Ria is the geographic term referring to a type of estuary at the mouth of a river where a valley has flooded. An estuary is a coastal body of water which opens to the sea and has one or more rivers flowing into it.
Rias are usually created by the flooding of a river valley where a river meets the ocean. They can be created either by a rise in sea level, or (less commonly) a sinking of land. Since a ria is at the seaward end of a river, either of these changes can cause a permanent geographic change when the local river valley is flooded. For this reason, rias are sometimes referred to as drowned river valleys. Rias look similar to fiords, but differ in that fiords are caused by glaciers, not flooding.
Many definitions of ria use the term "narrow," but this is only because the geography of a narrow valley is more prone to this type of flooding. But rias can also be broad and spread out, depending on the original river valley. The specifics of a ria are dictated by the previous geography of the valley, and its shape--while generally a funnel, getting larger toward the ocean--is usually irregular and can include small islands and inlets. Because of the method of their formation, rias can also seem disproportionately large to the rivers and streams which flow into them.
Rias can occur anywhere, although some areas have more than others, due to coastal geography. For example, they are quite common in Ireland and Spain. Sydney Harbor in Australia is actually a ria. In the United States, Chesapeake Bay and San Francisco Bay are two of the most well-known rias.