A spring is a natural place where groundwater flows out from underground and becomes surface water. It is part of the Earth's water system, called the hydrosphere, and plays a role in the water cycle. Humans have used springs for fresh water for a long time, especially in dry areas with little rain each year.

Springs are pushed to the surface by natural forces, such as gravity and water pressure. When hot groundwater flows out, it is called a hot spring. The amount of water that flows from a spring can vary greatly, from almost none to more than 14,000 liters per second (about 490 cubic feet per second) for the largest springs.

Formation

Springs form when groundwater moves to the surface. This often happens when the water table rises above the ground level or when the land drops suddenly. Springs can also form due to karst topography, aquifers, or volcanic activity. Springs have also been found on the ocean floor, where warm, low-salinity water flows directly into the ocean.

Springs that form because of karst topography are called karst springs. In these cases, groundwater moves through cracks and openings in the rock, such as spaces between grains or large caves, before emerging as a spring.

Groundwater can be forced to the surface when it is trapped in a confined aquifer. This occurs if the area where groundwater enters the aquifer is at a higher elevation than where it exits. Springs pushed to the surface by this pressure are called artesian wells. This can happen even if the exit point is inside a deep cave, as the higher ground pushes water through the cave like a hose.

Non-artesian springs may flow from higher ground through the Earth to lower ground, exiting as a spring. Other springs form due to pressure from underground sources, such as volcanic or magma activity. These can produce water that is hot and under high pressure, like hot springs or geysers.

Groundwater continuously dissolves permeable bedrock, such as limestone and dolomite, creating large cave systems.

• Depression springs are found in low areas like the bottom of valleys, basins, or valleys made of highly permeable materials.
• Contact springs are located on the sides of hills or mountains. They form when groundwater is below an impermeable layer of rock or soil, such as an aquiclude or aquifuge.
• Fracture springs occur when groundwater moving along an impermeable layer meets a crack or joint in the rock.
• Tubular springs form when groundwater flows from circular openings, such as those in caves (solution tubular springs) or in lava tube caves (lava tubular springs).
• Artesian springs are usually found at the lowest point in an area. They form when groundwater pressure is greater than atmospheric pressure, pushing water upward from the ground.
• Wonky holes are underwater openings where freshwater flows out from old river channels covered by coral and sediment.
• Karst springs are groundwater outflows that are part of a karst hydrological system.
• Thermal springs are heated by geothermal activity and have water temperatures much higher than the surrounding air. Geysers are a type of hot spring where steam forms underground from trapped hot water, causing regular eruptions of water and steam.
• Carbonated springs, such as Soda Springs Geyser, release naturally carbonated water due to dissolved carbon dioxide. They are sometimes called boiling springs or bubbling springs.
• "Gushette springs pour from cliff faces."
• Helocrene springs are widespread groundwater sources that support marshlands.

Flow

Spring discharge, or resurgence, depends on the spring's recharge basin. Factors that influence recharge include the size of the area where groundwater is collected, the amount of rainfall, the size of points where water enters the ground, and the size of the spring's outlet. Water can enter the underground system from many sources, such as soil that allows water to pass through, sinkholes, and streams that lose water to the ground. In some cases, entire creeks seem to disappear as water flows into the ground through the stream bed. Grand Gulf State Park in Missouri is an example of a creek that vanishes into the groundwater system. The water reappears 9 miles (14 km) away, forming part of the discharge from Mammoth Spring in Arkansas. Human activity can also affect a spring's discharge—removing groundwater lowers the pressure in an aquifer, reducing the amount of water that flows.

Springs are generally divided into three types: perennial (springs that flow continuously throughout the year), intermittent (springs that flow only after rainfall or during certain seasons), and periodic (springs, like geysers, that release water at regular or irregular times).

Springs are often grouped based on the amount of water they discharge. The largest springs are called "first-magnitude," meaning they release at least 2800 liters or 100 cubic feet (2.8 m³) of water every second. Some areas have many first-magnitude springs, such as Florida, which has at least 27 known springs of this size; the Missouri and Arkansas Ozarks, which have 10 known first-magnitude springs; and 11 more in the Thousand Springs area along the Snake River in Idaho. The scale for spring flow is as follows:

Water content

Minerals dissolve in water as it moves through rocks underground. The amount of minerals in water is measured as total dissolved solids (TDS). These minerals can affect the taste of the water and sometimes create carbon dioxide bubbles, depending on the type of rocks the water passes through. This is why spring water is often bottled and sold as mineral water. However, the term "mineral water" is sometimes used in misleading ways. Mineral water must contain at least 250 parts per million (ppm) of TDS. Springs with high mineral content are sometimes called "mineral springs." Springs with little to no minerals are sometimes called "sweet springs." Springs with large amounts of sodium salts, mostly sodium carbonate, are called "soda springs." Many resorts have been built around mineral springs and are known as spa towns. Mineral springs are said to have healing properties. Soaking in them is believed to allow the body to absorb minerals from the water. Some springs have arsenic levels higher than the 10 parts per billion (ppb) World Health Organization (WHO) standard for drinking water. When these springs flow into rivers, they can increase the arsenic levels in the rivers above WHO limits.

Water from springs is usually clear. However, some springs may have colors caused by dissolved minerals. For example, water with high levels of iron or tannins may appear orange.

In parts of the United States, a stream that carries water from a spring to another stream may be called a spring branch, spring creek, or run. Groundwater usually keeps a steady temperature similar to the aquifer it comes from. This means that water from a spring might be cooler than other water sources in summer but still not freeze in winter. The cool water in a spring and its branch can support fish like trout that usually can't live in warmer water.

• Sulfur springs have high levels of dissolved sulfur or hydrogen sulfide. Historically, they have been used to help ease symptoms of arthritis and other inflammatory diseases.
• Borax springs
• Gypsum springs
• Saline springs
• Iron springs (chalybeate springs)
• Radium springs (or radioactive springs) have detectable levels of radiation from natural radioactive decay.

Uses

Springs have been used for many human needs, such as drinking water, household water, irrigation, mills, transportation, and electricity. Today, springs are also used for activities like fishing, swimming, and floating; for therapy; to provide water for animals; in fish hatcheries; and for bottled mineral or spring water. Some people mistakenly believe that all springs are safe for drinking, but this is not always true. To use a spring properly, a complete water test is needed. Springs that are managed as spas usually already have this test.

Springs are often the source of bottled water. When buying bottled water labeled as "spring water," the company selling it may have the water test results on its website.

Springs have been used to provide water for irrigation using gravity. Indigenous people in the American Southwest built spring-fed acequias, which are canals that carry water to fields. Later, Spanish missionaries used this same method.

A sacred spring, or holy well, is a small body of water that comes from underground and is respected in some religious traditions, such as Christian, pagan, or others. In ancient Greece, many springs were considered sacred, like the Corycian, Pierian, and Castalian springs. In medieval Europe, pagan sacred sites often became Christian holy wells. The term "holy well" is used to describe any small water source, such as a pool or natural spring, that has importance in local stories. This may include a name, a legend, beliefs about the water's healing power, or rituals connected to the site. Christian stories often describe how a saint caused a spring to flow, a common theme in stories about Celtic saints.

Thermal springs are heated by geothermal energy and have water temperatures above the human body’s temperature, usually between 45–50 °C (113–122 °F), though some can be hotter. Springs with water slightly warmer than air temperature but cooler than body temperature are sometimes called warm springs.

Hot springs, or geothermal springs, have been used for thousands of years for treatments, bathing, and relaxation. Because of stories about their health benefits, some hot springs are now tourist attractions and places for physical therapy.

Hot springs have also been used as a heat source for thousands of years. In the 20th century, they became a renewable energy source for heating homes and buildings. The city of Beppu, Japan, has 2,217 hot spring well heads that supply the city with hot water. Hot springs have also been used to provide energy for greenhouses and to grow crops and flowers.

Cultural representations

Throughout history, springs have been shown in art, myths, and stories. The Fountain of Youth is a mythical spring said to restore youth to those who drank from it. Some say it was found in St. Augustine, Florida, by Juan Ponce de León in 1513. However, it has not proven to have this power, and most historians question the truth of this claim.

Pythia, also called the Oracle at Delphi, was a priestess at the Temple of Apollo. She gave prophecies while in a trance-like state caused by vapors rising from a rock chasm. These vapors are believed to have come from the Kerna spring at Delphi.

In Greek mythology, Narcissus was a young man who fell in love with his reflection in a still spring. He described the spring as "unmuddied, silvery, and untouched by animals or falling branches."

In the early 20th century, American photographer James Reuel Smith took many photos of New York City’s historical springs before they were covered by the city’s water system. Later, he photographed springs in Europe, which led to his book Springs and Wells in Greek and Roman Literature, Their Legends and Locations (1922).

In 1854, 19th-century Japanese artists Utagawa Hiroshige and Utagawa Toyokuni III created a wood-block print series called Two Artists Tour the Seven Hot Springs (Sōhitsu shichitō meguri).

The Chinese city Jinan is called "a City of Springs" (quán chéng) because of its 72 spring attractions and many small spring holes in its city center.

• Echo and Narcissus (1903) by John William Waterhouse
• Der Jungbrunnen (Fountain of Youth) (1546) by Lucas Cranach
• Sōhitsu shichitō meguri (1854) by Utagawa Toyokuni III and Utagawa Hiroshige, including Sokokura
• Oracle of Delphi, red-figure kylix by the Kodros Painter (440–430 BC), showing Pythia with a cup holding water from a spring
• A Woman Drinks at the Carmen Spring (c. 1897–1902) by James Reuel Smith, showing the Carmen Spring on West 175th Street and Amsterdam Avenue, New York City
• Belqais Spring Garden, Charam, Iran, a Persian garden where all water sources are springs within it.