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.

An analysis of island and active corridor dynamics is presented for a 16 km island-braided reach of the gravel-bed Tagliamento River (Italy) based upon information extracted, geocorrected and registered to a common base from three map The active corridor width showed a general decline over the study period but with some recent widening. Adjustments in active corridor width were achieved through processes of floodplain avulsion, island attachment and progressive encroachment of the edge of the active corridor across gravel areas. These adjustments were accompanied by the preferential creation of dissection (floodplain avulsion) islands during periods of widening and the construction of mid islands within the corridor during periods of narrowing. Changes in island extent were achieved by rapid island turnover, which reached a maximum rate of over 50% per annum when corridor narrowing was most rapid between 1970 and 1991. Very few island surfaces were found to persist for more than 24 years.Despite this enormous dynamism and apparent cyclic behaviour, between 1944/6 and 2005 the ratio of island area to active corridor area remained relatively constant at around 0.08 and supported a consistently high bankfull shoreline to downstream length ratio of around 6 km Á km À1 . These intrinsic properties of the dynamics of the study reach and other island-braided channels need to be recognized and maintained by river managers because they represent a characteristic habitat dynamism that is crucial to the maintenance of ecological integrity.These areas are calculated as the area of changed cover between surveys divided by the number of years between surveys and then expressed as a percentage of the area that persists as island between the two survey dates.

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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Rivers are natural systems whose planform pattern in alluvial reaches reflects a balance between three fundamental ingredients: flow energy, sediment calibre and supply, and vegetation. Whilst early research on river channel classification emphasised flow (stream power) and sediment controls, the impact of vegetation is now recognised in increasingly detailed classification schemes. Different planform patterns are more or less sensitive to changes in these three fundamental ingredients, which in the absence of human interventions all respond to changes in climate, allowing different morphological configurations to evolve and in some cases shift from one planform style to another. Multi-thread, braided and transitional river channel styles are common in European regions where conditions for the development of these planform styles, notably high bed material supply and steep channel gradients, exist. However, widespread, intense human impacts on European river systems, particularly over recent centuries, have caused major changes in river styles. Human activities impact on all three major controls on channel pattern: flow regime, sediment regime, vegetation (both riparian and catchment-wide). Whilst the mix of human activities may vary greatly between catchments, research from across Europe on the historical evolution of river systems has identified consistent trends in channel pattern change, particularly within rivers draining the Alps. These trends involve periods of narrowing and widening, and also switching between multi-thread and single-thread styles. Although flow regulation is often the key focus of explanations for human-induced channel change, our review suggests that human manipulation of sediment supply is a major, possibly the dominant, causal factor. We also suggest that “engineering” by riparian trees can accelerate transitions in pattern induced by flow and sediment change and can also shift transition thresholds, offering a new perspective for interpretation of channel change in addition to the focus on flow and sediment regime within existing models. Whilst the development of planform classifications of increasing complexity have been crucial in developing terminology and highlighting the main factors that control channel styles, additional approaches are needed to understand, predict and manage European Alpine river systems. A combination of field, laboratory and numerical modeling approaches are needed to advance the process understanding that is necessary to anticipate river landscape, particularly planform, changes and thus to make ecologically soundmanagement choices