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[1] In this work we have investigated the equilibrium configurations of a Y-shaped fluvial bifurcation through a laboratory analysis. Three series of experimental runs have been performed in a wide flume, where a symmetrical bifurcation has been constructed joining three branches with fixed banks and movable bed made of a well sorted quartz sand; the angle between the two downstream distributaries was equal to 30 degrees. The experiments have been carried out with different values of longitudinal bed slope and water discharge, in order to investigate a range of the relevant morphodynamic parameters typical of gravel bed braided rivers. The equilibrium configuration of the bifurcation has been characterized through the measure of the discharge partition in downstream branches and of the local bed structure at the node. The existence of unbalanced equilibrium configurations has been observed and the role of migrating alternate bars has been pointed out. The experimental results confirm the theoretical predictions which have been recently obtained through the simple model of Bolla Pittaluga et al. (2003). Moreover, interpreting the measured data in the light of the concept of morphodynamic influence provides a new perspective in the analysis of the equilibrium configurations of a bifurcation.

The role of riparian vegetation in shaping river morphology is widely recognized. The interaction between vegetation growth and riverbed evolution is characterized by complex nonlinear feedbacks, which hinder direct estimates of the role of key elements on the morphological evolutionary trajectories of gravel bed rivers. Adopting a simple theoretical framework, we develop a numerical model which couples hydromorphodynamics with biomass dynamics. We perform a sensitivity analysis considering several parameters as flood intensity, type of vegetation, and groundwater level. We find that the inclusion of vegetation determines a threshold behavior, identifying two possible equilibrium configurations: unvegetated versus vegetated bars. Stable vegetation patterns can establish only under specific conditions, which depend on the different environmental and species-related characteristics. From a management point of view, model results show that relatively small changes in water availability or species composition may determine a sudden shift between dynamic unvegetated conditions to more stable, vegetated rivers.

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. Figure 4. Quantification of the network steady state configuration: A) total braiding index TBI as a function of the dimensionless water discharge; B) total braiding index TBI as a function of the dimensionless stream power index; C) active braiding index ABI as a function of the dimensionless water discharge; D) active braiding index ABI as a function of the dimensionless stream power index.Figure 7. Comparison between the measured values of the total and active braiding indexes and values computed by the 1D numerical model, as a function of the dimensionless discharge and stream power.Figure 8. At-a-station variability of TBI and ABI, as computed by the numerical 1D model. Black dots represent the formative conditions.

1] This paper combines archived remotely sensed data (airborne lidar and digital color air photographs) with nonsynchronous ground observations (including observations of topographic form and vegetation cover and growth) to test the hypothesis that colonization of exposed river sediments by riparian trees has an impact on channel form and to quantify any impact that is identified. This is achieved along a 21 km reach of the braided, gravel bed Tagliamento River, in northeast Italy, where the width of the braided corridor typically exceeds 800 m. Lidar data are analyzed to extract a 2 m resolution digital evolution model (DEM) and determine riparian vegetation extent, height, and structure within the active corridor. Aerial photographs are used to map the topography of the submerged parts of the corridor. These data are divided into 1 km length subreaches, which possess strong contrasts in vegetation height and extent. Joint analysis of vegetation and morphological properties of these subreaches reveals significant associations between vegetation properties and reach morphology. Residuals from a gamma function fitted to the topographic data for each subreach show a good fit with poorly vegetated reaches, but a weakening fit with increasing vegetation cover, largely as a result of the appearance of secondary peaks in the elevation frequency distribution associated with the heavily vegetated areas. Furthermore, the overall skewness and kurtosis of the elevation frequency distribution within each of the subreaches are both significantly correlated with vegetation extent, height, median elevation, and growth rate, indicating a clear topographic signature of vegetation development along this braided river that reflects sediment accumulation within and around the vegetated patches.Citation: Bertoldi, W., A. M. Gurnell, and N. A. Drake (2011), The topographic signature of vegetation development along a braided river: Results of a combined analysis of airborne lidar, color air photographs, and ground measurements, Water Resour. Res., 47, W06525,

The Tagliamento River, Northeast Italy, represents an important Alpine to Mediterranean braided system, where interactions between river flows, sediment dynamics and vegetated landforms can be investigated within a relatively unconfined setting.We analysed data from contemporary and historical sources, including stage records, photographs and topographic surveys. From these we identified river stages at which thresholds in surface hydrological connectivity and biogeomorphological adjustment appeared to occur, contributing to a shifting habitat mosaic.Significant adjustments in landscape elements within the active tract commence at river stages well below bankfull with return periods of a few months. Flow pulse events with return periods from a few months to 2 years support a dynamic inundation pattern, ranging from a patchwork of isolated water bodies within a predominantly terrestrial landscape at low river stages to isolated vegetated islands within a fully connected aquatic landscape as the river approaches bankfull. Across this range, interactions between flow, sediment and vegetation lead to gradual and abrupt transitions in persistence, form and connectedness of different landscape elements. Bankfull flows (return period over 2.5 years) topple and disperse significant numbers of large trees, seeding the next generation of vegetated patches, and larger floods (return period around 10 years) induce significant turnover of established islands and floodplain surfaces.The results reported in this paper illustrate how extensive interdisciplinary research on a single river system can provide useful insights concerning the time scales and thresholds that characterize water-sediment-vegetation interactions in piedmont reaches of Alpine to Mediterranean braided systems. Anthropogenic effects on river systems are ubiquitous throughout Europe. However, systems such as the Tagliamento River that retain significant process dynamism and morphological integrity, provide a laboratory within which reference processes and process-form interactions can be investigated, understood and then incorporated into innovative restoration design on more impacted systems.

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Previous analyses have identified the active width of braided rivers, the bed area over which bed load flux and shortterm 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.