A marker of local identity, the Authie is a coastal river that marks the boundary between the departments of Pas-de-Calais and Somme. It runs parallel to the Canche and the lower bed of the Somme.
The valley is about 100 km long but it is not very wide, from 9 to 23 km maximum. Its geological formation explains this configuration. Oriented on a West/North-West axis, its sides are asymmetrical: on the right bank, the slope is gently sloping and on its left bank, the slope is much shorter and steeper.
The Authie has its source in Coigneux, about 100 m above sea level. After receiving the waters of its 3 tributaries, Kilienne, Grouche and Gézaincourtoise, it flows into the English Channel between Berck and Fort-Mahon via Authie Bay. The terms high, medium, low valleys and low fields are often used to find your way around the valley. It is according to the altitude at which the river is located that these four zones are delimited. Well-marked morphological characteristics correspond to these units. In the upper valley, between the source and the commune of Outrebois, the bottom of the Authie is narrow, the bed of the river is well marked and the slopes are quite steep. The middle valley which then runs to Dompierre sur Authie has a widening valley bottom. Up to the Falaise Morte near Colline-Beaumont, it is then the lower valley which crosses important marsh areas, the plateaus are low there. As you get closer to the bay of Authie, the slopes become less and eventually disappear so that the low-field area is finally practically at the same level as the sea.
Issues & Objectives
Floods in the Authie valley
There are different types of flooding: flooding by runoff, by marine submersion, by the rising of the water table or by the overflow of watercourses.