Analysis of the large scale dynamics of the Tagliamento River (Italy)

In the last century human activities have strongly affected the natural river behaviour often constraining complex patterns into more manageable single-thread channels with the aim to enhance conveyance of water flow and floods. In much of the developed world, the remarkable degree of spatio-temporal heterogeneity characterising riverine landscapes has been masked by a long history of river engineering. Floodplain reaches, which exhibit the highest heterogeneity in their natural state, have been the most severely altered. The present work is focused on braided rivers whose multiple-thread pattern is related to a highly complex eco-morphodynamism. The Tagliamento River (Italy), even though it has undergone morphological changes due to human activities, still displays semi-natural morphodynamic behaviour that would have characterised the pristine state of the lower valley sections of several alpine rivers. It, therefore, offers the rare opportunity to investigate the intrinsic dynamics of braided streams. This study is motivated by the need to recognize and maintain this eco-morphodynamism which prevents habitat degradation and is therefore crucial to the maintenance of ecological integrity. An analysis of island and active corridor dynamics is presented for a 16 km island-braided reach of the gravel-bed Tagliamento River based upon information extracted from three map and 9 aerial photograph sources, encompassing the period from 1803-present. The active corridor width showed a general decline over the study period but with some recent widening. Changes in island extent were achieved by rapid island turnover, which reached a maximum rate of over 50% per annum. Very few island surfaces were found to persist for more than 24 years. Despite this enormous dynamism and apparent cyclic behaviour, between 1944/6 and 2005 the ratio of island area to active corridor area remained relatively constant at around 0.08 and supported a consistently high bankfull shoreline to downstream length ratio of around 6 km·km-1.