According to a Pima Community College web page, a drowned river valley is called a coastal plain estuary. These are the more common types of estuaries. They developed at the end part of the last recorded ice age.
Estuaries developed from seawater that flooded river mouths and lowlands. A common example of a drowned river valley is the Chesapeake Bay. Estuaries are areas that are semi-enclosed where sea water and fresh water mix and meet up. This happens when rivers flow into the ocean. Other examples of drowned river valleys include the Thames River in England and the Murray River in Australia.