Such sites are usually found in "Rias" - flooded river valleys broadened and deepened by changes in sea level (Information). Rias tend to have broad, deep, estuaries at their mouths, funnelling inland to a standard river system.

In the case of many of these (the Solway Firth and Mersey, for example), they are also glacial fjords, "overdeepened" (excavated by glacial action) towards their mouth and infilled with sediment further inland.