the past several millions of years as the knickzones have propagated headward into the plateau.
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| 2. |
- Aronsson, Gunnar, et al.
(författare)
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Grand Canyon : a quantitative approach to the erosion and weathering of a stratified bedrock
- 1982
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 7:6, s. 589-599
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Tidskriftsartikel (refereegranskat)abstract
- A new method is proposed for solving a differential equation arising from weathering-limited development of a valley. Allowance is made for horizontal stratification and overhang can be allowed to develop. The model is applied to the Grand Canyon and comments are included on its relevance
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| 3. |
- Carling, Paul, et al.
(författare)
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Multichannel rivers : their definition and classification
- 2014
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 39:1, s. 26-37
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Tidskriftsartikel (refereegranskat)abstract
- The etymology and historic usage of such terms as anabranch', anastamose' and braided' within river science are reviewed. Despite several decades of modern research to define river channel typologies inclusive of single channels and multiple channel networks, typologies remain ill-conditioned and consequently ill-defined. Conventionally employed quantitative planform characteristics of river networks possibly cannot be used alone to define channel types, yet the planform remains a central part of all modern classification schemes, supplemented by sedimentological and other qualitative channel characteristics. Planform characteristics largely have been defined using non-standardized metrics describing individual network components, such as link lengths, braiding intensity and bifurcation angles, which often fail to separate visually-different networks of channels. We find that existing typologies remain pragmatically utilitarian rather than fundamentally physics-based and too often fail to discriminate between two distinctive and important processes integral to new channel initiation and flow-splitting: (i) in-channel bar accretion, and (ii) channel avulsion and floodplain excision. It is suggested that, first, if channel planform is to remain central to river typologies, then more rigorous quantitative approaches to the analysis of extended integral channel networks at extended reach scales (rather than network components) are required to correctly determine whether visually-different' channel patterns can be discriminated consistently; and, second, if such visually-different styles do in fact differ in their governing processes of formation and maintenance. A significant question is why do so many seemingly equilibrium network geometries possess a large number of anabranches in excess of predictions from theoretical considerations? The key research frontier with respect to initiating and maintaining multichannel networks remains the understanding and discrimination of accretionary-bar flow splitting versus avulsive processes. Existing and new knowledge on flow splitting processes needs to be better integrated into channel typologies.
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| 4. |
- Chandler, Benjamin M. P., et al.
(författare)
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Sub‐annual moraine formation at an active temperate Icelandic glacier
- 2020
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 45:7, s. 1622-1643
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents detailed geomorphological and sedimentological investigations of small recessional moraines at Fjallsjökull, an active temperate outlet of Öræfajökull, southeast Iceland. The moraines are characterised by striking sawtooth or hairpin planforms, which are locally superimposed, giving rise to a complex spatial pattern. We recognise two distinct populations of moraines, namely a group of relatively prominent moraine ridges (mean height ~1.2 m) and a group of comparatively low‐relief moraines (mean height ~0.4 m). These two groups often occur in sets/systems, comprising one pronounced outer ridge and several inset smaller moraines. Using a representative subsample of the moraines, we establish that they form by either (i) submarginal deformation and squeezing of subglacial till or (ii) pushing of extruded tills. Locally, proglacial (glaciofluvial) sediments are also incorporated within the moraines during pushing. For the first time, to our knowledge, we demonstrate categorically that these moraines formed sub‐annually using repeat uncrewed aerial vehicle (UAV) imagery. We present a conceptual model for sub‐annual moraine formation at Fjallsjökull that proposes the sawtooth moraine sequence comprises (i) sets of small squeeze moraines formed during melt‐driven squeeze events and (ii) larger push moraines formed during winter re‐advances. We suggest the development of this process‐form regime is linked to a combination of elevated temperatures, high surface meltwater fluxes to the bed, and emerging basal topography (a depositional overdeepening). These factors result in highly saturated subglacial sediments and high porewater pressures, which induces submarginal deformation and ice‐marginal squeezing during the melt season. Strong glacier recession during the summer, driven by elevated temperatures, allows several squeeze moraines to be emplaced. This process‐form regime may be characteristic of active temperate glaciers receding into overdeepenings during phases of elevated temperatures, especially where their englacial drainage systems allow efficient transfer of surface meltwater to the glacier bed near the snout margin.
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| 5. |
- Dargahi, Bijan
(författare)
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Three-dimensional flow modelling and sediment transport in the River Klaralven
- 2004
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 29:7, s. 821-852
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Tidskriftsartikel (refereegranskat)abstract
- A three-dimensional flow model that uses the RNG k-epsilon turbulence model and a non-equilibrium wall function was applied to the River Klaralven in the southwest part of Sweden. The objectives were to study the nature of the flow in the river bifurcation and to investigate the short-term sediment transport patterns in the river. The effectiveness of three-dimensional flow models depends upon: (1) how well the river geometry and it surface roughness are modelled; and (2) the choice of the closure model. Improvements were obtained by modelling the river in two parts: the entire river reach, and a selected part. Composite Manning coefficients were used to account for roughness properties. The method requires a calibration process that ensures the water surface profiles match the field data. The k-epsilon model under-predicted both the extent of flow separation zones and the number of secondary flow regions having a spiral motion, in comparison with the RNG k-epsilon model. The 3-D model could predict with good accuracy both the general and secondary flow fields in the river. The results agreed well with the 3-D velocity measurements using an acoustic Doppler current profiler. A conceptual model was developed that accounts for the development of secondary flows in a river bifurcation having two bends. The main flow feature in the river cross-sections was the existence of multiple counter-rotating spiral motions. The number of spiral motions increased as the river bends were approached. The river bends also caused vorticity intensification and increased the vertical velocities. The application of the 3-D flow model was extended by solving the sediment continuity equation. The sediment transport patterns were related to the secondary flow fields in the river. The sediment transport patterns at the river bifurcations are characterized by the growth of a sandbank.
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| 6. |
- De Angelis, Hernán, et al.
(författare)
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Palaeo-ice stream onsets: examples from the north-eastern Laurentide Ice Sheet
- 2008
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 33:4, s. 560-572
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we report on observations of glacial landscapes at the head of geomorphologically interpreted palaeo-ice streams, i.e. palaeo-ice-stream onsets. Our work is based on the results of systematic palaeoglaciological mapping of the portion of north-eastern Canada formerly covered by the Laurentide Ice Sheet. Four different cases are considered and analysed in detail: one is the Dubawnt Lake Ice Stream and the others are smaller (<70 km long) palaeo-ice streams. We found that in this region onset zones are generally characterized by a heterogeneous landform record and are more easily recognized when landscapes typical of partially frozen beds are present. Based on publicly available topographical and geological data as well as theoretical developments, we discuss the conditions and controls on the location of these onset zones and their potential stability. Except for the Dubawnt Lake Ice Stream, we found that, although topography and geology are important conditioning factors, the geomorphological imprint of palaeo-ice-stream onsets cannot be explained without invoking the role of the basal thermal boundary between cold- and warm-based ice. Finally, we observed that, except for the Dubawnt Lake Ice Stream, the onset zones of large ice streams are rarely preserved, and we suggest two possible explanations for this. We conclude that although the same qualitative processes are present under the majority of ice streams, conditions for the preservation of the onset zones are more likely to be met in the case of relatively small ice streams because these are more rapidly deglaciated.
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| 7. |
- Dietze, Michael, et al.
(författare)
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A seismic monitoring approach to detect and quantify river sediment mobilization by steelhead redd-building activity
- 2020
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Ingår i: Earth Surface Processes and Landforms. - : John Wiley & Sons. - 0197-9337 .- 1096-9837. ; 45:12, s. 2840-2849
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Tidskriftsartikel (refereegranskat)abstract
- The role of spawning salmonids in altering river bed morphology and sediment transport is significant, yet poorly understood. This is due, in large part, to limitations in monitoring the redd‐building process in a continuous and spatially extended way. A complementary approach may be provided through the use of a small seismic sensor network analysing the ground motion signals generated by the agitation of sediment during the redd‐building process. We successfully tested the viability of this approach by detecting and locating artificially generated redd signals in a reach of the Mashel River, Washington State, USA. We then utilize records of 17 seismic stations, in which we automatically detected seismic events that were subsequently manually checked, yielding a catalogue of 45 potential redd‐building events. Such redd‐building events typically lasted between 1 and 20 min and consisted of a series of clusters of 50–100 short energetic pulses in the 20–60 Hz frequency range. The majority (>90%) of these redd‐building events occurred within 11 days, predominantly during the early morning and late afternoon. The seismically derived locations of the signals were in agreement with independently mapped redds. Improved network geometry and installation conditions are required for more efficient detection, robust location and improved energetic insights into redd‐building processes in larger reaches. The passive and continuous nature of the seismic approach in detecting redds and describing fish behaviour provides a novel tool for fish biologists and fisheries managers, but also for fluvial geomorphologists, interested in quantifying the amount of sediment mobilized by this ecosystem engineer. When complemented with classic approaches, it could allow for a more holistic picture of the kinetics and temporal patterns (at scales from seconds to multiple seasons) of a key phase of salmonid life cycles.
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| 8. |
- Fu, Ping, et al.
(författare)
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Ice cap erosion patterns from bedrock Be-10 and Al-26, southeastern Tibetan Plateau
- 2019
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 44:4, s. 918-932
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Tidskriftsartikel (refereegranskat)abstract
- Quantifying glacial erosion contributes to our understanding of landscape evolution and topographic relief production in high altitude and high latitude areas. Combining in situ Be-10 and Al-26 analysis of bedrock, boulder, and river sand samples, geomorphological mapping, and field investigations, we examine glacial erosion patterns of former ice caps in the Shaluli Shan of the southeastern Tibetan Plateau. The general landform pattern shows a zonal pattern of landscape modification produced by ice caps of up to 4000 km(2) during pre-LGM (Last Glacial Maximum) glaciations, while the dating results and landforms on the plateau surface imply that the LGM ice cap further modified the scoured terrain into different zones. Modeled glacial erosion depth of 0-0.38 m per 100 ka bedrock sample located close to the western margin of the LGM ice cap, indicates limited erosion prior to LGM and Late Glacial moraine deposition. A strong erosion zone exists proximal to the LGM ice cap marginal zone, indicated by modeled glacial erosion depth >2.23 m per 100 ka from bedrock samples. Modeled glacial erosion depths of 0-1.77 m per 100 ka from samples collected along the edge of a central upland, confirm the presence of a zone of intermediate erosion in-between the central upland and the strong erosion zone. Significant nuclide inheritance in river sand samples from basins on the scoured plateau surface also indicate restricted glacial erosion during the last glaciation. Our study, for the first time, shows clear evidence for preservation of glacial landforms formed during previous glaciations under non-erosive ice on the Tibetan Plateau. As patterns of glacial erosion intensity are largely driven by the basal thermal regime, our results confirm earlier inferences from geomorphology for a concentric basal thermal pattern for the Haizishan ice cap during the LGM.
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| 9. |
- Goodfellow, Bradley W., et al.
(författare)
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Depth and character of rock weathering across a basaltic-hosted climosequence on Hawai` i
- 2014
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 39:3, s. 381-398
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Tidskriftsartikel (refereegranskat)abstract
- Using field observations and geochemical and digital terrain analyses, we describe the structure and thickness of the regolith across a climosequence on the island of Hawaii to gain insight into the relative roles of precipitation and the near-surface hydrologic structure in determining weathering patterns. In the wet portion of the climosequence, where the long-term water balance is positive, the regolith thickness reaches an observed maximum of similar to 40m and appears limited by the geomorphic base-level of the landscape. However, even within this thick regolith, distinct units of varying weathering intensity occur; the vertical ordering of which largely reflects differences in the initial permeability structure of the basalt flows rather than a systematic decrease in weathering intensity downwards from the ground surface. In the dry portion of the climosequence, where the long-term water balance is negative, the regolith thickness is confined to similar to 1m, is highly dependent on the inferred permeability structure of the basalt flows, and is independent of geomorphic base-level. Weathering intensity also varies according to permeability structure and decreases in this thin regolith with distance beneath the ground surface. The abrupt change in regolith depth and character that coincides with the transition from net-positive to net-negative long-term water balance implies that small changes in precipitation rates around a neutral water balance result in large changes in the distribution and depth of weathering. Together our observations indicate that the distribution and depth of weathering in basalts (and probably other lithologies) might be best understood by considering how precipitation interacts with the complicated near-surface permeability structure over regolith-forming timescales to weather rock in the vadose zone.
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| 10. |
- Gumbricht, Thomas, et al.
(författare)
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Channels, wetlands and islands in the Okavango Delta, Botswana, and their relation to hydrological and sedimentological processes
- 2004
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Ingår i: Earth Surface Processes and Landforms. - : Wiley. - 0197-9337 .- 1096-9837. ; 29:1, s. 15-29
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Tidskriftsartikel (refereegranskat)abstract
- The Okavango wetland in northern Botswana is one of the world's largest inland deltas. The delta is a dynamic environment with shifting channel routes, causing growth and decay of flanking wetlands, and giving birth to islands. Primary island nuclei are formed by fluvial processes and bioengineering, and subsequently grow into secondary larger islands of irregular shape by clastic and chemical sedimentation, and later by coalescence. This article presents classifications and quantitative estimations of channels, wetlands and islands of the Okavango Delta. Islands were classified dependent on composition, pattern of composition, shape and juxtaposition. 90 per cent of all islands in the entire wetland were identified, with a classification accuracy of 60 to 85 per cent. Smaller islands of the nucleus types dominate the upper parts of the delta, whereas larger secondary islands are more common in the distal part, a reflection of the age of the islands. Islands in the entry valley of the delta, the Panhandle, are larger in the top end - the primary region of recent elastic sedimentation. The overall size distribution of islands in the delta, however, shows no clumps, indicating that island growth is a uniform process over time and space. The total area flooded at least every decade is approximately 14 000 km(2), of which 9000 km(2) is classified as actual wetland. Channel meandering decreases from the Panhandle to the distal part of the delta, with the abandoned Thaoge channel as an exception. Occurrence of fluvially formed islands in the distal delta indicates that the water flow and area of inundation must once have been much larger.
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