Discovery of megadunes in the middle estuary of the St. Lawrence River, Québec, Canada
Three years of regional marine geology surveys in the St. Lawrence Estuary allow for the first time the detailed characterization of the seabed topography between Ile aux Coudres and Pointe-des-Monts, Québec, Canada. Multibeam bathymetric surveys complemented with high resolution geophysical surveys (seismic and side-scan sonar) and seabed sampling, are the basis for documenting megadune (10 m) fields up to 10 km in length and a few kilometers in width. The presence of these megadunes suggests a very dynamic environment as well as high sediment mobility. Two of these fields, located within the limits of the Saguenay‐St. Lawrence Marine Park, are described in detail in order to better understand the physical environment in which they were formed. The occurrence of internal waves in the upper St. Lawrence Estuary, a known and documented phenomenon, suggests that this may be an important factor in the development and mobility of the megadunes. However, seismic profiles over the megadune fields show that some of them may be a relict of a time when hydrodynamic conditions in the estuary were different, i.e. with a much stronger discharge and more important tidal currents. Some megadunes imaged on the seabed, with an amplitude of ten meters and an average wavelength of 200 m, have been observed on the seismic profiles under modern mud. Bedforms are observed at various scales, the largest being overridden by smaller ones, these ones perhaps representing modern hydrodynamic conditions. The modern activity of the megadunes needs to be evaluated with multi-year bathymetric surveys, which will allow quantifying their displacement characterization of changes in morphology associated with varying hydrodynamic conditions, and establish a direct relationship with modern hydrodynamic conditions.