and flows through the province from northeast to southwest.

The eastern part of the department is part of the Pays de France, an area of fertile plain traditionally used for agriculture (particularly cereals and sugar beet) based on its fine silty soils. This part is progressively diminishing in size as Paris expands. Part of Charles de Gaulle Airport falls in this eastern region, while other parts are in the departments of Seine-et-Marne and Seine-Saint-Denis.