• see also River
• see also Lake

Freshwater refers to naturally occurring water on the surface such as bogs, ponds, lakes, rivers and streams and underground in aquifers and underground rivers. Freshwater is characterised by having low concentrations of dissolved salts. The term specifically excludes seawater and brackish water.

Systems

Scientifically, freshwater habitats are divided into lentic systems which are the stillwaters including ponds, swamps and mires , lotic systems which are running water 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 groundwater.

Source

The source of almost all freshwater is precipitation from atmosphere in the form of mist, rain and snow. A very small proportion is emitted from active volcanoes. Freshwater 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 travelled. In industrialised areas rain is typically acid because of dissolved oxides of sulfur and nitrogen formed from burning of fossil fuels in cars, lorries, trains and aircraft and from the atmospheric emissions of industry. In extreme cases this causes acid rain which can has caused severe pollution of lakes and rivers in parts of Scandinavia, Scotland, Wales and the United States.

In coastal areas freshwater 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.

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 northern Africa. This effect can cause unwelcome contamination with dust from the Chernobyl disaster being spread across Europe in rain clouds

Very large amounts of freshwater are locked up in the ice-caps of the world, especially over Antarctica and Greenland

Uses

Freshwater is arguably the most important renewable resource, necessary for the survival of most terrestrial organisms, and is required by humans and domesticated animals for drinking and for agriculture. The use of freshwater as a source of drinking water and to carry away treated sewage effluent can be a limiting factor in human colonisation of the planet. The UN estimates that about 18 percent of the world's population lacks access to safe drinking water.