as the bottom of alluvial valleys, basins, or valleys made of highly permeable materials. Contact springs, which occur along the side of a hill or mountain, are created when the groundwater is underlaid by an impermeable layer of rock or soil known as an aquiclude or aquifuge Fracture, or joint occur when groundwater running along an impermeable layer of rock meets a crack (fracture) or joint in the rock. Tubular springs occur when groundwater flows from circular fissures such as those found in caverns (solution tubular springs) or lava tubular springs found in lava tube caves. Artesian springs typically occur at the lowest point in a given area. An artesian spring is created when the pressure for the groundwater becomes greater than the pressure from the atmosphere. In this case the water is pushed straight up out of the ground. Wonky holes are freshwater submarine exit points for coral and sediment-covered, sediment-filled old river channels. Karst springs occur as outflows of groundwater that are part of a karst hydrological system. Thermal springs are heated by geothermal activity; they have a water temperature significantly higher than the mean air temperature of the surrounding area. Geysers are a type of hot spring where steam is created underground by trapped superheated groundwater resulting in recurring eruptions of hot water and steam. Carbonated springs, such as Soda Springs Geyser, are springs that emit naturally occurring carbonated water, due to dissolved carbon dioxide in the water content. They are sometimes called boiling springs or bubbling springs. Flow Spring discharge, or resurgence, is determined by the spring's recharge basin. Factors that affect the recharge include the size of the area in which groundwater is captured, the amount of precipitation, the size of capture points, and the size of the spring outlet. Water may leak into the underground system from many sources including permeable earth, sinkholes, and losing streams. In some cases entire creeks seemingly disappear as the water sinks into the ground via the stream bed. Grand Gulf State Park in Missouri is an example of an entire creek vanishing into the groundwater system. The water emerges away, forming some of the discharge of Mammoth Spring in Arkansas. Human activity may also affect a spring's discharge—withdrawal of groundwater reduces the water pressure in an aquifer, decreasing the volume of flow. Classification Springs fall into three general classifications: perennial (springs that flow constantly during the year); intermittent