Endangered braided rivers

(left) The Brenta River (Italy) has experienced a severe reduction of its braidplain width over just a few decades. Vegetation has spread across the plain and the river configuration has shifted from braided (characterised by a rapidly evolving multple-channel pattern in a bare gravel bed) to transitional (with much more a stable, almost single-thread pattern and a few large vegetated islands).

Pictures originate from: N. Surian 2006. Effects of human impact on braided river morphology: examples from Northern Italy. In: Braided Rivers: Process, Deposits, Ecology and Management, Sambrook-Smith G. H., Best J. L., Bristow C.S., Petts G. E. (eds). Special publication 36 of the International Association of Sedimentologists: Blackwell, Oxford. UK. doi: 10.1002/9781444304374.ch16

(right) Alterations induced by damming explained using a flow chart. Alteration of the two major controls on the river morphology (flow regime and sediment transport) by the dam impacts a hierarchy of processes, finally determining a change in the 2D morphological configuration.

The Waitaki River (New Zealand) has experienced narrowing of the active braidplain and reduction of its morphological complexity following the spread of non-native vegetation (willows) initially introduced by settlers. Willow spread was favoured by the reduction in frequency of floods due to a modified flow regime following the construction of numerous hydropower dams. Nowadays, an artificial vegetation removal program is enforced a central fairway, which helps preserving some braiding there.

Pictures originate from: D. M. Hicks 2006. North bank tunnel concept–water consents: River geomorphology, sediment transport, coastal processes and flood hazard management. Prepared for Meridian Energy Ltd. NIWA client report CHC2006-090.