<P> In desert areas, or areas with impoverished or dusty soils, rain - bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing the freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils . Significant quantities of iron may be transported in this way including the well - documented transfer of iron - rich rainfall falling in Brazil derived from sand - storms in the Sahara in north Africa . </P> <P> Water is a critical issue for the survival of all living organisms . Some can use salt water but many organisms including the great majority of higher plants and most mammals must have access to fresh water to live . Some terrestrial mammals, especially desert rodents appear to survive without drinking but they do generate water through the metabolism of cereal seeds and they also have mechanisms to conserve water to the maximum degree . </P> <P> Out of all the water on Earth, saline water in oceans, seas and saline groundwater make up about 97% of it . Only 2.5--2.75% is fresh water, including 1.75--2% frozen in glaciers, ice and snow, 0.5--0.75% as fresh groundwater and soil moisture, and less than 0.01% of it as surface water in lakes, swamps and rivers . Freshwater lakes contain about 87% of this fresh surface water, including 29% in the African Great Lakes, 22% in Lake Baikal in Russia, 21% in the North American Great Lakes, and 14% in other lakes . Swamps have most of the balance with only a small amount in rivers, most notably the Amazon River . The atmosphere contains 0.04% water . In areas with no fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers . Most of the world's fresh water is frozen in ice sheets . Many areas suffer from lack of distribution of fresh water, such as deserts . </P> <P> Fresh water can be defined as water with less than 500 parts per million (ppm) of dissolved salts . </P>

What is the main reservoir for fresh water on earth
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