Role of the geomorphic setting in controlling groundwater–surface water exchanges in riverine wetlands: A case study from two southern Québec rivers (Canada)

Résumé 0

There is great interest worldwide to reconnect floodplain wetlands to their rivers. Whilst the surface water connection between rivers and wetlands is fairly well understood, the linkages via groundwater are not well known. In this study, it is hypothesized that the significance of the groundwater pathways between rivers and wetlands is largely determined by the geomorphic setting of the riverine corridor. This was tested by measuring the response of water levels and temperatures in floodplain groundwater and in wetlands to river pulses in two geomorphologically distinct riverine corridors in Southern Québec. In the De la Roche River (DLR; 145 km2), the floodplain is narrow and the alluvial sediments consist of sandy silt (wetland A; abandoned meander) or clayey silt (wetland B; stable floodplain), depending on the location. During within-channel floods, exchanges of water between the river and the floodplain are limited to some bank recharge where the alluvial sediments are permeable, and over-bank storage where the sediments are finer. Water levels in the DLR floodplain wetlands were controlled by a combination of over-bank flow and groundwater discharge from adjacent uplands. In the Matane River (1678 km2), the floodplain substrate is coarser, and the floodplain is wider and has a meandering planform geometry. The response of the Matane River wetland during floods shows storage of water due to a groundwater flood wave. This response of the wetland to within-channel flood pulses could play a role in downstream flood attenuation. In this river, the presence of river infiltration in this floodplain was also illustrated by the warming of floodplain groundwater during flood pulses. This study has shown with three distinct examples how riverine wetlands can be connected to their rivers via either a surface or subsurface pathway depending on the geomorphic setting of the riverine corridor.