Climate and human forcing of Alpine fluvial processes in the Anthropocene

English Abstract:

River flow in Alpine environments is likely to be highly sensitive to both: (a) climate change; and (b) human impacts (e.g. flow abstraction). Here we compare river flow and sediment flux of two Alpine drainage basins over the last 7 decades, one basin largely unimpacted by human activities (the Avançon), one strongly impacted by flow abstraction (the Borgne d’Arolla), so as to tease how climate forcing acts in isolation but also combines to impact upon fluvial processes.

First, reconstruction of the Avançon flow and sediment transport capacity shows that variability in temperature, through its effect upon snow accumulation and so the intra-annual distribution of runoff, significantly impacts upon sediment transport capacity. We observe a linkage between this increased river flow, river incision and terrace formation during runs of years that are snowier than average; and sediment accumulation and increases in the active width when warmer. Although changes in basin sediment delivery could also be responsible for these processes, we cannot exclude climate forcing of river flow as a plausible hypothesis for observed river morphodynamics.

Second, for the Borgne d’Arolla, abstraction of flow at intakes has very major impacts upon sediment deposition in the 20 km of studied river downstream. We show a slow-moving sediment wave linked to substantial reduction in the duration of possible sediment transport. But, this human forcing was hard to distinguish from climate forcing over the same period, and an observed increase in sediment transport capacity.

Thus, whilst both climate and human effects can have major impacts upon river flow and hence upon sediment flux and river channel morphodynamics, separating these effects is exceptionally difficult. Management systems have evolved continually both in response to climate forcing but also human forcing itself, and their effects on river dynamics. We need to understand climate and human forcing of Alpine rivers as co-evolving.