Decoding temporal and spatial patterns of fault uplift using transient river long profiles

We present detailed observations of rivers crossing active normal faults in the Central Apennines, Italy, where excellent constraints exist on the temporal and spatial history of fault movement. We demonstrate that rivers with drainage areas > 10 km² and crossing faults that have undergone an increase in throw rate within the last 1 My, have significant long-profile convexities. In contrast, channels that cross faults that have had a constant-slip rate for 3 My have concave-up profiles and have similar concavities and steepness indices to rivers that do not cross any active fault structures. This trend is consistent across the Central Apennines and cannot be explained by appeal to lithology or regional base level change. The data challenge the belief that active faulting must always be reflected in river profiles; instead, the long-profile convexities are best explained as a transient response of the river system to a change in tectonic uplift rate. Moreover, for these rivers we demonstrate that the height of the profile convexity, as measured from the fault, scales with the magnitude of the uplift rate increase on the fault; and we establish that this relationship holds for throw rate variation along strike for the same fault segment, as well as between faults. These findings are shown to be consistent with predictions of channel response to changing uplift rate rates using a detachment-limited fluvial erosion model, and they illustrate that analysis of the magnitude of profile convexities has considerable predictive potential for extracting tectonic information. We also demonstrate that the migration rate of the profile convexities varies from 1.5–10 mm/y, and is a function of the slip rate increase as well as the drainage area. This is consistent with n > 1 for the slope exponent in a classical detachment-limited stream-power erosion law, but could potentially be explained by incorporating an erosion threshold or an explicit role for sediment in enhancing erosion rates. Finally, we show that for rivers in extensional settings, where the response times to tectonic perturbation are long (in this case > 1 My), attempts to extract tectonic uplift rates from normalised steepness indices are likely to be flawed because topographic steady state has not yet been achieved.     

Regional and local controls on the spatial distribution of bedrock reaches in the Upper Guadalupe River, Texas

While studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades, few field studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach types. The objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the Upper Guadalupe River. We compare reach length, channel and floodplain width, sinuosity, bar length and spacing, bar surface grain size, and slope in alluvial and bedrock reaches to identify whether major differences exist between channel reach types. We find that local disturbances, interaction of the channel and valley sides, variation in lithology, and regional structural control contribute to the distribution of bedrock reaches in the largely alluvial channel. Alluvial and bedrock channel reaches in the Upper Guadalupe River are similar, particularly with respect to the distribution of gravel bars, surface grain size distributions of bars, and channel slope and width. Our observations suggest that the fluvial system has adjusted to changes in base level associated with the Balcones Escarpment Fault Zone by phased incision into alluvial sediment and the underlying bedrock, essentially shifting from a fully alluvial river to a mixed alluvial bedrock river.     

Testing links between river patterns and in-channel characteristics using MRPP and ANOVA

This study tests the assumption that the characteristics of channels within multiple channel rivers are different from those of single channel rivers. Some river restoration approaches propose radical transformation of river patterns, from multiple to single channels, based on the link between river patterns and their in-channel characteristics. Determining the links between river patterns and their in-channel characteristics is complicated by differences in geology, history, climate and discharge among rivers. Furthermore, multiple channel rivers are composed of a mosaic of channel types with a range of in-channel characteristics. This study minimizes these problems by analysing a single river containing neighbouring single and multiple channel patterns with little change in discharge downstream, and by analysing all channel types. The study addressed two objectives: to determine the hydraulic geometry, energy, and sediment mobility characteristics of neighbouring single and multiple channel river patterns, and to test for statistical differences in these characteristics between patterns. The Renous River shows a wandering pattern for 11.5 km, with multiple channels around semipermanent islands and abandoned channels in the flood plain. The river displays a single channel river pattern where channels are confined by their valley walls, upstream and downstream of wandering. The analysis was conducted at three scales. First, the confined single channel and wandering multiple channel patterns were compared (pattern scale). Second, the confined channel pattern was compared to single and multiple channel sections within the wandering pattern (section scale). Third, all channel types were compared (channel type scale). Multi response permutation procedure (MRPP) and analysis of variance (ANOVA) were used to analyze differences between channels. Difference tests found no simple discrimination between the single and multiple channel river patterns of the Renous River. Tests between the single confined and multiple wandering channel patterns found few differences in the in-channel variables. The tests did find differences between multiple channel sections within the wandering pattern and confined single channels; however, a greater number of differences were found between multiple channel and single channel sections within the wandering pattern, highlighting the variability within the wandering pattern. Two groups emerged when all channel types were tested for differences: perennial main-channels containing the thalweg, and ephemeral side-channels. Therefore, side-channels define the in-channel characteristics of wandering rivers because few differences were found among main-channels in either pattern. This analysis suggests that all channel types, not just main-channels, should be investigated to obtain a complete picture of a river pattern prior to any restoration efforts. Engineers must exercise caution when applying the link between river patterns and in-channel characteristics to river restoration efforts.     

Influence of vertical channel change associated with wood accumulations on delineating channel migration zones, Washington, USA

We combine hydraulic modeling and field investigations of logjams to evaluate linkages between wood-mediated fluctuations in channel-bed-and water-surface elevations and the potential for lateral channel migration in forest rivers of Washington state. In the eleven unconfined rivers we investigated, logjams were associated with reduced channel gradient and bank height. Detailed river gauging and hydraulic modeling document significant increases in the water-surface elevation upstream of channel-spanning wood accumulations. Logjams initiated lateral channel migration by increasing bed-or water-surface elevations above adjacent banks. Because the potential for a channel to avulse and migrate across its floodplain increases with the size and volume of instream wood, the area of the valley bottom potentially occupied by a channel over a specified timeframe — the channel migration zone (CMZ) — is dependent on the state of riparian forests. The return of riparian forests afforded by current land management practices will increase the volume and caliber of wood entering Washington rivers to a degree unprecedented since widespread clearing of wood from forests and rivers nearly 150 years ago. A greater supply of wood from maturing riparian forests will increase the frequency and spatial extent of channel migration relative to observations from wood-poor channels in the period of post-European settlement. We propose conceptual guidelines for the delineation of the CMZs that include allowances for vertical fluctuations in channel elevation caused by accumulations of large woody debris.     

Reorganization of a deeply incised drainage: role of deformation,sedimentation and groundwater flow

Deeply incised drainage networks are thought to be robust and not easily modified, and are commonly used as passive markers of horizontal strain. Yet, reorganizations (rearrangements) appear in the geologic record. We provide field evidence of the reorganization of a Miocene drainage network in response to strike–slip and vertical displacements in Guatemala. The drainage was deeply incised into a 50‐km‐wide orogen located along the North America–Caribbean plate boundary. It rearranged twice, first during the Late Miocene in response to transpressional uplift along the Polochic fault, and again in the Quaternary in response to transtensional uplift along secondary faults. The pattern of reorganization resembles that produced by the tectonic defeat of rivers that cross growing tectonic structures. Compilation of remote sensing data, field mapping, sediment provenance study, grain‐size analysis and Ar⁴⁰/Ar³⁹ dating from paleovalleys and their fill reveals that the classic mechanisms of river diversion, such as river avulsion over bedrock, or capture driven by surface runoff, are not sufficient to produce the observed diversions. The sites of diversion coincide spatially with limestone belts and reactivated fault zones, suggesting that solution‐triggered or deformation‐triggered permeability have helped breaching of interfluves. The diversions are also related temporally and spatially to the accumulation of sediment fills in the valleys, upstream of the rising structures. We infer that the breaching of the interfluves was achieved by headward erosion along tributaries fed by groundwater flow tracking from the valleys soon to be captured. Fault zones and limestone belts provided the pathways, and the aquifers occupying the valley fills provided the head pressure that enhanced groundwater circulation. The defeat of rivers crossing the rising structures results essentially from the tectonically enhanced activation of groundwater flow between catchments.     

Tectonics,geomorphology and water mill location in Scotland,and the potential impacts of mill dam failure

In this paper we assess the ways in which the topography of glaciated northern Britain has affected the siting and operations of water mills, and compare those factors and mill locations for mills in unglaciated southern Britain. We then explore the impacts of these findings on the potential downstream impacts of mill dam failure.We used a GIS to plot the locations of all 1712 localities in Britain's Ordnance Survey Gazetteer that include “mill”, “milton” (‘milltown’) and “miln” in their name. We then examined the geomorphology of mill locations in two study areas, one in northeast Scotland (glaciated; 421 localities) and one in southern England (unglaciated; 438 localities), assessing (i) mill location within the drainage net, and (ii) the steepness of an adjacent stream within a radius of 500 m of the mill locality. The large majority of mills are located within the first 10 km of the drainage net in both study areas, presumably on relatively stable bedrock channels. The data for most of the mills in both study areas indicate that catchment areas of less than 200 km² are sufficient to supply the water necessary for operation of a mill, but the higher rainfalls and runoff in Scotland (almost twice the values in the England study area) mean that mill dams in S England must have been higher and of higher capacity than those in NE Scotland. That finding is consistent with the results related to channel steepness, which show that mills in Scotland are associated with steeper channels than is the case in England. The generally greater channel steepness in Scotland (and the greater downstream extent of those steeper channels, as also confirmed by the data) reflect both the many glacially steepened bedrock channel reaches in Scotland and the steepening of Scotland's coastal bedrock channels as a result of glacio-isostatic rebound.The technical requirements of water mill operation favour situations where water can be delivered to the top of, or at least part-way up, the mill wheel. Scotland's steeper rivers and its higher rainfalls mean that Scotland's mills require smaller mill dams, if they are needed at all. It would therefore be expected that catastrophic or managed failure of mill dam walls in northern Britain would release lower volumes of trapped sediment to the downstream fluvial system. These lower volumes would in turn result in lower geomorphological impacts downstream of the dam, both in terms of changing channel patterns and burial of the bed. Such dam failure is a key current issue in geomorphology and one case study of a small failed mill dam in western Scotland confirms the minimal downstream impacts of that failure.     

Application of a new river classification scheme to Australia's tropical rivers

A revised typology of Australian tropical rivers was applied to the complete channel network (named and major rivers) shown on 1:250 000 topographic maps for three large drainage basins in northern Australia (Daly River, NT; Fitzroy River, WA; Flinders River, Qld). Reach mapping and classification were conducted using the revised typology. The 12 major river types proposed were: (1) bedrock rivers; (2) bedrock‐confined and ‐constrained rivers; (3) low sinuosity (straight) rivers; (4) meandering rivers; (5) wandering rivers; (6) anabranching rivers; (7) chains of ponds; (8) gullies; (9) floodouts; (10) lakes, swamps, billabongs and wetlands; (11) non‐channelized valley floors; and (12) estuarine rivers. The 12 major river types were developed based on river reach mapping for more than 264 000 km² of tropical Australian catchments. At scales larger than 1:250 000, subdivision of each major river type is recommended. In the Daly and Fitzroy catchments, confined and constrained rivers dominate, whereas in the Flinders and Fitzroy catchments, anabranching rivers dominate. The dominant river types need benchmarking with adequate numbers of control reaches so that channel changes induced by human and natural impacts can be measured by reference to the stability of these controls. Wandering rivers, floodouts and non‐channelized valley floors were rare for the 1:250 000 channel network in northern Australia but need inclusion in national parks.     

Anabranching in mixed bedrock-alluvial rivers: the example of the Orange River above Augrabies Falls, Northern Cape Province, South Africa

Anabranching is characteristic of a number of rivers in diverse environmental settings worldwide, but has only infrequently been described from bedrock-influenced rivers. A prime example of a mixed bedrock-alluvial anabranching river is provided by a 150-km long reach of the Orange River above Augrabies Falls, Northern Cape Province, South Africa. Here, the perennial Orange flows through arid terrain consisting mainly of Precambrian granites and gneisses, and the river has preferentially eroded bedrock joints, fractures and foliations to form multiple channels which divide around numerous, large (up to 15 km long and 2 km wide), stable islands formed of alluvium and/or bedrock. Significant local variations in channel-bed gradient occur along the river, which strongly control anabranching style through an influence on local sediment budgets. In relatively long (>10 km), lower gradient reaches (<0.0013) within the anabranching reach, sediment supply exceeds local transport capacity, bedrock usually only crops out in channel beds, and channels divide around alluvial islands which are formed by accretion in the lee of bedrock outcrop or at the junction with ephemeral tributaries. Riparian vegetation probably plays a key role in the survival and growth of these islands by increasing flow roughness, inducing deposition, and stabilising the sediments. Less commonly, channels may form by eroding into once-continuous island or floodplain surfaces. In shorter (<10 km), higher gradient reaches (>0.0013) within the anabranching reach, local transport capacity exceeds sediment supply, bedrock crops out extensively, and channels flow over an irregular bedrock pavement or divide around rocky islands. Channel incision into bedrock probably occurs mainly by abrasion, with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting. Mixed bedrock-alluvial anabranching also occurs in a number of other rivers worldwide, and appears to be a stable and often long-lived river pattern adjusted to a number of factors commonly acting in combination: (1) jointed/fractured granitoid rock outcrop; (2) erosion-resistant banks and islands; (3) locally variable channel-bed gradients; (4) variable flow regimes.     

Systematic variation of bedrock channel gradients in the central Oregon Coast Range: implications for rock uplift and shallow landsliding

In tectonically active regions, bedrock channels play a critical role in dictating the pace of landscape evolution. Models of fluvial incision into bedrock provide a means of investigating relationships between gradients of bedrock channels and patterns of active deformation. Variations in lithology, orographic precipitation, sediment supply, and erosional processes serve to complicate tectonic inferences derived from morphologic data, yet most tectonically active landscapes are characterized by these complexities. In contrast, the central Oregon Coast Range (OCR), which is situated above the Cascadia subduction zone, has experienced rock uplift for several million years, did not experience Pleistocene glaciation, boasts a relatively uniform lithology, and exhibits minor variations in precipitation. Although numerous process-based geomorphic studies suggest that rates of erosion across the OCR are relatively constant, it has not been demonstrated that bedrock channel gradients in the region exhibit spatially consistent values. Analysis of broadly distributed, small drainage basins (5–20 km) in the central OCR enables us to explore regional variability in bedrock channel gradients resulting from differential rock uplift or other sources. Consistent with previous studies that have documented local structural control of deformed fluvial terraces in the western portion of our study area, our data reveal a roughly 20-km-wide band of systematically elevated channel slopes (roughly twice the background value), roughly coincident with the strike of N–S-trending mapped folds. Although many factors could feasibly generate this pattern, including variable rock strength, precipitation gradients, or temporal or spatial variations in forearc deformation, the elevated bedrock channel slopes likely reflect differential rock uplift related to activity of local structures. Importantly, our analysis suggests that rock uplift and erosion rates may vary systematically across the OCR. Although our calculations were focused on the fluvial-dominated portion of study basins, our results have implications for upstream areas, including unchanneled valleys that often serve as source areas for long-runout debris flows. Zero-order basins (or topographic hollows) within the N–S-trending band of elevated channel slopes tend to be steeper than adjacent areas and may experience more frequent evacuation by shallow landsliding. Thus, this region of the OCR may be highly sensitive to land use practices and high-intensity rainstorms.     

Steady, balanced rates of uplift and erosion of the Santa Monica Mountains, California

Topographic change in regions of active deformation is a function of rates of uplift and denudation. The rate of topographic development and change of an actively uplifting mountain range, the Santa Monica Mountains, southern California, was assessed using landscape attributes of the present topography, uplift rates and denudation rates. Landscape features were characterized through analysis of a digital elevation model (DEM). Uplift rates at time scales ranging from 10⁴ to 10⁶ years were constrained with geological cross-sections and published estimates. Denudation rate was determined from sediment yield data from debris basins in southern California and from the relief of rivers set into geomorphic surfaces of known age. First-order morphology of the Santa Monica Mountains is set by large-scale along-strike variations in structural geometry. Drainage spacing, drainage geometry and to a lesser extent relief are controlled by bedrock strength. Dissection of the range flanks and position of the principal drainage divide are modulated by structural asymmetry and differences in structural relief across the range. Topographic and catchment-scale relief are ≈300–900 m. Mean denudation rate derived from the sediment yield data and river incision is 0.5±0.3 mm yr^?1. Uplift rate across the south flank of the range is ≈0.5±0.4 mm yr^?1 and across the north flank is 0.24±0.12 mm yr^?1. At least 1.6–2.7 Myr is required to create either the present topographic or the catchment-scale relief based on either the mean rates of denudation or uplift. Although the landscape has had sufficient time to achieve a steady-state form, comparison of the time-scale of uplift and denudation rate variation with probable landscape response times implies the present topography does not represent the steady-state form.     

Scaling in river corridor widths depicts organization in valley morphology

Landscapes have been shown to exhibit numerous scaling laws from Horton's laws to more sophisticated scaling in topography heights, river network topology and power laws in several geomorphic attributes. In this paper, we propose a different way of examining landscape organization by introducing the “river corridor width” (lateral distance from the centerline of the river to the left and right valley walls at a fixed height above the water surface) as one moves downstream. We establish that the river corridor width series, extracted from 1 m LIDAR topography of a mountainous river, exhibit a rich multiscale statistical structure (anomalous scaling) which varies distinctly across physical boundaries, e.g., bedrock versus alluvial valleys. We postulate that such an analysis, in conjunction with field observations and physical modeling, has the potential to quantitatively relate mechanistic laws of valley formation to the statistical signature that underlying processes leave on the landscape. Such relations can be useful in guiding field work (by identifying physically distinct regimes from statistically distinct regimes) and advancing process understanding and hypothesis testing.     

Investigating the surface process response to fault interaction and linkage using a numerical modelling approach

In order to better understand the evolution of rift‐related topography and sedimentation, we present the results of a numerical modelling study in which elevation changes generated by extensional fault propagation, interaction and linkage are used to drive a landscape evolution model. Drainage network development, landsliding and sediment accumulation in response to faulting are calculated using CASCADE, a numerical model developed by Braun and Sambridge, and the results are compared with field examples. We first show theoretically how the ‘fluvial length scale’, L_f, in the fluvial incision algorithm can be related to the erodibility of the substrate and can be varied to mimic a range of river behaviour between detachment‐limited (DL) and transport‐limited (TL) end‐member models for river incision. We also present new hydraulic geometry data from an extensional setting which show that channel width does not scale with drainage area where a channel incises through an area of active footwall uplift. We include this information in the coupled model, initially for a single value of L_f, and use it to demonstrate how fault interaction controls the location of the main drainage divide and thus the size of the footwall catchments that develop along an evolving basin‐bounding normal fault. We show how erosion by landsliding and fluvial incision varies as the footwall area grows and quantify the volume, source area, and timing of sediment input to the hanging‐wall basin through time. We also demonstrate how fault growth imposes a geometrical control on the scaling of river discharge with downstream distance within the footwall catchments, thus influencing the incision rate of rivers that drain into the hanging‐wall basin. Whether these rivers continue to flow into the basin after the basin‐bounding fault becomes fully linked strongly depends on the value of L_f. We show that such rivers are more likely to maintain their course if they are close to the TL end member (small L_f); as a river becomes progressively more under supplied, i.e. the DL end member (large L_f), it is more likely to be deflected or dammed by the growing fault. These model results are compared quantitatively with real drainage networks from mainland Greece, the Italian Apennines and eastern California. Finally, we infer the calibre of sediments entering the hanging‐wall basin by integrating measurements of erosion rate across the growing footwall with the variation in surface processes in space and time. Combining this information with the observed structural control of sediment entry points into individual hanging‐wall depocentres we develop a greater understanding of facies changes associated with the rift‐initiation to rift‐climax transition previously recognised in syn‐rift stratigraphy.     

Changes in bedrock channel morphology driven by displacement rate increase during normal fault interaction and linkage

We attribute changes in the morphology of relay ramp channels (increased slope and decreased width) to variations in displacement rate on ramp‐adjacent normal faults. We map the faults and fluvial channels associated with four sites in different stages of fault interaction and linkage on the Volcanic Tableland, a Late Pleistocene ash‐flow tuff in east‐central California. Because these channels are inactive today, we estimate downstream changes in channel width and depth using HEC‐RAS, a one‐dimensional open channel flow model. Our results show that channel slope must be greater than about 0.05 before there are substantial decreases in width or substantial increases in depth. Displacement rate increases during interaction between en echelon segments results in the increases in channel slope and decreases in channel width. Moreover, our data show that these changes begin to occur during the very early stages of fault interaction, well before the fault geometry would indicate ongoing or imminent linkage.     

Estimating extreme flood magnitude in bedrock-influenced channels using representative reach-based channel resistance data

Extreme flood events are considered by many researchers to be very important in controlling the development of semi‐arid bedrock‐influenced river systems. Accurate gauging of such events is often impossible, however, as gauges are drowned and often damaged during the event. A methodology for estimating flood discharge for bedrock‐influenced channels is presented that reconstructs hydrometric characteristics of the peak flow and relates these to the roughness character of the river channel in question. The method is evaluated using peak water‐surface slope data relating to the extreme floods of February 2000 along the Sabie and Letaba rivers, located respectively in the Mpumalanga and Northern Provinces, South Africa. The data, in the form of strandline measurements, were taken at hydraulically relevant points along the long profile of both rivers. The resultant data are utilised together with published high flow channel resistance figures, based on the channel morphology of the Sabie and Letaba rivers, to generate peak flow estimates for a number of locations along both rivers. Comparisons are made between the frictional discharge peak flow estimates, velocity‐area and hydrologic estimates of peak flow. These comparisons indicate that the method can produce discharge estimates with an accuracy of ±10% and ± 35% respectively.     

Quantifying the slip rates, spatial distribution and evolution of active normal faults from geomorphic analysis: Field examples from an oblique-extensional graben, southern Turkey

Quantifying the extent to which geomorphic features can be used to extract tectonic signals is a key challenge in the Earth Sciences. Here we analyse the drainage patterns, geomorphic impact, and long profiles of bedrock rivers that drain across and around normal faults in a regionally significant oblique-extensional graben (Hatay Graben) in southern Turkey that has been mapped geologically, but for which there are poor constraints on the activity, slip rates and Plio–Pleistocene evolution of basin-bounding faults. We show that drainage in the Hatay Graben is strongly asymmetric, and by mapping the distribution of wind gaps, we are able to evaluate how the drainage network has evolved through time. By comparing the presence, size, and distribution of long profile convexities, we demonstrate that the northern margin of the graben is tectonically quiescent, whereas the southern margin is bounded by active faults. Our analysis suggests that rivers crossing these latter faults are undergoing a transient response to ongoing tectonic uplift, and this interpretation is supported by classic signals of transience such as gorge formation and hill slope rejuvenation within the convex reach. Additionally, we show that the height of long profile convexities varies systematically along the strike of the southern margin faults, and we argue that this effect is best explained if fault linkage has led to an increase in slip rate on the faults through time from  0.1 to 0.45 mm/yr. By measuring the average length of the original fault segments, we estimate the slip rate enhancement along the faults, and thus calculate the range of times for which fault acceleration could have occurred, given geological estimates of fault throw. These values are compared with the times and slip rates required to grow the documented long-profile convexities enabling us to quantify both the present-day slip rate on the fault (0.45 ± 0.05 mm/yr) and the timing of fault acceleration (1.4 ± 0.2 Ma). Our results have substantial implications for predicting earthquake hazard in this densely populated area (calculated potential M_w = 6.0–6.6), enable us to constrain the tectonic evolution of the graben through time, and more widely, demonstrate that geomorphic analysis can be used as an effective tool for estimating fault slip rates over time periods > 10⁶ years, even in the absence of direct geodetic constraints.     

网状河流研究进展述评

网状河流作为一种新的冲积河流类型已经引起地貌学家、水利学家和沉积学家的关注 ,成为河流地貌领域、河流沉积领域以及河流水动力领域的研究热点之一。本文在介绍网状河流基本概念的基础上 ,综合国内外的研究成果 ,从河流的平面形态、边界条件、沉积特征、水动力条件以及在河型演化序列中的位置等方面 ,对网状河流的研究进展作一较全面的述评 ,并指出目前研究的薄弱环节 ,以利于研究者把握网状河流的研究现状 ,并推动对网状河流的进一步探讨。     

Understanding confluence dynamics in the alluvial Ganga–Ramganga valley, India: An integrated approach using geomorphology and hydrology

Confluence dynamics in the Ganga–Ramganga valley in the western Ganga plains of India has been studied through systematic mapping of channel configuration using multi-date remote sensing images and topographic sheets for a period spanning nearly 100 years (1911–2000). The study has been supplemented with a detailed analysis of the channel morphology, hydrology and sediment transport characteristics of the different rivers. Our study indicates that new confluences have been created during this period and that the confluence points have moved both upstream and downstream on a historical time scale. Apart from major avulsions, other processes that have controlled the confluence movements include river capture, cut-offs and aggradation in the confluence area. River capture occurs through lateral bank erosion and migration, encroachment by the master stream and beheading of smaller rivers resulting in upstream movement of the confluence point. Another process which influences the upstream migration of the confluence is an increase in sinuosity of one of the channels near the confluence and then a cut-off. Aggradation in the confluence area and local avulsions of the primary channel in a multi-channel system seem to be the major process controlling the downstream movement of the confluence point. Analysis of channel morphology, hydrology and sediment budget for the study period supports our interpretations.     

Channel Form, Bed Material and Sediment Sources of the Sprongdøla, Southern Norway: Evidence for a Distinct Periglacio-Fluvial System

A geomorphological and sedimentological case study is made of the Sprongdøla, a medium-sized river flowing in a predominantly bedrock channel in the mountains of southern Norway. Particular attention is given to bed material characteristics and potential sediment sources. Downstream patterns in clast size and shape, including sphericity and roundness, are poorly developed in the Sprongdøla. There is an apparent absence of down-stream fining and a large proportion of bedload in the lower channel remains in very angular and angular roundness categories. Covariance analysis of clast shape and roundness allows this to be attributed to the widespread input of coarse, angular rock particles derived from snow-avalanche activity on the valley sides and especially from frost-shattering of bedrock in the channel, which produces characteristically slabby clasts. The latter process is important in channel widening and in steepening the channel walls. Strictly fluvial processes, avalanche activity and frost-shattering are controlled in different ways by related annual environmental cycles. Their interaction in the context of the periglacial river channel produces a unique process-sediment-landform association that distinguishes alpine periglacio-fluvial systems from those associated with glacio-fluvial rivers and fluvial landscapes in the temperate zone.     

Steering of experimental channels by lateral basin tilting

A major issue in tectonics and sedimentation is the role of cross‐stream tectonic tilting in steering channels. The general idea is that channels will be attracted to lateral maxima in subsidence rate. A physical experiment performed in 1999 at the St. Anthony Falls Laboratory, however, was in conflict with the idea and showed that fluvial channels and resulting stratigraphy can be insensitive to even relatively strong lateral variation in subsidence. Here, we present results from an experiment which uses a simplified relay‐ramp geometry with laterally variable uplift and subsidence to test a hypothesis developed from the earlier experiment: Tectonic tilting steers channels only when the ratio of the time scales describing lateral channel mobility to tectonic deformation is sufficiently large. Occupation time by experimental channels and sand fraction in the deposit (a proxy for channel deposition) both increase with subsidence rate indicating strong steering of channels by tectonic forcing. We also found that, due to local incision, uplift lengthened the time scale for lateral channel migration relative to subsidence. Comparing channel mobility at the beginning of the experiment, with no tectonic forcing, to later tectonic stages of the experiment indicates that active tectonics increased the channel time scale. The interplay of channel steering with uplift and subsidence led to cyclic appearance and disappearance of an autogenic lake in the hanging‐wall basin. This lake was associated with alternation between channels going around vs. across the adjoining upstream uplifted footwall region. This creation and filling of the lake under constant tectonic forcing (constant fault slip rate) in the hanging wall created subaerial fan‐delta parasequences separated by fluvial deposits.