Groundwater, biogeomorphic succession and controls on river channel pattern

Abstract:

Strong feedbacks between river braiding and vegetation processes are now well-recognised. Recently, this has been illustrated in the notion of biogeomorphic succession, the transition from pioneer vegetation establishment to a fully-developed floodplain forest ecosystem. This succession also results in important vegetation-mediated feedbacks, through bank stabilisation and the capture of organic matter and fine sediments, stimulating soil formation and further enhancing the succession process itself. However, there are few studies that have addressed what this succession might mean for the evolution of channel planform, and almost no studies that have considered how this succession rates might be mediated by groundwater. The latter is a key concern for gravel-bed rivers with low water retention capacity. Here, we present results from a 2 km length of braiding-wandering river system in Switzerland (Allondon River). We show that the spatio-temporal dynamics of the groundwater table drives the biogeomorphic succession process at different rates, leading to very different river channel pattern responses. In the upper braiding-anastomosing part of the reach, the groundwater table is deeper. Here, dendrochronological data show that rates of pioneer vegetation growth are strongly dependent upon groundwater table fluctuations. Bank resistance modelling shows that vegetation-reinforcement of bank resistance is below its maximum. In the meandering lower part of the reach, with a mature floodplain forest, tree growth rates are independent of groundwater fluctuations, because trees can almost always access the higher groundwater table. Bank resistance is at its maximum. Through time, in response to disturbance frequency, the meandering tendency has migrated upstream. Thus, our results suggest that groundwater access modulates biogeomorphic succession processes in ways that determine the resultant river channel pattern.