Hi Max,
I come from this little village in Brittany. :) The river banks are indeed bordered with trees on almost all this section. You may also want to check out more detailed topographic maps and satellite images, e.g. at Géoportail (geoportail.gouv.fr).
However, I am new to OSM, so I may not have used a good representation for the river banks. If so, I am sorry and I would be pleased to correct this.
Regards,
Valentin