funnel-shaped estuary that occurs at a river mouth and is formed by the submergence of the lower portion of the river valley. Generally occurring along a rugged coast perpendicular to a mountain chain, many rias were formed by the rise in sea level after the melting of the vast continental glaciers. Rias are commonly very irregular and may have several branching tributaries; they usually are the major drainage systems of the adjacent areas. Their widening funnel shape and steadily increasing depth seaward usually cause an exaggerated tidal effect within the estuary.