Source de l'Aulne

A 140 km long coastal river, the Alder has its source in Lohuec, next to Penn Aon, east of the Monts d'Arrée. This coastal river flows in its upstream part from north to south, in the fault produced between Lannéanou and Landeleau by the eruption of the Huelgoat granite massif. Further downstream, from the confluence of the Hyères, it flows parallel to the Armorican direction of the folds, i.e. east-west. However, it meanders very steeply and steeply: between the confluence with the Hyères at Pénity-Saint-Laurent, in the commune of Landeleau, and Châteaulin, its course is 60 km long, whereas as the crow flies the distance is only 30 km.

Downstream, between Pont Triffen and Port-Launay, the Alder is channelled and forms part of the Canal de Nantes à Brest. The Alder ends in a ria on its last 18 km between Châteaulin and Rosnoën which is subject to the influence of the tides. This part of the river, from Port-Launay to the sea, is commonly called the maritime Alder and flows into the bay of Brest at Landévennec.

A meander of the Maritime Alder shelters the Landévennec ship cemetery.