Channel adjustments and vegetation cover dynamics in a large gravel bed river over the last 200 years

The timing and extent of the morphological changes that occurred in the last 200 years in a large gravel bed river (the Piave River, eastern Italian Alps) that was heavily impacted by human activities (training structures, hydropower schemes, and gravel mining) have been analyzed by historical maps, aerial photos, repeated topographic measurements, and geomorphological surveys. Results show that the channel underwent a strong narrowing during the twentieth century, but with a faster pace during the 1970s–1990s and with an associated shift from a dominant braided pattern to a wandering morphology. Bed incision up to 2 m — mostly from gravel mining — has been documented for this period. Large areas of the former active channel were colonized by riparian forests, both as islands and as marginal woodlands. The ceasing of gravel extraction in the late 1990s seems to have determined a reversal in the evolutionary trend, with evidence of vegetation erosion/channel widening even though a significant aggradation phase is not present. We conclude that alteration of sediment regime has played a major role on the long-term channel evolution. However, only relevant flood events (RI > 10–15 years) appear to determine substantial island erosion, and therefore the proportion of island vs. channel area fluctuates depending on flood history.