Longitudinal Stream Profiles Research Articles

The geomorphic impacts of culverts at paved forest roads: Examples from Carpathian headwater channels, Czech Republic

Road-stream crossings constructed with culverts have the potential to alter geomorphic processes in adjacent stream channels. We examined changes in channel geometry and bed sediments in longitudinal profiles related to culverts at five paved forest road crossings with perennial headwater streams and two gullies initiated by point releases of water through ditches. A dendrogeomorphic approach was used for dating of exposed tree roots and estimation of bank erosion rates in incised cross-sections downstream of road crossings. The studied forest road crossings concentrate surface runoff from the road net, act as stabilizing elements in stream longitudinal profiles and decelerate bedload transport in the case of culvert clogging, which caused notable upstream and downstream changes in fluvial processes. Notable fining of the coarsest fraction in upstream cross-sections was observed only for two culverts with the highest depositional tendencies. Much lower width-depth ratios (<4) and typically higher channel slopes were measured in channel reaches downstream of the culverts, compared with upstream cross-sections. Two road-induced gullies typically had very low width/depth ratios close to 2. At five of a total of seven studied culverts, the highest documented erosion rates occurred immediately downstream of these structures. The mean annual erosion rates reconstructed from cross-sections at the positions of individual tree root samples ranged from 0.2 to 6.5cm·year−1, and the erosion occurred over 2–5year intervals, depending on the studied location. The highest number of observed changes in cross-sectional geometry caused by channel erosion were connected with relatively high-magnitude flow events (with recurrence intervals >10years; specific discharge >1500l·s·km−2), whereas more frequent flows (with recurrence intervals of 2–10years) were not always recorded by exposed tree roots.

Stream Length-gradient Hotspot and Cluster Analysis (SL-HCA) to fine-tune the detection and interpretation of knickzones on longitudinal profiles

This work presents a new approach for fine-tuning the analysis of stream longitudinal profiles. We show that applying Hotspot and Cluster Analysis (HCA), based on the Getis-Ord Gi* statistic, to the stream length-gradient (SL) index improves the visualization of anomalous values, assisting in the identification of tectonic structures and large landslides. High positive Gi* values indicate the clustering of SL anomalies (hotspots), and mirror the occurrence of knickzones on the stream long-profiles. We applied this methodology to a mountainous sector of the eastern Emilia-Romagna region, in northern Italy. Remote sensing and field surveys conducted on hotspot sites indicate that large landslides are the main process associated to over-steepened long-profile segments along streams connected to the valley slopes. Along-stream changes in bedrock resistance accounts for the main anomalies within sectors where hillslopes and valley floors are disconnected. We demonstrate that specific relationships between geometry and intensity of SL hotspots are indicative of the process responsible for the knickzone formation and, in particular that tectonic structures generally provide the longest and highest anomalies. The results of this work suggest that SL-HCA maps are more advantageous for detecting and interpreting knickzones compared with traditional SL maps, since: i) they need less input data to be computed, thus making them useful to investigate regions poorly covered by detailed geological data and/or where field surveys are difficult to be carried out and ii) the hotspot geometry can help discriminate the knickzones attributable to gravitational mass movements from litho-structural ones.

Tectonic geomorphology of the Sudetes (Central Europe) – a review and re-appraisal

The Sudetes are a mountain range in Central Europe, which owes its emergence to the Cenozoic rejuvenation of an old Variscan orogen, subject to stresses from the Alps and the Carpathians. The gross morphological features of the Sudetes are typically explained as reflecting the superposition of the effects of long-term, rock-controlled denudation and Late Cenozoic differential uplift and subsidence. In this paper, early conceptual models, developed in the 1950s and 1960s and emphasizing alternating uplift and planation phases, are presented first. A review of more recent work focused on tectonic landforms and geomorphic indicators of tectonic movements follows, with special attention to fault-generated escarpments, valley morphology, stream longitudinal profiles, terraces and fans, drainage basin characteristics and regional geomorphometric studies. Attempts to provide a timeframe of tectonic relief differentiation are also summarized. In the closing part of the paper, the existing approaches and findings are re-evaluated in order to identify challenges and perspectives for future work. The availability of high-resolution digital terrain models creates a unique opportunity to quantify relief features and detect even the subtle topographic signatures of recent tectonics. A need to reconcile the results of geomorphological analysis with those emerging from other studies focused on faults is highlighted.

Analysis of the interaction between neotectonic and surface processes in a low-land intracratonic setting of South America

Abstract The Quaternary sedimentary record in the continental interiors has generally been studied taking into consideration the climatic forcing. However, episodes of uplift controlling the development of landforms have often been dismissed. The interiors of continents are typically considered to have low deformation rates, lower than the rates of exogenous processes. Deformation in the middle of the Pampean plains, an intracratonic setting of South America, has been observed to have generated an elevated area surrounded by lowland plains. This elevated area is susceptible to fluvial erosion. From this fact we developed a geomorphologic modelling constrained from geomorphic markers in order to estimate the relative incidence of the different factors that control erosion in lowland landscapes. An adapted erosion code was employed, which combines fluvial incision equation (the stream power law), an uplift function spatially distributed and the hillslope diffusion equation. A low-DEM was achieved for restoring an initial non-deformed topography affected by low fluvial incision. It represents a palaeosurface chrono-stratigraphically constrained. The estimation of the model parameters was achieved by applying the inversion method. The best-fit between the numerical solution (calculated topography) and the measured topography (simulated by means a DEM) was approached by comparison of the stream longitudinal profiles. The yield numerical approach suggests that a geomorphological modelling based on the stream power law is possible to be applied in low-land landscapes. For this, a large scale change of the magnitude of the processes modelling the landscape should be expected in comparison with mountain erosional environments. Remarkably low K empirical values ( K = 5 × 10 −6 –6 × 10 −6 yr −1 ) suggest that the extremely low slopes exert a larger influence on the stream power in low-land landscapes. Also the diffusion coefficient D values resulted largely lower (3 × 10 −4 –1.9 × 10 −3 cm 2 /yr) than presented in tectonically active landscapes (>0.03 cm 2 /yr). Regional uplift rates can be higher (0.35 mm/yr) than it is generally expected in low uplift rate settings (0.1–0.25 mm/yr). These estimations evidence highly dynamic surface and tectonic processes controlling the landscape evolution over intermediate time scales in an intracratonic setting.

Hack’s law in a drainage network model: A Brownian web approach

Hack [Studies of longitudinal stream profiles in Virginia and Maryland (1957). Report], while studying the drainage system in the Shenandoah valley and the adjacent mountains of Virginia, observed a power law relation $l\sim a^{0.6}$ between the length $l$ of a stream from its source to a divide and the area $a$ of the basin that collects the precipitation contributing to the stream as tributaries. We study the tributary structure of Howard's drainage network model of headward growth and branching studied by Gangopadhyay, Roy and Sarkar [Ann. Appl. Probab. 14 (2004) 1242-1266]. We show that the exponent of Hack's law is $2/3$ for Howard's model. Our study is based on a scaling of the process whereby the limit of the watershed area of a stream is area of a Brownian excursion process. To obtain this, we define a dual of the model and show that under diffusive scaling, both the original network and its dual converge jointly to the standard Brownian web and its dual.

Landscape response to recent tectonic deformation in the SW Pannonian Basin: Evidence from DEM-based morphometric analysis of the Bilogora Mt. area, NE Croatia

Bilogora Mt. area is a transpressional structure located in the NE of Croatia, linked with the tectonic evolution of the Drava Depression. The structure formed during Pliocene and Quaternary from the inversion of NW-striking Drava Depression Boundary Fault, which originated as a normal fault and was reactivated as a dextral strike-slip fault. During Pliocene and Quaternary time, Bilogora Mt. area underwent more than 400m of differential uplift, while still ongoing tectonic deformation is documented by historical and instrumental seismicity (3.5≤ML≤5.6). In this geomorphic study, the recent deformation in the Bilogora Mt. area, although obscured by landscape evolution variables, was assessed using DEM-based landscape morphometry and a set of morphometric indexes. Local relief and slope angle variability distributions were combined with morphometric indexes computed at the scale of the drainage basin. We analyzed hypsometric curves with the hypsometric integral (HI), basin asymmetry (AF) and parameters of longitudinal stream profiles (Cf, Cmax, Δl/L, θ, and ksn). Analyzed morphometric parameters were combined into a cumulative index of relative tectonic activity (RTA). This index summarizes drainage basin response to the possible recent tectonic activity. Results suggest that most of the tectonic activity is probably located along the NE front of the Bilogora Mt. area, in its NW and central part. This activity could be likely related to recent tectonic activity along the Kalnik Mt. Fault zone and Drava Depression Boundary Fault zones, respectively. A subordinate area characterized by recent tectonic activity could be suggested within the southernmost part of the study area. Quaternary activity in this area is probably related to NE-striking faults in vicinity of the town Daruvar that continue towards the northeast into the study area. Though methodology presented here represents a good identification tool of possible ongoing deformation, this study will be followed by detailed fieldwork and paleoseismological investigations at the identified locations.

Morphotectonic interpretation of the Makuyuni catchment in Northern Tanzania using DEM and SAR data

Landscapes in the East African Rift System are formed by complex effects of the active continental extension zone. These effects are caused by the Somalian micro-plate's eastward drift away from the Nubian plate, as well as by volcanic, erosional and depositional processes. Tectonic processes in this region have significantly contributed to the formation of the current drainage systems and landforms. This study focuses on the morphotectonics of the Makuyuni catchment with an analysis of topography, drainage networks, stream longitudinal profiles and lineaments. This analysis reveals a morphostructural control with an N–S trend for the uplifted Masai Block, as well as tectonic deformation in the Makuyuni catchment area (NE of Lake Manyara). Whereas basin asymmetry analysis shows basin tilting associated with active faulting and uplifting near the Essimingor volcanic cone, in this catchment the steepness and concavity indices, coupled with lineaments obtained from interpretations of Synthetic Aperture Radar satellite scenes, show an uplifting along micro-faults. Hypsometry curves reveal that subcatchments on the right side of the Makuyuni River are in a mature equilibrium phase, whereas those at the left side are in a younger stage of maturity. An investigation of base level and statistical moments of the hypsometric curves provides evidences for the spatial distribution of gully erosion phenomena. Such erosion processes are due to tectonic deformation in the northern parts of the Makuyuni catchment. These results of regional tectonic instability suggest that tectonic processes are a significant factor for the current landscape evolution in the Lake Manyara basin.

The effects of the North Anatolian Fault on the geomorphology in the Eastern Marmara Region, Northwestern Turkey

North-western Anatolia has been actively deformed since Pliocene by the right-lateral North Anatolian Fault (NAF). This transform fault, which has a transtensional character in its western end due to effects from the Aegean extensional system, is a major control on the regional geomorphologic evolution. This study applied some geomorphic analyses, such as stream longitudinal profiles, stream length-gradient index, ratio of valley floor width and valley height, mountain front sinuosity, hypsometry and asymmetry factor analyses, to an area just east of the Sea of Marmara in order to understand the tectonic effects on the area’s geomorphological evolution. The active and fastest northern branch of the NAF lies within a topographic depression connecting Sea of Marmara in the east to the Adapazarı Basin in the west. This depression filled with early Pleistocene and younger sediment after a series of pull-apart basins opened along the NAF. North of this depression lies the Kocaeli Peneplain, whose southern edge the NAF uplifted. Meandering streams on the central peneplain were incised possibly due to baselevel changes in the Black Sea. South of the depression, an E-trending mountainous area has a rugged morphology. Based on geomorphic analyses, uplifted Pliocene sediment, marine terraces, and recent earthquake activity, this area between northern and southern branches of the NAF is actively uplifting. The geomorphic indices used in this study are sensitive to vertical movements rather than lateral ones. The bedrock lithology that played an important role on the area’s geomorphologic evolution also affects the geomorphic indices used here.

Morphotectonics of a small river basin in the South Indian granulite terrain: An assessment through spatially derived geomorphic indices

ABSTRACTNeotectonic activity in Mahe (Mayyazhi) river basin and its implication in landform development and stream characteristics were investigated with the help of the digital elevation model-derived geomorphic indices. The analysis is based on the commonly used geomorphic indices such as hypsometric curve and integral, drainage basin asymmetry, transverse topographic symmetric factor (T), stream length (SL) gradient index and longitudinal profiles of the stream. Hypsometric integrals indicate that the basin has reached the base level and lesser amount of material remains for erosion and transportation. The basin is asymmetric and has a south-south-east directional tilt, but with a probable spin caused due to the presence of major strike slip fracture, the mouth portion of the Mahe river has been tilted towards north-north-west. The SL index with sudden and non-uniform variations supports the finding and indicates the relative tectonic activity and its influence over the river networks in the area. Longitudinal profile of the stream also reveals gradational changes in the profile with the presence of knick points. Cross comparison of the analysed geomorphic indices points towards neotectonic activity in the basin, which modified the river basin to the present morphology and is reflected in the characteristics of the river and the basin as a whole.

Longitudinal profile and sediment mobility as geomorphic tools to interpret the history of a fluviokarst stream system

The complex drainage systems within karst settings can result in atypical longitudinal profiles. Features, such as cave entrances, can be expressed as anomalous ‘bumps’ in the longitudinal profile of a stream if downcutting has continued upstream of the area in which the water is pirated to the subsurface. Horn Hollow, a fluviokarst valley located in Carter Caves State Park Resort in northeastern Kentucky, was examined for these types of features. The objectives of this study were to determine if sediment mobility can be used as a proxy for anomalous areas along the profile of the valley and if detailed cross-sections can reveal and/or differentiate areas of cave collapse from natural down-cutting of the system. To accomplish these objectives, the longitudinal profile of Horn Hollow and numerous cross-sections through the valley were surveyed. Armor point counts were performed at cross-sections unless the section was predominantly bedrock. Although Horn Hollow’s waters have been predominantly pirated to the subsurface, the longitudinal profile of the system is graded to that of a stream near equilibrium, but anomalous areas are present. The progression of sediment size along the length of the profile does not follow a typical fining-downstream pattern. Some of the largest sediments can be found within the lower segment of the profile. Taken together, the anomalous bumps and the sediment size suggest that the shape of the longitudinal stream profile is strongly influenced by karst processes such as stream piracy and cave collapse.

An algorithm to extract more accurate stream longitudinal profiles from unfilled DEMs

Morphometric features observed from a stream longitudinal profile (SLP) reflect channel responses to lithological variation and changes in uplift or climate; therefore, they constitute essential indicators in the studies for the dynamics between tectonics, climate, and surface processes. The widespread availability of digital elevation models (DEMs) and their processing enable semi-automatic extraction of SLPs as well as additional stream profile parameters, thus reducing the time spent for extracting them and simultaneously allowing regional-scale studies of SLPs. However, careful consideration is required to extract SLPs directly from a DEM, because the DEM must be altered by depression filling process to ensure the continuity of flows across it. Such alteration inevitably introduces distortions to the SLP, such as stair steps, bias of elevation values, and inaccurate stream paths. This paper proposes a new algorithm, called maximum depth tracing algorithm (MDTA), to extract more accurate SLPs using depression-unfilled DEMs. The MDTA supposes that depressions in DEMs are not necessarily artifacts to be removed, and that elevation values within them are useful to represent more accurately the real landscape. To ensure the continuity of flows even across the unfilled DEM, the MDTA first determines the outlet of each depression and then reverses flow directions of the cells on the line of maximum depth within each depression, beginning from the outlet and toward the sink. It also calculates flow accumulation without disruption across the unfilled DEM. Comparative analysis with the profiles extracted by the hydrologic functions implemented in the ArcGIS™ was performed to illustrate the benefits from the MDTA. It shows that the MDTA provides more accurate stream paths on depression areas, and consequently reduces distortions of the SLPs derived from the paths, such as exaggerated elevation values and negatively biased slopes that are commonly observed in the SLPs built using the ArcGIS™. The algorithm proposed here, therefore, could aid all the studies requiring more reliable stream paths and SLPs from DEMs.

Exploring landscape sensitivity to the Pacific Trade Wind Inversion on the subsiding island of Hawaii

AbstractThe interaction of the subsiding limb of the Hadley circulation and the easterly North Pacific Trade Winds establishes a persistent thermal inversion at about 2000 m above sea level in the subtropical Pacific. The inversion restricts convective rainfall to the lower elevations of the windward flank of the island of Hawaii, creating an order‐of‐magnitude vertical rainfall gradient, as well as high interbasin variability in precipitation. In the high‐rainfall zone, streams are incised tens to hundreds of meters below the surface of the volcanic shield. We use a digital elevation model and 1‐D numerical modeling to assess whether deep incision on the flank of Mauna Kea is tied to the elevation of the trade wind inversion. The 83 channels examined can be well fit in aggregate by all models that account for differences in precipitation between basins; specifically, the maximum depth of incision in each drainage is a power function of precipitation‐weighted drainage area with an exponent of ~ ½. Individual longitudinal stream profiles are generally better fit by models that acknowledge both along‐channel precipitation variability and the subsidence of the island through that gradient, but this relationship is not clearly demonstrable for many channels. We argue that island subsidence through the dry‐to‐wet precipitation gradient has resulted in less cumulative discharge in the lower reaches of the drainages over their lifetime, relative to a nonsubsiding case. This reduces differential erosion between the wet and dry reaches by 20–30% over 300 kyr, making the longitudinal profiles less sensitive to the very strong climate gradient.

Stream analysis of small drainage basins in an ancient landform, Korean Peninsula

Much of the topography of Korea is ancient, but many Quaternary fault outcrops and marine terraces have been observed in the southeastern part of the Korean Peninsula. Sufficient evidence of a Quaternary fault is lacking in the geological features of the commercially developed Jukbyun and Uljin areas. Knickpoints that develop on streams can be formed by tectonic activity such as faulting and folding, or can result simply from the differential erosion rates of bedrock. In this study, we investigated the relationship between stream steepness and faults in the Jukbyun area. Stream profile analyses of the Bugu and Namdae basins were performed using a digital elevation model to estimate Quaternary tectonic movements. Stream parameters obtained from analysis of the longitudinal stream profiles of the Bugu and Namdae drainage basins in the northeastern part of the Korean Peninsula indicated neotectonic movement. Thirty of the thirty-nine knickpoints that developed in the downstream areas of the fluvial channels corresponded to fault zones. It is thought that fault activity results in knickpoints in river systems. The normalized relative slope (Ksn) value (54.9) of the BS1 stream in the Bugu drainage basin was higher than that (28.8–36.3) of the other streams in both basins, despite a similar lithology to NS1 and NS2. Therefore, we concluded that stream steepness might be a result of tectonic forcing rather than a product of rock strength in the study area and that stream parameters could provide indirect evidence of Quaternary tectonics in ancient landforms.

Drainage anomalies as indicators of soft-linked deformation zones between neotectonic structural highs in the Precordillera, Central Andes of Argentina

Between 31°45′–32°15′ S, the western piedmont of the Ansilta block in the Precordillera of Argentina has been uplifted in Quaternary times through folds, blind faults and faulted blocks that form discrete and evident areas of neotectonic deformation (structural highs). In between, subtle tectonic perturbations of the fluvial landforms (drainage anomalies), such as diverted and entrenched streams, asymmetric basins, incision anomalies, convex-upward longitudinal stream profiles, dammed rivers and broom-shaped patterns allow the recognition of areas of incipient deformation. We combined the analyses of both the structural highs and the drainage anomalies areas to investigate the role of the latest as an expression of soft-linkage areas in fault systems. The structural highs would be interpreted as isolated features without the consideration of the drainage anomalies areas between them. Moreover, linking the structural highs using the drainage anomalies reveals the extent of major Quaternary deformation belts in their initial development stages.

Channel-reach morphology controls of headwater streams based in flysch geologic structures: An example from the Outer Western Carpathians, Czech Republic

A detailed measurement of 93 channel reaches that were classified with an adjusted Montgomery–Buffington (1997) reach-scale system provided comprehensive information of approximately 9 at-a-reach parameters: the channel gradient, the bankfull width, the bankfull depth, d90, the percentage of resistant rocks in the bed sediment, the number of pieces of large woody debris, valley confinement, direct sediment inputs and the presence of fluvial accumulations in the stream channel. In addition, the quantified intensity of sediment transport (i.e. ratio between sediment supply and transport capacity in longitudinal stream profiles) during flood events has been estimated by the one-dimensional bedload transport model (TOMSED), which was validated in two local streams. The principal component analysis of the at-a-reach parameters did not reveal significant groups of channel-reach morphologies; thus, the selected parameters that exclude sediment transport dynamics within stream longitudinal profile cannot reliably distinguish or predict individual channel reach morphologies. Nevertheless, the channel gradient represented the most significant single explanatory variable for stepped-bed morphologies. The addition of bedload transport parameters demonstrated that limited sediment supply streams and streams with limited transport capacities featured different successions of the channel reach morphologies in terms of the channel gradient and, subsequently, the fluvial continuity. The bedrock-cascades and step-pools were significant for the first case, whereas cascade and step-rapid morphology often occurred in higher sediment supply conditions.

Identification of tectonically active areas using DEM: a quantitative morphometric analysis of Mt. Medvednica, NW Croatia

Mt. Medvednica is an inselberg in NW Croatia which tectonically lies at the intersection between Southeastern Alps, Northwestern Dinarides and Tisza Mega-Unit of the Pannonian basin. Due to the Pliocene-Quaternary N-S directed transpression it experienced a differential uplift between 1500-2000 m and exposes pre-Neogene and Neogene tectonic and stratigraphic units surrounded by Pliocene-Quaternary sediments. This paper evaluates a set of methods in quantitative morphometry, used here for identification of tectonically active areas within a region characterized by low-rate active folding and faulting during the Pliocene-Quaternary times. Analysis employed extraction of hypsometric curves, calculation of hypsometric integral, asymmetry factor, and statistical parameters of normalized longitudinal main stream profiles of 36 drainage basins delineated from a 25-m resolution DEM of Mt. Medvednica area. Despite lithological heterogeneity, obtained morphometric parameters are considered as fairly good indicators for discriminating tectonically active from inactive areas. Based on calculated parameters, in combination with geological data, the most tectonically active areas during the Quaternary are found at the SW corner of the Mt. Medvednica related to an active segment of the North Medvednica Boundary Fault, and in the central part of the mountain related to the Kasina fault zone subdividing mountain into the NE and SW units.

Coarse Bed Sediments in a Headwater Channel as Indicators of Fluvial Processes and Slope-Channel Coupling: A Case Study from the Carpathian Mountains (Czech Republic)

Abstract The character of riverbed sediments usually reflects fluvial processes and the dynamics of sediment transport in fluvial systems. The approach in this study was based on the measurement of the largest boulders located within a bankfull channel, and on the observation of changes in their size in the longitudinal profile of a headwater stream in the Moravian-Silesian Beskids Mountains. The resulting trends in a particle-size index reflect the character of sediment delivery into channel segments and the recent channel-forming processes. The largest boulders were observed in channel sections with a strong interaction of slope and fluvial processes, and a slight coarsening of sediments was recorded in the incised downstream sections of the longitudinal stream profile. In contrast, the refining of bed sediments was typical of the transitional zone between slope-channel coupled reaches and an alluvial cone characterised by a tendency to material aggradation.

Lateral sediment sources and knickzones as controls on spatio‐temporal variations of sediment transport in an Alpine river

AbstractModern mixed alluvial‐bedrock channels in mountainous areas provide natural laboratories for understanding the time scales at which coarse‐grained material has been entrained and transported from their sources to the adjacent sedimentary sink, where these deposits are preserved as conglomerates. This article assesses the shear stress conditions needed for the entrainment of the coarse‐bed particles in the Glogn River that drains the 400 km2 Val Lumnezia basin, eastern Swiss Alps. In addition, quantitative data are presented on sediment transport patterns in this stream. The longitudinal stream profile of this river is characterized by three ca 500 m long knickzones where channel gradients range from 0·02 to 0·2 m m−1, and where the valley bottom confined into a &lt;10 m wide gorge. Downstream of these knickzones, the stream is flat with gradients &lt;0·01 m m−1 and widths ≥30 m. Measurements of the grain‐size distribution along the trunk stream yield a mean D84 value of ca 270 mm, whereas the mean D50 is ca 100 mm. The consequences of the channel morphology and the grain‐size distribution for the time scales of sediment transport were explored by using a one‐dimensional step‐backwater hydraulic model (Hydrologic Engineering Centre – River Analysis System). The results reveal that, along the entire trunk stream, a two to 10 year return period flood event is capable of mobilizing both the D50 and D84 fractions where the Shields stress exceeds the critical Shields stress for the initiation of particle motion. These return periods, however, varied substantially depending on the channel geometry and the pebble/boulder size distribution of the supplied material. Accordingly, the stream exhibits a highly dynamic boulder cover behaviour. It is likely that these time scales might also have been at work when coarse‐grained conglomerates were constructed in the geological past.

Headward retreat of streams in the Late Oligocene to Early Miocene Swiss Alps

AbstractThis study uses the widths, the spacing and the grain‐size pattern of Oligo/Miocene alluvial fan conglomerates in the central segment of the Swiss Alpine foreland to reconstruct the topographic development of the Alps. These data are analysed with models of longitudinal stream profile development, to propose that the Alpine topography evolved from an early transient state where streams adjusted to rock uplift by headward retreat, to a mature phase where any changes in rock uplift were accommodated by vertical incision. The first stage comprises the time interval between ca 31 Ma and 22 Ma, when the Alpine streams deposited many small fans with a lateral spacing of &lt;30 km in the north Alpine foreland. As the range evolved, the streams joined and the fans coalesced into a few large depositional systems with a lateral spacing of ca 80 to 100 km at 22 Ma. The models used here suggest that the overall elevation of the Alps increased rapidly within &lt;5 Myr. The variability in pebble size increased either due to variations in sediment supply, enhanced orographic effects, or preferentially due to a change towards a stormier palaeoclimate. By 22 Ma, only two large rivers carried material into the foreland fans, suggesting that the major Alpine streams had established themselves. This second phase of stable drainage network was maintained until ca 5 Ma, when the uplift and erosion of the Molasse started and streams were redirected both in the Alps and in the foreland. This study illustrates that sedimentological archives of foreland basins can be used to reconstruct the chronology of the topographic development of mountain belts. It is suggested that the finite elevation of mountainous landscapes is reached early during orogeny and can be maintained for millions of years, provided that erosion is efficient.

Global climate controls on the denudation of glaciated mountain

This paper presents semi-quantitative data concerning the pattern of erosion under present-day humid climatic conditions for a ca. 10-km2 large soil-mantled drainage basin located in the northern foothills of the Central Swiss Alps. The present-day pattern of erosion was presumably initiated at ca. 15 ka at the time when the last glacial period was terminated.Present-day erosional processes are characterized by fluvial dissection and hillslope mass failure, resulting in linear geomorphic features. As fluvial dissection proceeds, the rivers tend to adapt steady state longitudinal stream profiles. In this case, the stream profiles display power–law relationships between channel gradient and upstream size of the drainage basin. Our study shows that establishment of steady state longitudinal stream profiles requires a balanced mass flux between processes on hillslopes and in channels. This situation is interpreted for locations where the downslope flux of mass on hillslopes does not exceed the erosional potential and the transport capacity in channels. Consequently, the development of steady state drainage systems appears to be controlled by erosional processes in channels as they dictate the elevation of the base level. In contrast, systems that are at transient states of longitudinal stream profile development very often display non-balanced mass flux between processes in channels and on hillslopes. Specifically, low mechanical strength of the bedrock (e.g. glacial till, marls) results in a downslope flux of mass that exceeds the transport capacity and the erosional potential in the channel, causing the elevation of the channel floor to increase. In this case, processes on hillslopes are not necessarily coupled with erosion in channels and appear to control the evolution of the drainage basin for a limited time interval.Besides interpretations of controls on surface erosion, this paper also estimates post-glacial sediment-yield for the analyzed drainage basin. These estimates are achieved by determination of the difference between the elevation of the glacial and the present-day surface. The data show that ca. 30% of the analyzed drainage basin experiences present-day surface erosion. Furthermore, average post-glacial sediment yield measures ca. 4.6 m/ka if the post-glacial volume of sediment is related to the area that has experienced surface erosion, or ca. 1.4 m/ka if it is related to the whole drainage basin. Interestingly, the erosional efficiency appears to be significantly higher for systems that display steady state stream profiles than for transient rivers.