Author(s): D. Garcia De Jalon; E. Lora; C. Lestienne; L. Garcia; Bejarano Md.

Linked Author(s): Diego García de Jalón

Keywords: Sandy River; Islands; Vegetation; Braided channel; Willows

Abstract: Braided segments in European rivers are increasingly scarce because of human activities (dams, reservoirs, eutrophication, pipelines, gravel mining) resulting into a decrease in the circulating flow of water and sediments. Hydro-bio-geomorphic processes that lead to the formation of islands in rivers, including those involving vegetation and woody debris, are well known in gravel braided rivers where a conceptual model of island development is already available. However, the formation and functioning of islands in sandy braided rivers is less known. The Tietar River presents a sandy braided morphology with many bars and islands along its channel, covered by woody vegetation in its middle-low reaches. The study area consists of two 2 and 5 km length river segments that were sampled in 2017 and 2022. A total of 36 islands were monitored by measuring their woody species composition and diameter (DBH) and analyzing a selection of ring growths. Each island was analyzed divided into 4 areas: an upstream area facing water current, two middle areas at both sides, and the final area further downstream. In order to assess the dynamic evolution of different islands and channel trajectory, a sequential comparison of aerial photographs (1956 to 2018) was done. Also, changes of channel topography and island heights were measured based on two terrain digital models (2010 and 2018). Additionally, we characterized the flow regime of the last decades. Our results show that islands are formed following spring floods along incised areas which remain moist during summer, and where willow seeds accumulate, germinate and get established. Willow seedlings increase the roughness of these areas favoring the accumulation of sand. If this accretion process is maintained in successive years, an island may arise. Colonization by other species may occur on the top part of the new island. These results show a clear pattern of island formation, starting with the establishment of a single pioneer species (i. e., Salix salvifolia) at once, and if the island resists the floods for more than six years, continuing with the colonization of other native and exotic species (i. e., Fraxinus angustifolia, Acer negundo) as part of an ecological succession. Size increases (at landscape scale) of the island correspond to the growth of colonizing woody plants, while size reductions to individuals dead or which have been uprooted and carried away by the current. Older islands generally have greater heights and supported greater biodiversity, while younger islands were only covered by willows. Overall, we found that woody vegetation coverage of islands is continuously increasing in both studied segments, since almost absence in 1956 until reaching 60 and 80% of the active channel of analyzed river segments, respectively, in 2018.

DOI: https://doi.org/10.3850/978-90-833476-1-5_iahr40wc-p0896-cd

Year: 2023