Knickpoint Distribution Research Articles

Geomorphic index-based landscape evolution study for the extraction and interpretation of knickpoint and channel steepness in the mandakini catchment, western himalaya.

Geomorphometric analysis using geomorphic indices is essential to comprehend the evolution of a river basin including denudation, surface runoff, subsurface infiltration, differential erosion, lithological variations, possible surface tilting, landslides, and the influence of geological formations and structure. Research in morphometric measurements continues to face many challenges and difficulties despite all the effort carried out. These include the inaccuracy of morphometric measurements and the time it takes to obtain the expected results in large basins. Under such condition, the purpose of the study is to conduct an analysis for the group of indices which includes SL index, transversetopographic symmetry factor, and hypsometry curve along with its integral value in the Mandakini Catchment. Examining the spatial distribution of knickzones has not been well documented, particularly in the Mandakini Catchment; hence, we further analyzed the spatial distribution of knickpoints, channel steepness index, and chi-index along with the longitudinal river profile. Through this analysis, we aim to determine how these indices collectively contribute to the comprehensive characterization of the landscape evolution within the study area and to find the landscape signatures of the uplift by comparing different river profiles. Various knickpoints were found mainly in the upper reaches at higher elevation, validated through aerial imagery and then through detailed field observation. During the field investigation, various geomorphic indicators such as fluvial terraces, entrenched river meandering, active landslides, extensive toe erosion, and waterfalls associated were observed. The study also found out that the places near the Kedarnath, Sonprayag, and Kalimath-Kotma, show high SL index and high steepness index that may correlate with the presence of active thrust and faults.

The interplay of lithology, tectonics and climate in the morphology of Corsica

The morphology of the landscape on the island of Corsica in the western Mediterranean Sea is transient with a clear topographic asymmetry. The geology of the island can be divided into two units – Hercynian Corsica and Alpine Corsica – with contrasting lithology, climate and structural controls between them. We used topographic analysis to assess the influence of each of these factors on landscape form. Disequilibrium in the landscape is illustrated by analysing χ and Gilbert metrics, which indicate active migration of the drainage divide, along with a new metric that quantifies tributary heights above the main stem channel. The climate in the region has fluctuated dramatically from the Messinian salinity crisis through to Quaternary glaciation. However, analysis of the size and distribution of knickpoints across the island does not reflect specific base-level changes. Instead, the basement structures control valley orientation and the conditioning of the landscape by glaciation exerts strong controls on drainage evolution.

Differential surface uplift and knickpoint evolution along the transient Teesta river in the eastern Himalayas

Tectonic and erosional processes that contribute to the gradual process of topographic uplift and erosion have shaped the Himalayan terrain. We used geomorphic proxies and previous published rates of erosion and exhumation across the Teesta River catchment to model the presence of transient segments. This helped us understand how the regional landscape has been changing over time due to surface uplift and channel incision. The Celerity model has been used to evaluate the knickpoint distribution and headward migration along the river system. Based on the river profile and Chi-(χ) analysis, the observed major knickpoints at about ∼ 2500–4000 m elevation comprise the fluvial erosion process, while higher elevation knickpoints suggest the glacial process. The strong significance of the model is shown by the observed correlation coefficient of 0.94 between the predicted and observed horizontal knickpoints. The analysis of relict reach for the paleo-channel reconstruction across the transient drainage channels shows a differential surface uplift of 3823 ± 252 m and 2372 ± 48 m with a corresponding relief gain. The rates of erosion and exhumation were spatially correlated with topographic metrics, precipitation intensity, knickpoint distribution, and surface uplift. The observation suggests that the rate of erosion tends to be more significant and spatially more varied than the long-term rate of exhumation. The rate of exhumation has been found to decline through topographic timescales from south to north, with rates as high as ∼ 1.0 mm/y in the southwest and as low as ∼ 0.4 mm/y in the northernmost Tessta catchment.

Geomorphic interpretation on the formation of strike-slip basins along the Northern Sumatran fault

This study describes geomorphic expressions and constructs the schematic evolution of the Northern Sumatran Fault based on the development of transverse drainage basins and streams. This fault is a 400-km NW-SE right-lateral strike-slip fault with three segments, namely the Aceh and Seulimeum Faults in the northern section and the Tripa Fault in the southern section. The two faults at the northern section are sub-parallel and they link at the southeast termination of the latter fault. The examination on the geomorphic expressions comprised the fault configuration, stream deflections, and the delineation of landforms based on their genesis and geometry. This study applied drainage basin relief ratio (Rh), drainage basin volume-to-area ratio (Rva), and transverse stream profile analysis (normalized stream profile, qualitative interpretation of the profile shapes, stream concavity index (SCI), stream gradient, and knickpoint distribution) for investigating the development of transverse drainage basins and streams. For constructing the schematic evolution, this study evaluates drainage maturity level from the applied methods to interpret relative timing of basin formation. This study suggests that the Aceh and Tripa Faults constituted the initial configuration and they propagated to the southeast and northwest, respectively, before merging. The Seulimeum Fault, which formed subsequently, propagated to the northwest after merging with the Aceh Fault at its southeast termination. This study also infers that fault section with lower drainage maturity coincides with greater numbers of earthquakes.

Identification of morphostructures by utilizing Knickpoint Ranks, a method to avoid the camouflage effect in knickpoint analysis

Knickpoints are relief features characterized by a convex-up abrupt change in the equilibrium profile of a given river. In the field they can be recognized as rapids or waterfalls. Their presence indicate geological conditions or processes that modify erosive rates, such as contrasting lithologies, base-level changes, or active structures, and, therefore, especially relevant in tectonic geomorphology studies. The computation of knickpoints for classification purposes is made dividing the Slope-Extension Ratio (RDE) by the Total Slope-Extension Ratio (RDT), which are derived from the Stream-Gradient index (SL). Traditionally, the classification of knickpoints has been treated in two ways, (1) by the simple division into first and second order, according to a limit value of RDE/RDT of 10, and (2) by the statistical division of values into classes, mainly with natural breaks. Recent studies have used both divisions and correlated the RDE/RDT values with the size of the knickpoints in the field. However, there are studies on morphometric analysis that point to the existence of gaps that occur in specific cases, such as the so-called knickpoint camouflage. In order to eliminate these gaps and with the aim of improve the classification technique, we propose the determination of fixed intervals of distribution of knickpoints which, based on the division according to the RDE/RDT values, use the strengths of the statistical divisions as a means to define limits of RDE/RDT values. The objective of this methodological development is to prevent errors during the classifications as well as to facilitate the interpretation of the obtained knickpoints. The new classification method was applied in 20 hydrographic basins with their RDE/RDT values checked in 44 knickpoints in the field. The analyses carried out point to the coherence of the new proposed classification and confirm the effectiveness of both the fixed intervals and the tools used in the cartography of knickpoints.

The bedrock river profile fitting and the indicative significance: a case study in Gyirong watershed of the Middle Himalaya

Area-slope (AS) Model focuses on quantitative study of bedrock river which has been widely used in the longitudinal profile simulation of bedrock and alluvial rivers, but many applications have failed to carefully consider the mechanism, fitting effect and applicability, resulting in unsatisfactory simulation results. In order to verify the channel profile analysis capability, the study analyzes the oretical basis of AS model and applies to the river channel fitting and knickpoint identification in the Gyirong Watershed in the middle Himalayan Orogen. The results show that, for rivers with different equilibrium status, the fitting curves have different splattering patterns. More than 80% knckpoints locate on the elevation between 4000 –5050 m where coincided with Tibetan Himalayan Unit. Although climate affects the spatial distribution of knickpoints, the structure of Gyirong watershed have developed before modern climate pattern.

Drivers of landscape evolution in eastern Tibet

The eastern margin of the Tibetan Plateau is characterized by two end-member morphologies: 2–3-km-deep canyons that are juxtaposed against extensive high elevation, low relief landscapes. Past researchers have highlighted topographic evidence for mobile drainage divides in this region. Active thrusting is thought to drive river gorge incision originating from the east, but within the eastern Tibetan plateau, there is no evidence for focussed thrusting, and no significant post-Cenozoic shortening structures are evident at the surface upstream of the Three Rivers regions. In this region, researchers have attributed the evident landscape transience to different mechanisms, including upward propagation of an incision signal, regional uplift, and local uplift caused by strike-slip motion. Here, we quantify topographic metrics to document the geomorphic response to the proposed tectonic forcing. Specifically, we quantify channel steepness and identify knickpoints to assess evidence for landscape transience and potential accelerated incision triggered in eastern Tibet. We find that ksn and slope systematically increase downstream along the Tongtianhe River (TR) which bisects the study area from NW to SE and is part of the Yangtze River system, but not along adjacent main stem rivers with similar orientation, therefore ruling out the effect of a regional uplift gradient. The spatial distribution of knickpoints does not cluster along or around fault lines, which we interpret to mean that local faulting is not likely a significant factor causing the systematic variations in the topographic metrics. We find that tributaries of the TR exhibit large non-lithological channel convexities, and that the elevation of these knickpoints above the tributaries' mouth increases towards the SE, in the TR's downstream direction. This observation can be best explained by a transient signal propagating upstream along the TR. An additional large non-lithological knickpoint along the TR, as well as a strong local drainage divide disequilibrium between the TR's tributaries and adjacent basins, suggest that waves of incision, potentially set by regional uplift, are propagating up the TR but not up the adjacent major river systems (the Mekong and eastern branch of Yangtze rivers). We propose these waves of incision are the first-order driving force for river arrangements in eastern Tibet. Accordingly, we reconstruct the evolution processes of landscapes in eastern Tibet.

Progressive evolution of thrust fold topography in the frontal Himalaya

The bulk of the crustal shortening across the Himalayas is accommodated across the Main Frontal Thrust (MFT), forming the topographic ridges of the Siwalik Hills. Here, we quantify topographic metrics to document the geomorphic response to progressive shortening. We use the integral method of channel profile analysis to assess the topographic response to the development of thrust anticline ridges in the eastern and western Churia, Dundwa, and Mohand Ranges. River channel steepness (ksn), the distribution of knickpoints, and channel concavity (θ) in the Siwalik Hills indicate geomorphic evolution during ongoing shortening across the deformation front. Mean channel steepness indices increase with increasing shortening over the Quaternary amongst the eastern Churia (~3.8 km shortening), Mohand (~6.9 km shortening), and Dundwa Ranges (~9 km shortening). Higher channel steepness at the western tip of the Churia Range (~0.4 km shortening) than in the eastern Churia and Mohand Ranges may indicate that the topography is still responding to the initial stages of shortening and relative uplift. Balanced cross sections reveal that there is spatial variability in the growth of these ranges, and as shortening increases, the drainage divide migrates southward (away from the mountains) relative to the flat-ramp transition of the underlying thrust; this results in steepening of the southward-draining catchments (basinward facing), and elongation of the northward flowing catchments. Basinward facing channels also have more prominent knickpoints and greater channel concavities caused by ongoing shortening between their fluvial base level and the drainage divide; this contrasts with the mountain facing channels where there is net growth of the distance between the drainage divide and the base level. Despite the asymmetry of erosional processes, the southward displacement of the drainage divide over the underlying thrust ramp drives topographic advection into the foreland succession which eventually leads to the propagation of new thrust-cored ridges.

Quaternary drainage network reorganization in the Colombian Eastern Cordillera plateau

AbstractDramatic drainage reorganization from initial longitudinal to transversal domains has occurred in the Eastern Cordillera of Colombia. We perform a regional analysis of drainage basin geometry and transformed river profiles based on the integral form of the slope‐area scaling, to investigate the dynamic state of drainage networks and to predict the degree of drainage reorganization in this region. We propose a new model of drainage rearrangement for the Eastern Cordillera, based on the analyses of knickpoint distribution, normalized river profiles, landforms characteristic of river capture, erosion rates and palaeodrainage data. We establish that the oldest longitudinal basin captured by the Magdalena River network was the Suárez Basin at ≈409 ka, inferring the timing of abandonment of a river terrace using in situ produced cosmogenic beryllium‐10 (10Be) depth profiles and providing a first estimation of incision rate of 0.07 mm/yr. We integrate published geochronologic data and interpret the last capture of the Sabana de Bogotá, providing a minimum age of the basin opening to the Magdalena drainage at ≈38 ka. Our results suggest that the Magdalena basin Increased its drainage area by integrating the closed basins from the western flank of the Eastern Cordillera. Our study also suggests that the Magdalena basin is an aggressor compared to the basins located in the eastern flank of the orogen and provides a framework for examining drainage reorganization within the Eastern Cordillera and in similar orogenic settings. The results improve our understanding of headward integration of closed basins across orogenic plateaux. © 2020 John Wiley & Sons, Ltd.

Drainage network dynamics and knickpoint evolution in the Ebro and Duero basins: From endorheism to exorheism

The study of fluvial network rearrangement provides a key to understand past and future landscape evolution. Large perturbations of hydrographic basins such as the change from endorheism to exorheism have repercussions in the steady or disequilibrium state of the basins and their drainage divides. Such transitions from internal to external drainage imply a major lowering of the geomorphological base level causing a major retreating knickpoint wave that separates the upstream low-relief area (inherited from the endorheic period) from the downstream incised area. Subsequently, the water divide migrates to reach an equilibrium in which erosion rates at both sides of the divide are similar. Previous fluvial analyses suggest that both the Duero and Ebro drainage networks, the two largest catchments in Iberia, experienced a change from endorheism to exorheism sometime between the Late Miocene and the Pliocene. Fluvial capture evidence argues for a Pliocene westward migration of the Ebro-Duero divide implying an area decrease for the Duero fluvial network (victim) in favor of the Ebro (aggressor). We used river profiles, knickpoint distribution and Chi-map calculation to understand the different degree of erosion of the Duero and the Ebro catchments and the dynamics of their drainage network. The results show an equilibrated Ebro drainage network in contrast with a disequilibrium in the erosional state of the Duero drainage, which remains reorganizing and adapting to the newly-imposed Atlantic base level. We identified at least two knickpoint wave trains in the Duero drainage resulting from the onset of exorheism: a fast-propagating wave through the cover and a low-propagating wave affecting the bedrock. Field evidence and topographic analysis suggest a westward migration of the Ebro-Duero divide, resulting in an ongoing headward erosion of the Ebro against the Duero catchment. Chi analysis provides the degree of disequilibrium of the drainage network indicating a large-scale aggressor role for the Duero and a victim role for the Ebro. We interpreted this seeming contradiction as the result of a different time scale perspective: local divide observations indicate a victim Duero in the short-term, whereas basin-scale dynamics support a victim role for the Ebro fluvial network in the long-term (multi-million-year time-scales).

Examination of Tectonic Activity Based on Knickpoint Distribution and Movement Potential of Faults in Khuzestan Province, South West Zagros, Iran

Although the distribution of fluvial knickzones, as an important geomorphic feature in bedrock river morphology, has been studied by many investigators, the role of them in examination of tectonic activity has not been well investigated [1], specially based on its comparision with movement potential faults over a broad area. This study examines the tectonic activity of Khuzestan province, South West Zagros, by considering two different parameters: distribution of the fluvial knickzones along Mountain Rivers and evaluation of faults activity in the study area. A segment of a river long-profile that is steeper than adjacent segments is commonly referred to as a knickzone or a knickpoint if it is visibly steeper than the trend of the longitudinal profile. Knickzones are often observed along bedrock rivers and the most visible form is a waterfall. Knickzones are supposed to be a response to base-level changes or to alternations of local lithology [2]. Upstream migration of knickzones has been argued to cause rapid river incision and result in the formation of terraces and instability of valley-side slopes. Knickpoint evolution on a river can provide evidence for uplift of plate margins [3]. Knickpoints can be used as geomorphic markers in steep, rapidly eroding landscapes that commonly lack datable river terraces [4]. In this paper we use the method that was proposed by Hayakawa and Oguchi [5] to extract knickzones in broad areas using DEMs (Digital Elevation Models) and GIS (Geographical Information System). DEM analysis of the longitudinal profiles of rivers permits quantitative, reproductible, and efficient identification of knickzones. The obtained inventory of knickzones will provide a basis for objective analyses of the distribution of knickzones.

Normal fault evolution and coupled landscape response: examples from the Southern Apennines, Italy

AbstractWe present new data addressing the evolution, activity and geomorphic impact of three normal faults in the Southern Apennines: the Vallo di Diano, East Agri and Monti della Maddalena faults. We show that these faults have minimum total throws of ca. 1000–2000 m, and throw rates of ca. 0.7–1 mm year−1 for at least the last ca. 18 ka. We demonstrate that for the Vallo di Diano and East Agri faults, the landscape is effectively recording tectonics, with relief, channel and catchment slopes varying along fault strike in the same manner as normal fault activity does, with little apparent influence of lithology. We therefore use these data to reconstruct the time‐integrated history of fault interaction and growth. From the distribution of knickpoints on the footwall channels, we infer two episodes of base level change, which we attribute to fault interaction episodes. We reconstruct the amount of throw accumulated after each of these events, and the segments involved in each, from the fault throw profiles, and use fault interaction theory to estimate the magnitude of the perturbations and past throw rates. We estimate that fault linkage events took place 0.7 ± 0.2 Ma and 1.4 ± 0.3 Ma in the Vallo di Diano fault, and 1 ± 0.1 in the East Agri Fault, and that both faults likely started their activity between 3 and 3.5 Ma. These fault linkage scenarios are consistent with the observed knickpoint heights. This method for reconstructing fault evolution could potentially be applied for any normal faults for which there is information about throw and throw rates, and in which channels are transiently responding to tectonics.

Relict landscape resistance to dissection by upstream migrating knickpoints

Expanses of subdued topographies are common at high elevation in mountain ranges. They are often interpreted as relict landscapes and are expected to be replaced by steeper topography as erosion proceeds. Preservation of such relict fragments can merely reflect the fact that it takes time to remove any preexisting topography. However, relict fragments could also possess intrinsic characteristics that make them resilient to dissection. We document here the propagation of a wave of dissection across an uplifted relict landscape in Puerto Rico. Using 10Be-26Al burial dating on cave sediments, we show that uplift started 4 Ma and that river knickpoints have since migrated very slowly across the landscape. Modern detrital 10Be erosion rates are consistent with these long-term rates of knickpoint retreat. Analysis of knickpoint distribution, combined with visual observations along the streambeds, indicates that incision by abrasion and plucking is so slow that bedrock weathering becomes a competing process of knickpoint retreat. The studied rivers flow over a massive stock of quartz diorite surrounded by an aureole of metavolcanic rocks. Earlier studies have shown that vegetation over the relict topography efficiently limits erosion, allowing for the formation of a thick saprolite underneath. Such slow erosion reduces streambed load fluxes delivered to the knickpoints, as well as bed load grain size. Both processes limit abrasion. Compounding the effect of slow abrasion, wide joint spacing in the bedrock makes plucking infrequent. Thus, the characteristics of the relict upstream landscape have a direct effect on stream incision farther downstream, reducing the celerity at which the relict, subdued landscape is dissected. We conclude that similar top-down controls on river incision rate may help many relict landscapes to persist amidst highly dissected topographies.

Transient fluvial landscape and preservation of low-relief terrains in an emerging orogen: Example from Hengchun Peninsula, Taiwan

The oblique collision of Taiwan orogen led to a progressive southward uplift and emergence of a submarine accretionary wedge. The Hengchun Peninsula located at the southernmost tip of Taiwan island represents the most recently emerged landform of an antecedent submarine surface. In this study we examine the geomorphic evolution of this newly emerged and uplifted landscape. The extraction of geomorphic parameters (knickpoint distribution, steepness index of rivers) indicates a transient nature of the landscape but shows that a simple model of regressive erosion consecutive to sudden uplift is unsatisfactory. Knickpoint migration modeling shows that only the upstream Sizhong basin can be explained by this single process. On the other hand, the occurrence of a relict landscape (Mutan Ponds low relief) developed at low elevations and, today uplifted in the core of the peninsula, has strongly influenced the drainage evolution. Numerous knickpoints are linked to capture processes around the Mutan Ponds area, indicating a dynamic reorganization of the drainage system. We propose that the landscape of the Hengchun Peninsula is in a transient stage and results from a combination between rejuvenation and drainage rearrangement caused by the late stage of emergence of the south of Taiwan.

Quaternary landscape evolution over a strike-slip plate boundary: Drainage network response to incipient orogenesis in Sakhalin, Russian far east

This study focuses on the fluvial and tectonic landscape of the North Sakhalin Basin (Russia), where 5 km of Neogene deltaic sediments were deposited across the Okhotsk-Amur plate boundary. The homogeneous, poorly lithified sedimentary sequence created a flat landscape without structural inheritance. These sediments are now being actively deformed by oblique compression. This allows us to investigate the early stages of orogenesis in a strike-slip plate boundary and the response of drainage networks in such a setting, and to construct a model for the topographic evolution along 220 km of the plate boundary. We use fluvial geomorphological indicators (planform morphology, concavity, steepness indices, and knickpoint distribution) as evidence for active landscape deformation. Tectonics and topography are strongly coupled, and neotectonic activity can be observed directly from the landscape. Knickpoints are mostly located on fault planes, suggesting geologically recent activity, or in areas of drainage capture, where they are associated with low concavity indices. Geomorphic indications from longitudinal river profiles and planform morphology suggest that creation of anticlines and disruption of drainage patterns appears to be diachronous in the North Sakhalin Basin, with deformation propagating north and east through time. Minimum uplift and strike-slip displacement rates in the northeast of Sakhalin are 0.63 mm a -1 and 1.95 mm a -1 , based on exhumed stratigraphy and offset drainage networks, respectively.

Knickpoint retreat and transient bedrock channel morphology triggered by base-level fall in small bedrock river catchments: The case of the Isle of Jura, Scotland

A sudden drop in river base-level can trigger a knickpoint that propagates throughout the fluvial network causing a transient state in the landscape. Knickpoint retreat has been confirmed in large fluvial settings (drainage areas >100km2) and field data suggest that the same applies to the case of small bedrock river catchments (drainage areas <100km2). Nevertheless, knickpoint recession on resistant lithologies with structure that potentially affects the retreat rate needs to be confirmed with field-based data. Moreover, it remains unclear whether small bedrock rivers can absorb base-level fall via knickpoint retreat. Here we evaluate the response of small bedrock rivers to base-level fall on the isle of Jura in western Scotland (UK), where rivers incise into dipping quartzite. The mapping of raised beach deposits and strath terraces, and the analysis of stream long profiles, were used to identify knickpoints that had been triggered by base-level fall. Our results indicate that the distance of knickpoint retreat scales to the drainage area in a power law function irrespective of structural setting. On the other hand, local channel slope and basin size influence the vertical distribution of knickpoints. As well, at low drainage areas (~4km2) rivers are unable to absorb the full amount of base-level fall and channel reach morphology downstream of the knickpoint tends towards convexity. The results obtained here confirm that knickpoint retreat is mostly controlled by stream discharge, as has been observed for other transient landscapes. Local controls, reflecting basin size and channel slope, have an effect on the vertical distribution of knickpoints; such controls are also related to the ability of rivers to absorb the base-level fall.

Knickpoint and knickzone formation and propagation, South Fork Eel River, northern California

The South Fork Eel River, northern California (United States), displays a prominent knickzone in its longitudinal profile that may represent a perturbation that is propagating upstream. We investigated two tributary basins (Standley and Bear Pen Creeks) located downstream from this major trunk-stream knickzone to document the presence of knickzones within tributary and subtributary streams and to explore their correlation to the South Fork Eel River knickzone. We utilized LIDAR (light detection and ranging) derived digital elevation models to identify more than 100 major knickpoints and knickzones along 103 streams within these 2 tributary basins. Major knickpoints are located at clear inflection points separating two reaches of concave-upward stream profiles. These knickpoints can be delineated at breaks in the regression relation of channel slope versus drainage area for these two tributaries. Using the slope-area relation, we recreate paleolongitudinal profiles to represent the pre-incision profile of main stem tributary channels, as well as the pre-incision elevations of subtributary outlets. Knickpoint distribution throughout the two basins indicates that the channels are responding to pulses of incision initiated through base-level fall along the South Fork Eel River. However, most of the major knickpoints identified do not correlate with the current, prominent knickzone along the South Fork Eel River. Rather, knickpoint distribution within the study area indicates that there have been multiple instances of base-level fall along the South Fork Eel River, each triggered by the upstream passage of knickzones that are no longer preserved in the South Fork Eel River profile.

Gully and tunnel erosion in the hilly Loess Plateau region, China

Although gully development is frequently observed to be associated with piping and tunnel erosion in many parts of the world, there is a lack of detailed studies on their spatial interactions. Such studies not only contribute to developing geomorphic theories on the evolution of both hillslopes and drainage networks but also are useful to design effective control measures of erosion. The present study aims to examine the interactions between gully development and tunnel erosion in the hilly Loess Plateau region of northern China. Firstly, gullies, tunnel inlets and mass movements in the 9.1km2 Wangjiagou watershed were mapped through aerial photo interpretation and reconnaissance surveys. Secondly, detailed field studies were carried out to investigate the interactions between gully development and tunnel formation processes in the selected gullies. Thirdly, the physiographic variables of gullies, derived from GIS and field measurements, were analyzed to identify differences between tunneled and untunneled gullies.A total of 704 channels, 967 tunnel inlets and 547 mass movements were identified in the study watershed. On the basis of their location and morphology, all the channels were classified into four types: headwater gullies, hillside gullies, valleyside gullies and ephemeral river channels. Tunnels are associated with 79% of headwater gullies, 48% of hillside gullies, 3% of valleyside gullies and none of ephemeral river channels. Mass movements are dominated by falls in headwater gullies, falls and slides in hillside gullies, and soil creeps in ephemeral stream channels. Statistical tests indicate that there are significant differences in physiographic variables between tunneled and untunneled gullies. Tunnel formation in gullies is intricately affected by topographic conditions, land uses, knickpoint distribution, soil materials and mass movements.

Initiation and recession of the fluvial knickpoints: A case study from the Yalu River-Wangtian’e volcanic region, northeastern China

Initiation and recession of the knickpoints are significant boundary condition for processes of fluvial system. The distribution and recession rates of knickpoints contain information that provides a fundamental understanding of geomorphic processes. In the Yalu River-Wangtian’e volcanic region of northeastern China, broadly distributed flat lava terrain provides an ideal site to study the recession of fluvial knickpoints because knickpoints and waterfalls are well preserved here. Here we describe the distribution of knickpoints in the Yalu River-Wangtian’e volcanic region by combining DEM analysis and numerical modeling. Furthermore, we present a knickpoint celerity model, derived from stream-power incision model, to relate knickpoint recession rate to drainage area. We calibrate important empirical coefficients with our knickpoint celerity model; the best fit erosion coefficient (K) is 1.32×10−8, and the best fit drainage area exponent (m) is 0.69. Error analysis indicates a close correspondence between synthetic and real knickpoints. Finally, we show that knickpoint recession rates in the Yalu River-Wangtian’e volcanic region are ∼1–10 mm/a during the early stages of transient incision, and that the present rates are ∼1–6 mm/a. Our recession rate results are in good agreement with previous findings from the Aso Volcano and volcanoes near Boso Peninsula (Japan), which have a similar geologic history to the Yalu River-Wangtian’e volcanic region. Our present effort provides new insight into landscape evolution in the Yalu River-Wangtian’e volcanic region in northeastern China.

Knickpoint initiation and distribution within fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New Zealand

If knickpoints transmit signals of base level fall in river networks, then improvements in our understanding of their retreat rate and basin wide distribution helps constrain the transient response following perturbation. Many studies of knickpoint retreat focus on the response of trunk streams to base level fall. Here we examine the response of an entire fluvial network, as recorded by 236 active knickpoints distributed within the Waipaoa River on the North Island of New Zealand. Base level fall within the Waipaoa catchment initiated 18,000 years ago in response to a climatically triggered and tectonically exacerbated pulse of incision. Using observations from field work, aerial photo analysis and a digital elevation model (DEM), we study the knickpoint positions within the network. We find that ∼ 70% of the knickpoints are located at drainage areas between 1 × 10 5 m 2 and 1 × 10 6 m 2 and more than half are < 1 km upstream of a large change in drainage area. For the knickpoints < 1 km upstream of large tributary junctions, we find that the retreat distances were well correlated with the tributaries' drainage areas. In order to determine how a pulse of incision distributes itself throughout a fluvial network, we develop two simple, end-member models and compare their behavior to the observed knickpoint distribution in the Waipaoa. In the first model, we propose that a knickpoint initiated at the basin outlet retreats upstream and distributes the signal throughout the network at a rate that is a power law function of drainage area. In the second model, we propose that knickpoints form near a threshold drainage area, below which channels cannot incise with the same efficiency as possible in downstream reaches. Though neither model addresses the along-stream variability in substrate or knickpoint form, the misfit between the modeled and the observed knickpoints' along-stream positions are surprisingly low (< 1 km; ∼ 1.25% of the total stream length) for knickpoints with drainage areas < 1 × 10 6 m 2. Large misfits (up to 3.5 km) are observed for knickpoints with present-day drainage areas greater than 1 × 10 6 m 2. The large, single step in channel elevation that characterizes knickpoints presently observed in tributaries of the Waipaoa River may not characterize the base level fall signal that propagated through the trunk streams. Evidence for progressive (rather than instantaneous) incision in the trunk streams, the knickpoints’ vicinities to tributary junctions and the equivalent success of the two-end member models lead us to conclude that the present positions of the 236 observed knickpoints are largely a consequence of thresholds in channel incision at low drainage areas.