The Allondon River: Decadal Planform Changes Under Changing Boundary Conditions

Abstract

The Allondon River (Canton of Geneva) is a historically well-documented piedmont river system that reflects interactions between an evolving climate, a changing catchment and local management practices and the suite of internal biogeomorphological responses and feedbacks that result. Over the last century, the river has been evolving from a braided to a wandering system. This appears to be driven by two key factors: (i) changes in hydrological forcing (notably in flood seasonality and flood power) coupled to a decrease in sediment availability, which has increased river incision; (ii) the angle of the shallow groundwater table, which creates drier conditions in the upper reach and wetter conditions in the lower reach. The coupling of both effects influences vegetation encroachment rates and the associated channel stabilisation. Hence the change in the rate of disturbance versus the rate of stabilisation along the river reach causes the morphological evolution to be spatially heterogeneous and variably resilient to extreme events, validating observations of the effects of vegetation made in flume studies.