Medium and short term riparian vegetation, island and channel evolution in response to human pressure in a regulated gravel bed river (Piave River, Italy)

- The evolutionary trends of type and structure of riparian vegetation were assessed.
- The effects of human pressure produced more stable riparian vegetation.
- Riparian vegetation gets older, protecting the banks.
- The cessation of gravel mining is not sufficient to revert a degradation trend.
- Regulated rivers need high flood intensity to promote restoration trajectories.

During the last decades, several Italian gravel-bed rivers suffered from different types and levels of human pressures that changed their morphological and vegetation patterns. This study aims to analyze the medium- and short-term evolution of vegetation cover, fluvial islands and main channel characteristics along a gravel-bed reach of the Piave River (North-eastern Italy), with special emphasis on the changes in the vegetation cover type and structure. The Piave River experienced different types of human disturbance such as gravel mining, hydropower schemes, and land use changes. A sequence of aerial photographs (1960, 1970, 1982, 1991, 1999, 2006, 2010 and 2012) have been analyzed to detect the medium-term (about 50 yr) evolution of riparian vegetation along a reach about 30 km-long in the middle course of the Piave River. In addition, LiDAR data (2003) and seven repeated topographic surveys (done between 2007 and 2011) have been used to quantify changes in three cross sections on a sub-reach about 2 km-long, in order to analyze the channel evolution over a short period (1-8 yr). The medium-term analysis revealed that changes in the river evolutionary trend depended on the variations in human activities both in the main channel and at basin scale. In fact, during the last five decades there has been a consistent and continuous increase of riparian vegetation within the river corridor, from around 50% up to 68%, which corresponds to a continuous decrease from around 46% to 29% in the area of exposed gravel and low flow channels. Considering the different vegetation cover types, there was a predominant increase of stable and tall vegetation from around 34% up to a maximum of around 67% (1999), then a slight decrease to around 62% (2010 and 2012). After a slight recovery phase subsequent to the cessation of gravel mining in the late 1990s and associated to flood events in the early 2000s, the Piave River appears to have been in an equilibrium phase in terms of bed elevation, planform morphology and areas of vegetation, which is becoming taller and more mature, as a result of the combination of flow regulation, reduced bedload input from upstream and lack of relevant flood events. These results suggest that in highly regulated - in terms of flow and sediment fluxes - rivers the cessation of gravel mining alone is not sufficient to revert a degradation trend and thus restore prior morphological patterns.

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