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On an average day 1 million m³ of water issue from Big Spring in Missouri at a rate of 12,000 L/s.

A natural spring on Mackinac Island in Michigan.

A spring is a point where groundwater flows out of the ground, and is thus where the aquifer surface meets the ground surface.

Dependent upon the constancy of the water source (rainfall or snowmelt that infiltrates the earth), a spring may be ephemeral (intermittent) or perennial (continuous).

Water issuing from an artesian spring rises to a higher elevation than the top of the confined aquifer from which it issues. When water issues from the ground it may form into a pool or flow downhill, in surface streams. Sometimes a spring is termed a seep.

Minerals become dissolved in the water as it moves through the underground rocks. This may give the water flavour and even carbon dioxide bubbles, depending upon the nature of the geology through which it passes. This is why spring water is often bottled and sold as mineral water, although the term is often the subject of deceptive advertising. Springs that contain significant amounts of minerals are sometimes called 'mineral springs'. Springs that contain large amounts of dissolved sodium salts, mostly sodium carbonate, are called 'soda springs'. Many resorts have developed around mineral springs known as spa towns.

A stream carrying the outflow of a spring to a nearby primary stream is called a spring branch. The cool water of a spring and its branch may harbor species such as certain trout that are otherwise ill-suited for a warmer local climate.

Water emanating from karst topography is another type of spring, often called a resurgence as much of the water may come from one or more sinkholes at a higher altitude. Karst springs generally are not subjected to as great a degree of ground filtering as spring water which may have continuously passed through soils or a porous aquifer.

Springs are often classified by the volume of the water they discharge. The largest springs are called "first-magnitude," defined as springs that discharge water at a rate of at least 2800 L/s. The scale for spring flow is as follows:

Water Portal

• Chalk stream
• Drinking water
• Geyser
• Hot spring
• Karst
• Pond
• Seep
• Water cycle
• Water well
• mineral water
• spa towns
• list of spa towns

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• "The Science of Springs" (via Wayback Machine)
• "What Is A Spring?"
• List of First-Magnitude springs in Florida
• Technical and educational material on mineral springs and mineral water in Victoria