In General, Plain-River area based groups away from the inlet or the sea not far more water level in the River, the area is usually located in the tide influence due to sea level at all times.
Generally flat area - valley along the river, away from the river mouth. Or sea, not far The water level in the river area Often in the tidal influence stemming from sea level to support all the time.
In general, the plains - the group with the two side of the river, away from the estuary, or sea water level in the river near the area. Usually in the tidal influence due to the sea level on all the time.