The Arc is an 83 km long river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 in the south of France

France

The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

. It arises at an altitude of 470 m, close to the village of Pourcieux

Pourcieux

Pourcieux is a commune in the Var department in the Provence-Alpes-Côte d'Azur region in southeastern France.The small village provides a typical image of the Provence of Frédéric Mistral, with vine-grower's houses built around a castle, and a town square and fountain shaded by hundred year-old...

. It then passes through Aix-en-Provence

Aix-en-Provence

Aix , or Aix-en-Provence to distinguish it from other cities built over hot springs, is a city-commune in southern France, some north of Marseille. It is in the region of Provence-Alpes-Côte d'Azur, in the département of Bouches-du-Rhône, of which it is a subprefecture. The population of Aix is...

 before flowing into the Étang de Berre

Étang de Berre

The Étang de Berre is a body of water adjacent to the Mediterranean, about 25km north-west of Marseille.-Geography:Created by the rise in water levels at the end of the last ice age, this small inland sea is composed of...

, a lake to the west of Marseille

Marseille

Marseille , known in antiquity as Massalia , is the second largest city in France, after Paris, with a population of 852,395 within its administrative limits on a land area of . The urban area of Marseille extends beyond the city limits with a population of over 1,420,000 on an area of...

.

Its drainage basin

Drainage basin

A drainage basin is an extent or an area of land where surface water from rain and melting snow or ice converges to a single point, usually the exit of the basin, where the waters join another waterbody, such as a river, lake, reservoir, estuary, wetland, sea, or ocean...

, with a surface area of 727 km², is divided between two départments, Var and Bouches-du-Rhône

Bouches-du-Rhône

Bouches-du-Rhône is a department in the south of France named after the mouth of the Rhône River. It is the most populous department of the Provence-Alpes-Côte d'Azur region. Its INSEE and postal code is 13.-History of the department:...

. The risk of flood, always present in the basin, has been aggravated by urban development that has sometimes been lacking in foresight, with Aix-en-Provence particularly threatened.