The Aulne valley is deep and strongly influenced by rainfall with the maritime influence visible up to Châteaulin. The valley has a powerful landscape, environmental and identity marker of the area. The downstream part of the Aulne is channelled and forms the western part of the Nantes-Brest canal.
The municipalities along the Aulne valley are affected by frequent flooding and were particularly badly affected during the floods of 1995 and 2000. The catchment area has a flood prevention action programme. The lower part of the valley, where the majority of the issues are concentrated, already has a flood risk prevention plan.