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Evolution of the Active Channel and the Vegetation Islands in a Sandy River

Author(s): Lourdes Garcia; Diego Garcia De Jalon

Linked Author(s): Diego García de Jalón

Keywords: No Keywords

Abstract: River Tietar is a braided river with multiple channels that intertwine, forming rods of material inside the channel and which act as natural obstacles to flowing water. In these bars, as well as in other points of the channel, fluvial islands of vegetation can be created. The sediments of this river are composed almost entirely by sand, and thus river flows allow a greater dynamic character due to the easy drag of this type of sediment during floods. In this study, we have analyzed through digitalization of aerial photographs the evolution over time of the river channel, the vegetation islands and the riparian vegetation that surrounds it, in order to describe how has been its evolution over time. Also, the composition and structure of vegetated islands was measured in field work. On the other hand, a hydrological analysis was carried out to analyze the causes of observed changes and to forecast the evolution of this ecosystem in the future. Results show that the River Tietar has been increasingly colonized by woody riparian vegetation (mainly Salix salvifolia and Fraxinus angustifolia) both within its lateral channel bars and in the island bars since1956, when most of the channel was bare sand. According to hydrological analyzes, this increase in vegetation is facilitated by a reduction in average flows and a lower frequency and intensity of large floods/floods, together with an increase of summer flows from nearby irrigation farming fields. Vegetated Islands show a dynamic pattern of movement inside the channel similar to those made by ‘amoebas’. Islands showed an ecological succession from only pioneer S. salvifolia in the early stages to a more diverse mature stage including also F. angustifolia, Sambucus nigra and Celtis australis. With these data, we have been able to conclude that future trends show almost total riparian colonization of the active channel by vegetation, thus creating a new ecosystem of continuous vegetation bands without islands. We predict that the vegetation encroachment within the channel will reduce the mobility or river the dynamism since the vegetation root system would not allow the channel to change position after the floods. Therefore, the present braided river would be transformed into a single channel meandering one before 18 years.


Year: 2018

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