Abstract
Over the last two centuries, the vast majority of rivers in the European Alps have undergone significant hydromorphological modifications due to land reclamation, flood mitigation, hydropower production and gravel mining. However, only few investigations have quantified such changes addressing also their impact on riparian vegetation growth.
The Ahr river (Eastern Italian Alps, drainage area 630 km2, 25 km2 covered by glaciers) featured an anabranching pattern with the presence of large riparian areas during the 19th century, until the first half of the 20th century. Since the 1960s, the channel underwent intense variations as a result of gravel mining and sediment retention due to a hydropower dam and several check-dams along its tributaries. Bed incision followed by bank stabilization works led to a hydrological and morphological disconnection of the floodplain from the channel. Moreover, cross-section narrowing and deepening of the riverbed brought about bed armouring and reduction of morphological diversity, until several reaches of the river were restored by widening and raising the bed in the period 2003-2011.
Planimetric changes occurred in the Ahr were determined by the interpretation of 10 maps and aerial photos covering the period 1820-2011. Cross-sections derived from topographic surveys and from Lidar-DTM led to estimate the elevation of the different surfaces presents in the river corridor and thus to estimate the extent of vertical changes during the degradation phase. The effect of morphological degradation and restoration on the growth of the adjacent riparian forest is being monitored since spring 2011 through dendrochronological analysis of 3 tree species. Preliminary results indicate a different response to bed incision by the different species in terms of radial growth, whereas no effects are evident after the restoration works. However, these have increased the morphological diversity in terms of morphological units such bars and islands.