<P> Fresh water habitats are divided into lentic systems, which are the stillwaters including ponds, lakes, swamps and mires; lotic, or running - water systems; and groundwater which flows in rocks and aquifers . There is, in addition, a zone which bridges between groundwater and lotic systems, which is the hyporheic zone, which underlies many larger rivers and can contain substantially more water than is seen in the open channel . It may also be in direct contact with the underlying underground water . </P> <P> The majority of fresh water on Earth is in ice caps . </P> <P> The source of almost all fresh water is precipitation from the atmosphere, in the form of mist, rain and snow . Fresh water falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled . In industrialized areas rain is typically acidic because of dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, factories, trains and aircraft and from the atmospheric emissions of industry . In some cases this acid rain results in pollution of lakes and rivers . </P> <P> In coastal areas fresh water may contain significant concentrations of salts derived from the sea if windy conditions have lifted drops of seawater into the rain - bearing clouds . This can give rise to elevated concentrations of sodium, chloride, magnesium and sulfate as well as many other compounds in smaller concentrations . </P>

Most of the earth's fresh water is found in