Defining and measuring braiding intensity

Geomorphological studies of braided rivers still lack a consistent measurement of the complexity of the braided pattern. Several simple indices have been proposed and two (channel count and total sinuosity) are the most commonly applied. For none of these indices has there been an assessment of the sampling requirements and there has been no systematic study of the equivalence of the indices to each other and their sensitivity to river stage. Resolution of these issues is essential for progress in studies of braided morphology and dynamics at the scale of the channel network. A series of experiments was run using small‐scale physical models of braided rivers in a 3 m ∞ 20 m flume. Sampling criteria for braid indices and their comparability were assessed using constant‐discharge experiments. Sample hydrographs were run to assess the effect of flow variability. Reach lengths of at least 10 times the average wetted width are needed to measure braid indices with precision of the order of 20% of the mean. Inherent variability in channel pattern makes it difficult to achieve greater precision. Channel count indices need a minimum of 10 cross‐sections spaced no further apart than the average wetted width of the river. Several of the braid indices, including total sinuosity, give very similar numerical values but they differ substantially from channel‐count index values. Consequently, functional relationships between channel pattern and, for example, discharge, are sensitive to the choice of braid index. Braid indices are sensitive to river stage and the highest values typically occur below peak flows of a diurnal (melt‐water) hydrograph in pro‐glacial rivers. There is no general relationship with stage that would allow data from rivers at different relative stage to be compared. At present, channel count indices give the best combination of rapid measurement, precision, and range of sources from which measurements can be reliably made. They can also be related directly to bar theory for braided pattern development. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical investigation of avulsions in gravel-bed braided rivers

Gravel-bed braided rivers are highly energetic fluvial systems characterized by frequent in-channel avulsions, which govern the morphodynamics of such rivers and are essential for them to maintain a braided planform. However, the avulsion mechanisms within natural braided rivers remain unclear due to their complicated hydraulic and morphodynamic processes. Influenced by neighbouring channels, avulsions in braided rivers may differ from those of bifurcations in single-thread rivers, suggesting that avulsions should be studied within the context of the entire braid network. In this study, braiding evolution processes in gravel-bed rivers were simulated using a physics-based numerical model that considers graded bed-load transport by dividing sediment particles into multiple size fractions and vertical sediment sorting by dividing the riverbed into several vertical layers. The numerical model successfully produced braiding processes and avulsion activities similar to those observed in a laboratory river. Results show that bend evolution of the main channel was the fundamental process controlling the occurrence of avulsions in the numerical model, with a cyclic process of channel meandering by lateral migration that transitioned to a straight channel pattern by avulsion. The radius of bend curvature for triggering avulsions in the numerical model was measured and it was found that the highest probability for a channel bend to generate an avulsion occurs when its radius of curvature is approximately 2.0–3.3 times the average anabranch width. Other types of avulsion were also observed that did not occur specifically at meander bends, but upstream meander evolution indirectly influenced such avulsions by altering channel pattern and discharge to those locations. This study explored the processes and mechanisms of several types of avulsion, and proposed factors controlling their occurrence, namely increasing channel curvature, high shear stress, tributary discharge, riverbed gradient and upstream channel pattern, with high shear stress being a direct indicator. Furthermore, avulsions in a typical gravel-bed braided river, the Waimakariri River in New Zealand, were analysed using sequential Google Earth maps, which confirmed the conclusions derived from the numerical simulation.     

Comparison of hydraulic geometry between sand‐ and gravel‐bed rivers in relation to channel pattern discrimination

A comparison has been made between the hydraulic geometry of sand‐ and gravel‐bed rivers, based on data from alluvial rivers around the world. The results indicate a signi?cant difference in hydraulic geometry among sand‐ and gravel‐bed rivers with different channel patterns. On this basis, some diagrams for discrimination of meandering and braided channel patterns have been established. The relationships between channel width and water discharge, between channel depth and water discharge, between width–depth ratio and water discharge and between channel slope and water discharge can all be used for channel pattern discrimination. The relationship between channel width and channel depth can also be used for channel pattern discrimination. However, the accuracy of these relationships for channel pattern discrimination varies, and the depth–discharge relationship is a better discriminator of pattern type than the classic slope–discharge function. The cause for this difference has been explained qualitatively. To predict the development of channel patterns under different natural conditions, the pattern discriminator should be searched on the basis of independent or at least semi‐independent variables. The relationship between stream power and bed material grain size can be used to discriminate channel patterns, which shows a better result than the discriminator using the slope–discharge relationship. Copyright © 2004 John Wiley & Sons, Ltd.     

Regime theories in gravel‐bed rivers: models,controlling variables,and applications in disturbed Italian rivers

Downstream hydraulic geometry relationships describe the shape of alluvial channels in terms of bankfull width, flow depth, flow velocity, and channel slope. Recent investigations have stressed the difference in spatial scales associated with these variables and thus the time span required for their adjustment after a disturbance. The aim of this study is to explore the consequences in regime models considering the hypothesis that while channel width and depth adjust quickly to changes in water and sediment supply, reach slope requires a longer time span. Three theoretical models were applied. One model incorporates an extremal hypothesis (Millar RG. 2005. Theoretical regime equations for mobile gravel‐bed rivers with stable banks. Geomorphology 64 : 207–220), and the other two are fully physically based (Ikeda S, Parker G, Kimura Y. 1988. Stable width and depth of straight gravel rivers with heterogeneous bed materials. Water Resources Research 24 : 713–722; Parker G, Wilcock PR, Paola C, Dietrich W, Pitlick J. 2007. Physical basis for quasi universal relations describing bankfull hydraulic geometry of single‐thread gravel‐bed rivers. Journal of Geophysical Research 112 , DOI: 10.1029/2006JF000549). In order to evaluate the performance of models introducing the slope as an independent variable, we propose two modifications to previous models. The performance of regime models was tested against published data from 142 river reaches and new hydraulic geometry data from gravel‐bed rivers in Patagonia (Argentina) and north‐eastern Italy. Models that assume slope as a control (Ikeda et al., 1988; or Millar, 2005) predict channel depth and width reasonably well. Parker et al.'s (2007) model improved predictions because it filters the scatter in slope data with a relation slope–discharge. The extremal hypothesis model of Millar (2005) predicts comparably to the other physically based models. Millar's model was chosen to describe the recent changes in the Piave and Brenta rivers due to human intervention – mainly in‐channel gravel mining. The change in sediment supply and recovery was estimated for these rivers. This study supports the interpretation that sediment supply is the key factor guiding morphological changes in these rivers. Copyright © 2013 John Wiley & Sons, Ltd.     

Bar dynamics and bifurcation evolution in a modelled braided sand‐bed river

Morphodynamics in sand‐bed braided rivers are associated with simultaneous evolution of mid‐channel bars and channels on the braidplain. Bifurcations around mid‐channel bars are key elements that divide discharge and sediment. This, in turn, may control the evolution of connected branches, with effects propagating to both upstream and downstream bifurcations. Recent works on bifurcation stability and development hypothesize major roles of secondary flow and gradient advantage. However, this has not been tested for channel networks within a fully developed dynamic braided river. A reason for this is a lack of detailed measurements with sufficient temporal and spatial length, covering multiple bifurcations. Therefore we used a physics‐based numerical model to generate a dataset of bathymetry, flow and sediment transport of an 80 km river reach with self‐formed braid bars and bifurcations. The study shows that bar dissection due to local transverse water surface gradients is the dominant bifurcation initiation mechanism, although conversion of unit bars into compound bars dominates in the initial stage of a braided river. Several bifurcation closure mechanisms are equally important. Furthermore, the study showed that nodal point relations for bifurcations are unable to predict short‐term bifurcation evolution in a braided river. This is explained by occurrence of nonlinear processes and non‐uniformity within the branches, in particular migrating bars and larger‐scale backwater‐effects, which are not included in the nodal point relations. Planform morphology, on the other hand, has predictive capacity: bifurcation angle asymmetry and bar‐tail limb shape are indicators for near‐future bifurcation evolution. Remote sensing data has predictive value, for which we developed a conceptual model for interactions between bars, bifurcations and channels in the network. We conducted a preliminary test of the conceptual model on satellite images of the Brahmaputra. Copyright © 2015 John Wiley & Sons, Ltd.     

Channelization of a large Alpine river: what is left of its original morphodynamics?

The Adige River drains 12 200 km² of the Eastern Alps and flows for 213 km within this mountain range. Similar to other large rivers in Central Europe, the Adige River was subject to massive channelization works during the 19th century. Thanks to the availability of several historical maps, this river represents a very valuable case study to document the extent to which the morphology of the river changed due to channelization and to understand how much is left of its original morphodynamics. The study was based on the analysis of seven sets of historical maps dating from 1803–1805 to 1915–1927, on geomorphological analysis, on the application of mathematical morphodynamic theories and on the application of bar and channel pattern prediction models. The study concerns 115 km of the main stem and 29 km of its tributaries. In the pre‐channelization conditions, the Adige River presented a prevalence of single‐thread channel planforms. Multi‐thread patterns developed only immediately downstream of the main confluences. During the 19th century, the Adige underwent considerable channel adjustment, consisting of channel narrowing, straightening, and reduction of bars and islands. Multi‐thread and single‐thread reaches evolved through different evolutionary trajectories, considering both the channel width and the bar/vegetation interaction. Bar and channel pattern predictors showed good correspondence with the observed patterns, including the development of multi‐thread morphologies downstream of the confluences. Application of the free‐bar predictor helped to interpret the strong reduction – almost complete loss – of exposed sediment bars after the channelization works, quantifying the riverbed inclination to form alternate bars. This morphological evolution can be observed in other Alpine rivers of similar size and similar massive channelization, therefore, a simplified conceptual model for large rivers subjected to channelization is proposed, showing that a relatively small difference in the engineered channel width may have a strong impact on the river dynamics, specifically on bar formation. Copyright © 2017 John Wiley & Sons, Ltd.     

Active width of gravel‐bed braided rivers

Previous analyses have identified the active width of braided rivers, the bed area over which bed load flux and short‐term morphological change occurs, as an important element of braiding dynamics and predictions of bed load flux. Here we compare theoretical predictions of active width in gravel‐bed braided rivers with observations from Sunwapta River, and from a generic physical model of gravel braided rivers, to provide general observations of the variation in active width, and to develop an understanding of the causes of variation. Bed topography was surveyed daily along a 150 m reach of the pro‐glacial Sunwapta River for a total of four weeks during summer when flow was above threshold for morphological activity. In the laboratory, detailed digital elevation models (DEMs) were derived from photogrammetric survey at regular intervals during a constant discharge run. From the field and flume observations there is considerable local and circumstantial variation in active width, but also a general trend in average active width with increasing discharge. There is also a clear relationship of active width with active braiding index (number of active branches in the braided channel network), and with dimensionless stream power, which appears to be consistent across the range of data from field and physical models. Thus there is a link between active width and the river morphology and dynamics, and the possibility of a general relationship for estimating active width from channel pattern properties or reach‐scale stream power values, from which approximate bedload flux calculations may be made. The analysis also raises questions about differences between hydraulically‐based numerical model computations of instantaneous active width and observation of time‐integrated morphological active width. Understanding these differences can give insight into the nature of bedload transport in braided rivers and the relationship to morphological processes of braiding. Copyright © 2011 John Wiley & Sons, Ltd.     

Intra‐event scale bar–bank interactions and their role in channel widening

Channel bars and banks strongly affect the morphology of both braided and meandering rivers. Accordingly, bar formation and bank erosion processes have been greatly explored. There is, however, a lack of investigations addressing the interactions between bed and bank morphodynamics, especially over short timescales. One major implication of this gap is that the processes leading to the repeated accretion of mid‐channel bars and associated widenings remain unsolved. In a restored section of the Drau River, a gravel‐bed river in Austria, mid‐channel bars have developed in a widening channel. During mean flow conditions, the bars divert the flow towards the banks. One channel section exhibited both an actively retreating bank and an expanding mid‐channel bar, and was selected to investigate the morphodynamic processes involved in bar accretion and channel widening at the intra‐event timescale. We repeatedly surveyed riverbed and riverbank topography, monitored riverbank hydrology and mounted a time‐lapse camera for continuous observation of riverbank erosion processes during four flow events. The mid‐channel bar was shown to accrete when it was submerged during flood events, which at the subsequent flow diversion during lower discharges narrowed the branch along the bank and increased the water surface elevation upstream from the riffle, which constituted the inlet into the branch. These changes of bed topography accelerated the flow along the bank and triggered bank failures up to 20 days after the flood events. Four analysed flow events exhibited a total bar expansion from initially 126 m² to 295 m², while bank retreat was 6 m at the apex of the branch. The results revealed the forcing role of bar accretion in channel widening and highlighted the importance of intra‐event scale bed morphodynamics for bank erosion, which were summarized in a conceptual model of the observed bar–bank interactions. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydro‐morphodynamic evolution in a 90° movable bed discordant confluence with low discharge ratio

Confluences with low discharge and momentum ratios, where narrow steep tributaries with high sediment load join a wide low‐gradient main channel that provides the main discharge, are often observed in high mountain regions such as in the upper‐Rhone river catchment in Switzerland. Few existing studies have examined the hydro‐morphodynamics of this type of river confluence while considering sediment discharge in both confluent channels. This paper presents the evolution of the bed morphology and hydrodynamics as observed in an experimental facility with a movable bed. For that purpose, one experiment was carried out in a laboratory confluence with low discharge and momentum ratios, where constant sediment rates were supplied to both flumes. During the experiment, bed topography and water surface elevations were systematically recorded. When the bed topography reached a steady state (so‐called equilibrium) and the outgoing sediment rate approximated the incoming rate, flow velocity was measured at 12 different points distributed throughout the confluence, and the grain size distribution of the bed surface was analyzed. Typical morphodynamic features of discordant confluences such as a bank‐attached bar and a flow deflection zone are identified in this study. Nevertheless, the presence of a marked scour hole in the discordant confluence and distinct flow regimes for the tributary and main channel, differ from results obtained in previous studies. Strong acceleration of the flow along the outer bank of the main channel is responsible for the scour hole. This erosion is facilitated by the sediment discharge into the confluence from the main channel which inhibits bed armoring in this region. The supercritical flow regime observed in the tributary is the hydrodynamic response to the imposed sediment rate in the tributary. Copyright © 2015 John Wiley & Sons, Ltd.     

The variability in the morphological active width: Results from physical models of gravel‐bed braided rivers

The morphological active width, defined as the lateral extent of bed material displacement over time, is a fundamental parameter in multi‐threaded gravel‐bed rivers, linking complex channel dynamics to bedload transport. Here, results are presented from five constant discharge experiments, and three event hydrographs, covering a range of flow strengths and channel configurations for which morphological change, bedload transport rates, and stream power were measured in a physical model. Changes in channel morphology were determined via differencing of photogrammetrically‐derived digital elevation models (DEMs) of the model surface generated at regular intervals over the course of ~115 h of experimental runs. Independent measures of total bedload output were made using downstream sediment baskets. Results indicate that the morphological active width increases with total and dimensionless stream power and is strongly and positively correlated with bulk change (total volume of bed material displaced over time) and active braiding intensity (ABI). Although there is considerable scatter due to the inherent variability in braided river morphodynamics, the active width is positively correlated with independent measurements of bedload transport rate. Active width, bulk change, and bedload transport rates were all negligible below a dimensionless stream power threshold value of ~ 0.09, above which all increase with flow strength. Therefore, the active width could be used as a general predictor of bulk change and bedload transport rates, which in turn could be approximated from total and dimensionless stream power or ABI in gravel‐bed braided rivers. Furthermore, results highlight the importance of the active width, rather than the morphological active depth, in predicting volumes of change and bedload transport rates. The results contribute to the larger goals of better understanding braided river morphodynamics, creating large high‐resolution datasets of channel change for model calibration and validation, and developing morphological methods for predicting bedload transport rates in braiding river systems. Copyright © 2018 John Wiley & Sons, Ltd.     

River channel and bar patterns explained and predicted by an empirical and a physics‐based method

Our objective is to understand general causes of different river channel patterns. In this paper we compare an empirical stream power‐based classification and a physics‐based bar pattern predictor. We present a careful selection of data from the literature that contains rivers with discharge and median bed particle size ranging over several orders of magnitude with various channel patterns and bar types, but no obvious eroding or aggrading tendency. Empirically a continuum is found for increasing specific stream power, here calculated with pattern‐independent variables: mean annual flood, valley gradient and channel width predicted with a hydraulic geometry relation. ‘Thresholds’, above which certain patterns emerge, were identified as a function of bed sediment size. Bar theory predicts nature and presence of bars and bar mode, here converted to active braiding index (B_i). The most important variables are actual width–depth ratio and nonlinearity of bed sediment transport. Results agree reasonably well with data. Empirical predictions are somewhat better than bar theory predictions, because the bank strength is indirectly included in the empirical prediction. In combination, empirical and theoretical prediction provide partial explanations for bar and channel patterns. Increasing potential‐specific stream power implies more energy to erode banks and indeed correlates to channels with high width–depth ratio. Bar theory predicts that such rivers develop more bars across the width (higher B_i). At the transition from meandering to braiding, weakly braided rivers and meandering rivers with chutes are found. Rivers with extremely low stream power and width–depth ratios hardly develop bars or dynamic meandering and may be straight or sinuous or, in case of disequilibrium sediment feed, anastomosing. Copyright © 2010 John Wiley & Sons, Ltd.     

Hydraulic visibility: Using satellite altimetry to parameterize a hydraulic model of an ungauged reach of a braided river

What hydraulic information can be gained from remotely sensed observations of a river's surface? In this study, we analyze the relationship between river bed undulations and water surfaces for an ungauged reach of the Xingu River, a first‐order tributary of the Amazon river. This braided reach is crosscut more than 10 times by a ENVISAT (ENVironmental SATellite) track that extends over 100 km. Rating curves based on a modeled discharge series and altimetric measurements are used, including the zero‐flow depth Z ₀ parameter, which describes river's bathymetry. River widths are determined from JERS (Japanese Earth Ressources Satellite) images. Hydrodynamic laws predict that irregularities in the geometry of a river bed produce spatial and temporal variations in the water level, as well as in its slope. Observation of these changes is a goal of the Surface Water and Ocean Topography satellite mission, which has a final objective of determining river discharge. First, the concept of hydraulic visibility is introduced, and the seasonality of water surface slope is highlighted along with different flow regimes and reach behaviors. Then, we propose a new single‐thread effective hydraulic approach for modeling braided rivers flows, based on the observation scales of current satellite altimetry. The effective hydraulic model is able to reproduce water surface elevations derived by satellite altimetry, and it shows that hydrodynamical signatures are more visible in areas where the river bed morphology varies significantly and for reaches with strong downstream control. The results of this study suggest that longitudinal variations of the slope might be an interesting criteria for the analysis of river segmentation into elementary reaches for the Surface Water Ocean Topography mission that will provide continuous measurements of the water surface elevations, the slopes, and the reach widths.     

Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude

This work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north‐eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954–2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic‐vegetation dynamics and interactions were addressed: (i) long‐term (1954–2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986–2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in‐channel vegetation persisting for less than 5–6 years and only 10% of vegetation persisting for more than 18–19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1–2.5 years, although some differences exist between sub‐reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel‐bed rivers. Besides flow regime, other factors seem to play a significant role for in‐channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in‐channel vegetation trends. Copyright © 2014 John Wiley & Sons, Ltd.     

Numerical study on the effects of floodplain vegetation on river planform style

The effects of floodplain vegetation on river planform have been investigated for a medium‐sized river using a 2D morphodynamic model with submodels for flow resistance and plant colonization. The flow resistance was divided into a resistance exerted by the soil and a resistance exerted by the plants. In this way it was possible to reproduce both the decrease in bed shear stress, reducing the sediment transport capacity of the flow within the plants, and the increase in hydraulic resistance, reducing the flow velocities. Colonization by plants was obtained by instantaneously assigning vegetation to the areas that became dry at low water stages. This colonization presents a step forward in the modelling of bank accretion. Bank erosion was related to bed degradation at adjacent wet cells. Bank advance and retreat were reproduced as drying and wetting of the computational cells at the channel margins. The model was applied to a hypothetical case with the same characteristics as the Allier River (France). The river was allowed to develop its own geometry starting from a straight, uniform, channel. Different vegetation densities produced different planforms. With bare floodplains, the river always developed a braided planform, even if the discharge was constant and below bankfull. With the highest vegetation density (grass) the flow concentrated in a single channel and formed incipient meanders. Lower vegetation density (pioneer vegetation) led to a transitional planform, with a low degree of braiding and distinguishable incipient meanders. The results comply with flume experiments and field observations reported in the literature.     

Experimental study on the influence of river flow confluences on the open channel stage–discharge relationship

ABSTRACT

Accurate assessment of stage–discharge relationships in open channel flows is important to the design and management of hydraulic structures and engineering. Flow junctions commonly occur at the confluence of natural rivers or streams. The effect of flow junctions on the stage–discharge relationship at mountain river confluences was found by measuring velocity fields and water levels in experimental models. The results show that the backwater and accumulation–separation at flow junctions affect the flow structures and patterns in the channel; also, flow confluences may induce complex flow characteristics of backwater and flow separation at river junctions, indicating potential submerged flooding disasters within the confluence zone. The impacts of flow junctions on the stage–discharge relationship are investigated for two physical confluence models built from river confluence prototype systems in southwest China. The results show that the presence of tributary river inflows tends to increase the water level of the main river. This is important for flood control, flood-risk evaluation and engineering (e.g. hydropower station construction) in mountain rivers. Finally, a comparative quantitative analysis based on flow motion equations is conducted to evaluate the stage–discharge relationship in both uniform and regular confluence systems. The results indicate that more accurate prediction can be made when taking into account the flow non-uniformity induced by flow separation, backwater and distorted bed in the junction region.     

Quantification of braided river channel change using archival digital image analysis

Historical archives of grey‐scale river channel imagery are extensive. Here, we present and test a methodology to extract detailed quantitative topographic data from such imagery of sand‐bed rivers. Extracting elevation information from rivers is difficult as they are characterized by a low relative relief (<4 m); the area of interest may be spatially extensive (e.g. active channel widths >500 m in large braided rivers); the rate of change of surface elevation is generally low except in the vicinity of individual channel banks where the rate of change is very high; there is the complication that comes from inundation; and there may be an added complication caused by blockage of the field of view by vegetation. Here, we couple archival photogrammetric techniques with image processing methods and test these for quantification of sand‐bed braided river dynamics, illustrated for a 500 m wide, 3 km long reach of the South Saskatchewan River, Canada. Digital photogrammetry was used to quantify dry areas and water edge elevations. A methodology was then used to calibrate the spectral signature of inundated areas by combining established two media digital photogrammetric methods and image matching. This allowed determination of detailed depth maps for inundated areas and, when combined with dry area data, creation of complete digital elevation models. Error propagation methods were used to determine the erosion and deposition depths detectable from sequential digital elevation models. The result was a series of elevation models that demonstrate the potential for acquiring detailed and precise elevation data from any historical aerial imagery of rivers without needing associated calibration data, provided that imagery is of the necessary scale to capture the features of interest. We use these data to highlight several aspects of channel change on the South Saskatchewan River, including bar movement, bank erosion and channel infilling. Copyright © 2010 John Wiley & Sons, Ltd.     

Environmental change,hydromorphological reference conditions and the restoration of Polish Carpathian rivers

In the twentieth century Polish Carpathian rivers were considerably modified by channelization and gravel mining, with significant detrimental effects to their ecological integrity, vertical stability of the streambeds and flood hazard to downstream river reaches. Restoration of the rivers is thus necessary to improve their ecological status and re‐establish geomorphic dynamic equilibrium conditions. Various approaches to defining hydromorphological reference conditions, proposed to date in river restoration literature, have serious deficiencies. In particular, environmental changes that took place in the catchments of Carpathian rivers during the twentieth century invalidate the historical state of the rivers as reference for their restoration. This is illustrated by a change from bar‐braided to island‐braided channel pattern that occurred in the past century in unmanaged sections of the Czarny Dunajec in response to a reduction in flow and sediment dynamics of the river. We indicate that reference conditions should be defined as those which exist or would exist under present environmental conditions in the catchment but without human influence on the channel, riparian zone and floodplain of the river which is to be restored. This assumption was tested through the evaluation of hydromorphological river quality of the Czarny Dunajec according to the European Standard EN‐14614. The evaluation confirmed a high‐status hydromorphological quality in an unmanaged channel section, which can thus be used as a reference for restoration of impacted river sections. Copyright © 2012 John Wiley & Sons, Ltd.     

Physics‐based numerical modelling of large braided rivers dominated by suspended sediment

Physics‐based models have been increasingly developed in recent years and applied to simulate the braiding process and evolution of channel units in braided rivers. However, limited attention is given to lowland braided rivers where the transport of suspended sediment plays a dominant role. In the present study, a numerical model based on the basic physics laws of hydrodynamics and sediment transport is used to simulate the evolution process of a braided river dominated by suspended load transport. The model employs a fractional method to simulate the transport of graded sediments and uses a multiple‐bed‐layer approach to represent the sediment sorting process. An idealized braided river has been produced, with the hydrodynamic, sediment transport and morphological processes being analysed. In particular, the formation process of local pool–bar units in the predicted river has been investigated. A sensitivity analysis has also been undertaken to investigate the effects of grid resolution and an upstream perturbation on the model prediction. A variety of methods are applied to analyse the geometrical and topographical properties of the modelled river. Self‐organizing characteristics related to river geometry and topography are analysed by state‐space plots, which indicate a close relationship with the periodical erosion and deposition cycles of braiding. Cross‐sectional topography and slope frequency display similar geometries to natural rivers. Scaling characteristics are found by correlation analysis of bar parameters. Copyright © 2014 John Wiley & Sons, Ltd.     

Planform dynamics of braided streams

The high dynamism and complexity of braided networks poses a series of open questions, significant for river restoration and management. The present work is aimed at the characterization of the morphology of braided streams, in order to assess whether the system reaches a steady state under constant flow conditions and, in that case, to determine how it can be described and on which parameters it depends. A series of 14 experimental runs were performed in a laboratory physical model with uniform sand, varying the discharge and the longitudinal slope. Planimetric and altimetric configurations were monitored in order to assess the occurrence of a steady state. A set of parameters was considered, such as the braid‐plain width and the number and typology of branches and nodes. Results point out that a relationship exists between braiding morphology and two dimensionless parameters, related to total water discharge and stream power. We found that network complexity increases at higher values of water discharge and a larger portion of branches exhibits morphological activity. Results are then compared to the outputs of a simple one‐dimensional model, that allows to easily predict the average network complexity, once the bed topography is known. Model computations permit also the investigation of the effect of water discharge variations and to compare different width definitions. The at‐a‐station variability of planimetric parameters shows a peculiar behaviour, both regarding number of branches and wetted width. In particular, the analysis of the relationship between width and discharge highlighted relevant differences in comparison to single thread channel. Copyright © 2009 John Wiley & Sons, Ltd.     

Water temperature dynamics in High Arctic river basins

Despite the high sensitivity of polar regions to climate change and the strong influence of temperature upon ecosystem processes, contemporary understanding of water temperature dynamics in Arctic river systems is limited. This research gap was addressed by exploring high‐resolution water column thermal regimes for glacier‐fed and non‐glacial rivers at eight sites across Svalbard during the 2010 melt season. Mean water column temperatures in glacier‐fed rivers (0.3–3.2 °C) were lowest and least variable near the glacier terminus but increased downstream (0.7–2.3 °C km^–1). Non‐glacial rivers, where discharge was sourced primarily from snowmelt runoff, were warmer (mean: 2.9–5.7 °C) and more variable, indicating increased water residence times in shallow alluvial zones and increased potential for atmospheric influence. Mean summer water temperature and the magnitude of daily thermal variation were similar to those of some Alaskan Arctic rivers but low at all sites when compared with alpine glacierized environments at lower latitudes. Thermal regimes were correlated strongly (p < 0.01) with incoming short‐wave radiation, air temperature, and river discharge. Principal drivers of thermal variability were inferred to be (i) water source (i.e. glacier melt, snowmelt, groundwater); (ii) exposure time to the atmosphere; (iii) prevailing meteorological conditions; (iv) river discharge; (v) runoff interaction with permafrost and buried ice; and (vi) basin‐specific geomorphological features (e.g. channel morphology). These results provide insight into the potential changes in high‐latitude river systems in the context of projected warming in polar regions. We hypothesize that warmer and more variable temperature regimes may prevail in the future as the proportion of bulk discharge sourced from glacial meltwater declines and rivers undergo a progressive shift towards snow water and groundwater sources. Importantly, such changes could have implications for aquatic species diversity and abundance and influence rates of ecosystem functioning in high‐latitude river systems. Copyright © 2012 John Wiley & Sons, Ltd.