The mouth of the river is a ria (or ), eroded along a fault line in the Precambrian mica-schist rocks of the Armorican Massif which define the north coast of the Bay of Bourgneuf. It can be considered as the southernmost aber in Brittany. Formed mainly before the Pliocene era, it was inundated by the rise in sea level due to the Flandrian transgression and has gradually silted up since. The navigable part of the river upstream from the port was considered to be 11km or 12km, but the maximum length from source to port is . The distance between the causeway of the port and the mouth of the ria at the adds another .
