The term ‘brackish water’ is used in several senses and, therefore, I want at first to explain what I consider to be brackish water and what not. Redeke (1933) defined brackish water as a mixture of fresh water and sea water sensu stricto. This definition excludes the continental salt waters which have a different origin. There exist, however, salt waters which have lost long ago their contact with the sea and have now a chemical composition completely aberrant from sea water, due to the joint effect of continual inflow of new electrolytes with river water, evaporation, and precipitation. As examples the Caspian and the Aral Sea may be mentioned. Other investigators prefer to include on grounds of hydrological and hydrochemical similarities also the continental salt waters which at no time in their history have been in contact with the sea, e.g. the Great Salt Lake in Utah, U.S.A. They regard as brackish all salt waters which have a lower salinity than the sea. According to Schmitz (1959) the differences between the continental and marine salt waters are only of a relative value as both categories have the following four hydrological characters in common: 1. The total salinity is generally between that of fresh and sea water. 2. The waters show often salinity stratification. 3. Horizontal differences in salinity occur e.g. where a river discharges in a salt lake. 4. Large seasonal fluctuations in salinity occur.