Type of wetland ecosystem characterized by poorly drained mineral soils and by plant life dominated by grasses. The latter characteristic distinguishes a marsh from a swamp (q.v.), whose plant life is dominated by trees. Marshes are common at the mouths of rivers, especially where extensive deltas have been built. The river brings a steady supply of water. The gradient of the river approaches zero at the sea, where flow is sluggish. Because the delta is deposited by sediment settling from the river water, the land that is built will be poorly drained at its driest and will often be underwater. Sediment supplied by the river has often been eroded from the surface soils of the drainage basin and is thus very rich. The combination of water supplied steadily at a low rate over a waterlogged but rich soil creates a perfect environment for marsh grasses.
Source: Taken from Encyclopaedia Britannica 2002