Heavily impacted rivers

Morphological changes in (formerly) braided rivers have been increasingly reported in the recent geomorphological literature. Most contributions regard rivers in Europe, and, within Europe, the Alpine and pre-Alpine region (e.g., Italy, France, Austria, Switzerland), which reflects the overall magnitude of changes observed in that region, where most of braided reaches have been heavily impacted. Furthermore, other contributions from other regions of Europe (e.g., the Polish Carpathians, Spain, Scotland, Corsica) and of the Earth (e.g., the South Island of New Zealand) document similar stories.

Two examples of heavily modified braided rivers are shown in this page.

(left). The Brenta River (Italy) has significant reduced its floodplain width over just a few decades, as aerial images show. Furthermore, it has experience significant spread of vegetation across its floodplain, which has determined a dramatic change in its planform shape. While in the 1950s the river planform was braided, showing multiple wetted channels forming a network subject to fast morphological evolution, in the 1990s the river has become transitional, i.e., almost single-thread, with a well defined meandering channel and stable vegetated islands.

Source of images:

N. Surian. (2006) Effects of human impact on braided river morphology: examples from northern Italy. In: Braided rivers: Processes, deposits, ecology and management. G. H. Sambrook Smith, J. L. Best, C. S. Bristow, G. E. Petts (eds.), Special Publication 36 of the IAS, Blackwell, UK.

(right). Morphological changes induced by dams are explained through a conceptual model. Dams represent and alteration of two fundamental controls on the river shape: the flow regime, and sediment transport. Flow regime changes also encourage colonisation by vegetation. The combined action of these changes in controls finally determines the observed 1D and 2D morphological changes

The Waitaki River (South Island, New Zealand) is the major source of hydropower in New Zealand, and is impacted by at least 7 dams. Dams reduced the occurrence of floods able to mobilise the bed and completely block sediment feed from the higher to the lower Waitaki (the lower course river still recovers gravel from tributaries and from its own bed).

In addition to these modifications, the lower Waitaki experienced spread of invasive vegetation (crack willows, introduced by settlers, which are non-native of New Zealand) which is way more capable than the native vegetation of colonising the floodplain. At present, a vegetation control program is enforced in a central 500 meter wide fairway.

Aerial images of a reach of the Waitaki (above) show that, as a result of these changes in controls, while in 1936 the river bed was bare and wind swept and showed a braided multiple-channel pattern, by 2001 it has become significantly less braided, showing two main channels separated by vegetated islands.

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