Braid-scale channel geometry changes in a sand-bedded river: Significance of low stages

This study focuses on the importance of different stages of bed formation and transportation processes along the partly managed lowland section of Maros River (South Hungary). The role of different stages was determined, applying a series of cross-sectional data. Besides, major morphological zones of over-widened, braided sections were defined, along with their role and function in sediment storage and transport processes. During the rising stage of floods a great volume of sediment is eroded from and transported through the cross-sections. On the falling stage accumulation will be the dominant process. However, concerning the total amount of sediment deposited during failing stages, braids store overwhelmingly more, than straight, narrow reaches. During long-lasting, low-stage periods the cross-sectional area increases, which means net erosion and an increase in mean depth. Therefore, braids have a significant role in falling stage sediment storage and low-stage release. Nevertheless, the location of accumulation within braids is different, influenced by their structure. In braids which are characterised by islands, the sediment is deposited at their upstream end. Islands create a transportation zone towards the downstream end of the braid by increasing stream power. Over-widened sections without islands are more likely to experience deposition at their downstream end. Constant narrowing of the studied reach was also observed. Braids on unmanaged reaches proved to be the most prominent places in this sense. Changes in the regime of the river, and altered land-use help to explain this process, as the frequency of floods has decreased, while the length of extreme low stages has increased. Therefore, the need for the sediment storing and releasing function of braids is less necessary, which leads to their narrowing. (c) 2006 Elsevier B.V. All rights reserved.