Can we incorrectly link armouring to damming? A need to promote hypothesis-driven rather than expert-based approaches in fluvial geomorphology

Abstract

Present-day river forms and processes are in many cases conditioned by the consequences of anthropogenic modifications such as dams, embankments and gravel mining. Fluvial geomorphologists have typically investigated the effects of these human impacts using a so-called expert-based approach, whereby observed association or synchronicity between geomorphological changes and a given, preidentified impact, are interpreted as evidence of causation. This approach has important limitations when the effects of multiple human interventions interact along the same river corridor or overlap with the legacy of natural changes affecting the sediment - water balance. In such situations, the establishment of causal links between channel morphology and single impacts is not as straightforward as commonly assumed and the conclusions are susceptible to ‘confirmation biases’. In this paper we highlight this risk through an assessment of human impacts on the Rhône River within a multi-driver context. The French Rhône is an excellent example of an Anthropocene river impacted by two main development phases during the twentieth century: embankments (1890s–1930s) followed by a series of multiple dams (1950s–1990s). We began by laying out several geomorphologically consistent hypotheses for the geomorphological trajectory of the Rhône over the twentieth century. Next, we tested these hypotheses against grain size data collected in the field in a structured and hypothesis-oriented way. Using this hypothesis-driven and deductive attribution analysis we identified the relative impacts of the different development phases on the present-day grain size distribution and in particular on armouring in the Rhône River, and proposed a hierarchy of dominant drivers of geomorphological change along the Rhone over the last century and a half. Our results led us to conclude that in the case of the Rhône, the effect of dams on armouring was negligible compared to a legacy of natural heritages and embankments.