Many gravel bed rivers in the European Alpine area suffered different ranges and types of human pressure that modified their morphology and altered their processes. This work presents the case of the middle portion of the Brenta River, historically impacted by human activities such as floodplain occupations, bank protection, gravel mining, hydropower schemes and water diversion. Dam operation and gravel mining have produced considerable modifications in the natural sediment regime generating important morphological channel responses (narrowing and incision). Large areas of the former active channel have been colonized by riparian vegetation, both as islands and as marginal woodlands. Overall, the river changed its morphological pattern from braided to wandering. The present study analyses the timing and extent of the planform morphological changes that occurred over the last 30 years along the middle portion of the river (20 km long) through the examination of aerial photos, repeated topographic measurements and hydrological data. A series of recent aerial photos (1981, 1990, 1994, 1999, 2003, 2006, 2008, 2010 and 2011) have been used to assess the medium and short‐term morphological changes of the floodplains and the active channel area. As to the medium‐term modification, the recent changes in in‐channel gravel mining have determined a new trend of active channel widening through erosion of vegetated areas. The analysis has also allowed to assess the morphological effect of single flood events. Only floods with recurrence interval higher than 8–10 years appear to be able to determine substantial erosion of floodplain and island margins. Copyright © 2013 John Wiley & Sons, Ltd.

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Until very recently, rivers have been considered as the result of the interaction between water and sediments, thus simplifying this very complex system. In doing so, one important component was missed. This is vegetation, namely trees growing on banks, floodplains, and bars/islands. The pattern of riparian vegetation in gravel bed rivers depends on the climate, hydrological regime, floods, sediment transport and the morphological settings of the river. Also, the marked spatial variability of density, height, species diversity, age, and rates of growth reflects the very complex nature of bed colonization, the strong influence of sequences and magnitude of floods, and the feedbacks between morphology, vegetation and hydraulics. Furthermore, a wide array of human impacts acting at either the basin or river network scales can influence substantially the morphodynamics and thus the characteristics, types and distribution of vegetation within the river corridor. The aim of the work is to analyze the relationship between the vegetation structure and the morphological characteristics of two sub-reaches of the Piave river which suffered both floods and intense and multiple human impacts especially due to dam building and inchannel gravel mining. Six cross sections were surveyed and vegetation, soil variables and sediment deposited were measured on 214 plots, (4 x 4 m each one and 10 m spaced). Each vegetation plot were recognized on a recent aerial photo (2009) and its ages were calculated performing both a multi-temporal analysis of older photos (1960, 70, 80, 91, 99, 2006 and 2009) analysis. The Piave River shows a complex pattern of vegetation distribution along the cross-sections with no clear relationship between the elevation and the plant communities. However later successional plant communities cover older surfaces and the thickness of the sand layer helps explaining differences in areas dominated by different species (e.g. Salix alba and Salix eleagnos).

River islands could be defined as discrete areas of woodland vegetation surrounded by either water-filled channels or exposed gravel which exhibits some stability, and remain exposed during bunk-full flows. Islands are important from morphological, functional, and ecological points of view. The aim of the present study is to analyze the dynamics of stable, young and pioneer islands in the Piave River, which suffered intense and multiple human impacts especially due to dam building and in-channel gravel mining. Plan-form changes of river features from 1960 were analyzed using aerial photos, and a LiDAR was used to derive the maximum, minimum and mean elevation of island surfaces and the maximum and mean height of the island vegetation. The results suggest that stable islands lie at higher elevation than young and pioneer islands, and are characterized by a thicker layer of fine sediments deposited on their surfaces after big floods. There was a progressive decrease in the active corridor area from 1960 to 2006. After the 1966 exceptional flood (RI> 200 years) there was a moderate increase of island extend and numbers, followed by a further increase from 1991, due to a succession of flood events, in 1993 and 2002 with RI> 10 years, as well as a change in the anthropic management characterized by a control relating to mining activities. The narrowing trend (1960-1999) of the morphological plan-form, certainly enhanced the chance of the island to become stable and this explains the reduction of active channel, the increase of stable island and the reduction of pioneer island. Even though a general channe

The extent and amount of erosion and deposition processes experienced by the Brenta river after the flood events of November and December 2010, have been assessed on the basis of DTMs derived from LiDAR surveys made before (2010) and after (2011) these floods. The study, carried out on three sub-reaches of this regulated gravel-bed river took advantage, beside LiDAR elevation data, two relatively novel approaches for channel bathymetry and uncertainty calculation. The first one allowed us to obtain the elevations within the wetted channels while the second method allowed the quantification of uncertainty related to the various phases of the topographic representation process. The erosion-deposition volumetric results derived from the two years DoD differencing models highlighted the impact of the two floods. In all the sub-reaches, erosion processes have dominate and featured a decreasing downstream trend, being the lower reach in a nearly equilibrium condition. The quantitative results of morphological changes have emphasized the evidence that relevant flood events (RI > 10 years) in high regulated gravel bed-rivers, encourage erosion rather than deposition patterns.