The ionic composition of inland waters reflects both the geochemistry of the watershed and the processes of rainfall and evaporation. There are three major classes of inland waters: dilute low salinity waters in rainfall dominated watersheds where the ionic composition reflects the rainfall, medium salinity waters where carbonate and bicarbonate frequently predominate due to reaction with the underlying rocks, and high salinity waters in watersheds where evaporative processes predominate and where sodium and chloride are the most common ions. Where evaporative processes predominate the ionic composition of the water follows a predetermined sequence of concentration and precipitation which has geochemical and biological significance. The ecological effects of changing salinity are complex with the species diversity generally declining with increased salinity. Stenohaline freshwater organisms usually are found at salinities below 3000 ppm, a range a of euryhaline species between 3000–10 000 ppm and a small number of specialized salt water tolerant species in the range of 10 000–35 000 ppm and above. There is evidence for a physiological basis for the ecological effects of the changing mono- to divalent cation ratio as waters become more concentrated due to evaporation and precipitation. Salt may be used as a conservative tracer of water flow in surface and groundwaters and much new information is being gained from high frequency monitoring of the fluctuations in salinity in streams draining small experimental watersheds. In arid and semi-arid watersheds under natural vegetation salt which originated from rainfall becomes stored in the soil profile. When the vegetation is cleared for agriculture the alterations in the hydrological balance lead to increased infiltration of water and the mobilization of the salt. This can result in widespread salinization and reductions in crop growth.