Where the Allier has still mostly a natural morphology, the Grensmaas is a regulated river without a broad riverbed. Logically the Allier functions as a natural reference for the developments that can be expected along the future Grensmaas. Therefore, a comparison between the vegetation of both systems is interesting. In this article we review the pioneervegetation along side both riviers. We have determined the vegetation in 21 plots along the Allier and 24 sides along the river Grensmaas.
High water gulleys, with effects of groundwater.
Where the Allier has a diverse and species-rich vegetation on gravel and sandbanks, we miss certain aspects of a natural gravel river along side the Grensmaas. The relation between the morphodynamic situation and the development of riparian forest along the Border Meuse was analysed on the basis of a literature survey and a field survey of the gravel banks in 1998 and 1999. The correlation between the level of freedom afforded to morphodynamic processes and the development of woody vegetation was very clear. Those riverbanks where erosion was allowed provided the necessary sediment for the settlement of riparian forest on emerging bars and islands. The forest development itself, and especially the characteristic species Salix purpurea and Populus nigra, was found to play an important role in the development of bars in the riverbed.