The Aa is an 89 km long river in northern France. Its source is near the village Bourthes. It flows through the following départements and cities:
The Aa flows into the North Sea near Gravelines, very close to the north-eastern limit of the English Channel. The river has two characters. From its source to Saint-Omer, it is a small chalk stream; a small version of the Somme. From Saint-Omer seawards, it is an artificial navigation with branch canals leading towards Calais and Dunkirk and the Canal de Neufossé heading inland into the French canal system.
Saint-Omer formerly lay at the head of its estuary while to seaward; Calais lay on its western margin and Bergues, now inland from Dunkirk, on its eastern one. By the time of the Viking settlements on this coast, Dunkirk was developing on the dunes, offshore across the estuarine marsh from Bergues. Gravelines was the later port at the seaward end of the river as it became, after the area of the estuary was reclaimed. The dates of these events are imprecise but the modern pattern was firmly established by 1588, the time of the Spanish Armada, when an approximation to the modern course of the lowland river formed the boundary between the Spanish Netherlands