Estuary (lat. Aestuarium, "low estuary") is the name for a estuary shaped like a funnel. Estuaries are rarer than delta-shaped estuaries. Such forms of estuaries have, for example, the rivers: Thames, Elbe, Ob, Yenisei. Highly stratified estuaries predominate in Croatia. This means that the surface layer of low salinity is clearly separated from the bottom layer of high salinity. Examples of such rivers in Croatia are the Krka and Zrmanja rivers. In 1967, Pritchard first defined estuaries as areas where freshwater river mass has a permanent connection with sea mass, within which water is of measurably lower salinity. Estuaries are the lower parts of river valleys that are exposed to the influence of tides. They differ from shallow bays and coves due to the distinct influence of fresh water. Mixing fresh and salt water, and reduced flow in some places in the estuary cause sediment deposition: sand and silt. Deposited sediments can form a delta at the mouth, an example of such a delta in Croatia is the river Neretva. The subtype of estuaries characteristic of Croatia is the type of estuaries of karst rivers that formed after the melting of glaciers and rising sea levels after the last ice age.