Legacy sediments in a European context: The example of infrastructure-induced sediments on the Rhône River

Abstract

The concept of legacy sediments (LS) as sediments periodically produced by anthropogenic disturbances has been introduced in the early 2010s and has been increasingly used since then, primarily in the USA. It is a key concept to characterize river corridors health and design relevant restoration schemes. Surprisingly, examples of legacy sediments in European rivers remain scarce in the literature, despite the long history of human influences in this part of the world. In this viewpoint paper, we argue that while land-use changes within upstream basins seem to be mostly responsible for legacy sedimentation along downstream river sections in New World countries like the USA or Australia, reach-scale engineering is the main driver of legacy sedimentation along large Alpine European rivers during the Anthropocene era. We give an example of LS induced by navigation infrastructures (groynes and lateral training walls) present along the Rhône River downstream of Lyon (France), and assume that analogous LS might exist in most engineered rivers. Those infrastructure-induced sediments differ from the classic examples of American LS in terms of formation mechanisms and characteristics, which leads us to suggest a broadened definition of legacy sediments in order to make the concept more applicable to the European context.

Depositional environments and historical contamination as a framework to reconstruct fluvial sedimentary evolution

“…section 3.3) and the floodplain (PDR-C10a) can be used as a date marker. Indeed, a similar change has already been observed in sediment cores from the Rhône floodplain, in this study area and two other sites along the river corridor (Vauclin et al, 2019;Vauclin et al, 2020). In those cases, it was proven that the abrupt grain-size modification was due to the first period of river engineering (~1880) that created a partial disconnection between the channel and the floodplain, and sediments above the limit were therefore defined as infrastructure-induced legacy sediments.…”

“…Other subparallel reflectors are apparent below the continuous reflector while the GPR image is smooth above: this filling is characteristic of legacy sediments induced by the implementation of river training infrastructures in the 1880s in the area, as demonstrated in Vauclin et al (2020). The continuous reflector also coincides with a grain-size limit observed in the corresponding sediment core (PDR-C10a).…”

“…Bigus et al, 2014, Dendievel et al, 2020 or other anthropogenic impacts such as pre-historic gullying (Dotterweich, 2005;Vanwalleghem et al, 2006), agriculture intensification (Brooks and Brierley, 1997;Fuller et al, 2015) or river manipulations (Surian and Rinaldi, 2003;Vauclin et al, 2019). The diversity of anthropogenic disturbances potentially recorded in sediments even led to the establishment of the term "legacy sediments" to refer to sediments resulting from past or contemporary human activities such as land-use, contamination (Walter and Merritts, 2008;James, 2013;Wohl, 2015) or river engineering (Vauclin et al, 2020). However, in order to use wisely the information contained in sediment deposits, it is critical to understand their recording ability depending on their geographical and geomorphological context.…”

“…-An overbank floodplain location that used to be flooded regularly prior to the bypass implementation but is entirely disconnected from the river since the 1980s (Vauclin et al, 2019). Note that part of the results from this site have already been published (Vauclin et al, 2019;Vauclin et al, 2020) but are used in this study for a broader comparison of different depositional environments.…”

Can vegetation clearing operations and reprofiling of bars be considered as an ecological restoration measure? Lessons from a 10‐year vegetation monitoring program (Loire River, France)

“…By disconnecting floodplains from rivers, these developments have induced legacy effects that constrain the expression of hydrogeomorphic processes (e.g. Vauclin et al 2020) and orient riparian communities toward more advanced successional stages (e.g. Janssen et al 2020).…”