On the coasts of inland half-open seas, such as the Baltic, there is observed a phenomenon of periodic intrusions of sea waters into land waters. They are caused by a current meteorological and hydrological situation and not by tides. However, due to the similarity of the processes of water exchange and the results brought about by them in the geographical environment, they are described as quasi-estuary processes. The course, scope and results of the phenomenon depend on the hydrography of the coast. In order to illustrate this dependency there were chosen four characteristic hydrographic situations which occur on the coast of the southern Baltic. These are: a deltaic system, a cut off distributary arm of a big European river, a system in which the mouth section of a river connects a large coastal lake with an open sea, and marshes. The consequence of the occurrence of the phenomenon of quasi-estuary is such problems which must be taken into consideration in the use of the geographical environment of the southern Baltic coast. This necessity becomes especially vital with the prospect of a rising of sea levels related to global climatic changes.