5.2. Isère River landscape: dominance of biological forcing in-between dikes in a highly anthropized context
The intricate interplay between hydrogeomorphological processes and riparian vegetation can play a pivotal role in shaping the fluvial landscapes within highly-confined rivers. Corenblit et al. (2020a) studied the reciprocal coupling between river morphodynamics and vegetation succession in the Isère River, France, where human intervention in the form of lateral confinement with embankments and straightening had a transformative impact on riverine landscape dynamics (see also Vautier, 2000; Vautier et al., 2002).
Using GIS analysis of historical aerial photographs spanning nearly five decades, from 1948 to 1996, the authors showed that a remarkable biogeomorphological transformation unfolded as the once mobile transverse gravel bars began a gradual but significant colonization by a diverse array of vegetation types. This situation presented an opportunity to understand the temporal adjustments between fluvial landforms and vegetation succession stages, ultimately culminating in the establishment of mature riparian forests on stabilized alternate bars. This biogeomorphological trajectory of change was a direct consequence of feedbacks between hydraulics, sediment transport, and vegetation dynamics in a highly impacted river reach. The mosaic of vegetated bars that emerged was an outcome of the strong bioconstruction and biostabilization effects exerted by pioneer vegetation on bars of differing ages, sizes, and mobility within the riverine landscape.
Field measurements undertaken in 1996 provided tangible evidence of a robust positive feedback loop between sediment dynamics and the ongoing riparian vegetation succession (Vautier, 2000). In the complex balance between physical and biotic adjustments, the vegetated bars evolved as emergent biogeomorphological entities specifically resulting from the biogeomorphological feedbacks of sediment accretion, topographic raise, and vegetation growth and succession. Co-inertia analysis (CoIA), a type of ordination statistical technique used for exploring and understanding bi-directional relationships between two or more data tables or matrices, permitted the quantification of the intricate relationships. A highly significant statistical association between geomorphological and vegetation variables (as exemplified by an RV value of 0.41, p < 0.001) was found. This association explained a staggering 95% of the variability along just one axis, demonstrating the existence of an exceedingly robust feedback loop between geomorphological changes and vegetation succession.
To appreciate the full significance of these findings, we must turn our gaze to the broader context of the theoretical model of fluvial biogeomorphological succession based on positive feedback loops between fluvial morphodynamics and vegetation succession (FBS model; Corenblit et al., 2007). The empirical observations from the Isère and Tech Rivers both allowed the validation of the FBS model, suggesting that this model is applicable to natural but also channelized rivers.