Medium- and short-term channel and island evolution in a disturbed gravel bed river (Brenta River, Italy)

The timing and extent of the morphological and island changes that have occurred in the last thirty years in a gravel bed river that has been heavily impacted by human activities were analysed by nine sets of aerial photographs, repeated topographical measurements and morphological- vegetation surveys. Dam operations and gravel mining activities have produced modifications in the natural sediment regime that have generated important morphological responses in the channel. Large areas of the formerly active channel were colonised by riparian forest, both as islands and as marginal woodlands. The cessation of gravel extraction in the late 1990s seems to be causing incipient reversion of this pattern, with evidence of vegetation erosion/channel widening. Alteration of sediment regime has played a major role in the medium- and short-term channel evolution. However, only relevant flood events (recurrence interval >10 years) appear to determine substantial island erosion and, therefore, the proportion of islands versus channel fluctuates depending on flood history. Smaller scale analysis (sub-reach level) was more effective in describing morphological responses and relationships with the sediment dynamics within the 20 km study reach.