Caves, springs, and sinkholes are all features of karst regions. Much of Missouri is a karst landscape of porous limestone and dolomite with deep fissures.
The Karst Cycle
Slightly acidic groundwater flows through cracks in limestone or dolomite, slowly dissolving the rock. The cracks widen to form cavities and eventually a subterranean drainage system. The creeks that flow through wet caves come from surface water that has seeped downward.
Where a cave is below the water table, it is filled entirely with water. (Well-drillers search for these water-filled pockets.) When the cave is above the water table, the cave has air in it and its water, an underground stream, flows ever downward. Wherever cave water exits the rock and flows into the open air, it is called a spring.
Some of our largest caves formed ages ago as completely water-filled cavities. Over geologic time, the Ozark Plateau was uplifted, and rivers carved valleys ever deeper into the bedrock, creating bluffs and causing cave passages to be above the water table and to drain.
As the slightly acidic water continues its etching of the rocks, sinkholes can occur where the roof of a cave, and the soil above it, collapse downward into the cave system.
Inside a cave, the formation of speleothems (stalactites, stalagmites, soda straws, dripstone, flowstone, and so on) represent the cave’s old age, as water crystallizes previously dissolved minerals, basically reversing the cave-making process, slowly filling the cave back up with rock.
All the land through which water moves into groundwater or into springs or caves is called a recharge area. For a single cave system, this area can be many miles wide. Pollutants, such as agricultural chemicals and animal waste, roadway runoff, leaking septic tanks, contaminants from trash thrown into sinkholes, and even excess silt from a variety of construction and agricultural earthworks, can seep into the groundwater, polluting caves, springs, and well water.
Groundwater habitats in karst geology are fascinating, unique, and fragile. Humans have only a limited understanding of aquatic cave species and their interconnections.
Many animals living in aquatic caves lack body pigments, are sightless or nearly so, and are adapted to economize their energy expenditures, since their food supplies can be unsteady.
There are many remarkable animals that are so specialized for living in caves that they cannot survive outside the cave environment. These include cavefishes, cave crayfishes, the grotto sculpin and grotto salamander, and several species of flatworms, cave snails, arachnids, amphipods, copepods, isopods, millipedes, insects, and more.
Because they cannot live outside the cave, and because several species are limited to a single cave system, a single pollution event affecting the groundwater seeping into the cave can potentially wipe out an entire population, which might constitute all the members of a species.
Wet Cave Diversity
Though there is a lot of overlap, wet cave communities can be described based on the way water flows within them.
A cave spring is a cave (with air in it) that has a stream (spring) flowing from the entrance. This water flow is often permanent or nearly permanent. Cave springs are usually positioned at the base of steep slopes or bluffs along streams. The water's source is usually rainwater, which flows into the cave through sinkholes or through the permeable rocks (limestone, dolomite) above. Usually, the passages branch into the cave like tree branches. The networks of passageways can total more than 26 miles in length.
Most of our major bat caves are cave springs, which are sometimes called effluent caves. The ecosystem in these caves usually begins with the nutrients in bat guano (manure): Bats eat insects outside the cave, then fly into the cave to roost. The guano that collects below them is broken down and digested by microorganisms and tiny animals, which then are eaten by increasingly larger animals.
Swallets, sometimes called swallet holes or influent caves, can have an intermittent or perennial flow of water, but in this case the water, usually a stream, enters the cave system through a “swallow hole.” The swallow hole can be a horizontal entrance, a sinkhole, or a crack in a stream. A “pirated stream” is when a surface stream is “pirated” as it is diverted underground via a sinkhole or cave within its valley. Passageways of influent caves are relatively simple and can be hundreds of feet to several miles long.
The ecosystem of influent caves tends to have more nutrients fed into it than effluent caves do. Large amounts of organic material (eroded soil, branches, leaves) can be flushed into the cave by rainwater, surface streams, flooding, and other runoff.
Wet Pit Caves
Pit caves have vertical cave entrances, which are called shafts or pits. These shafts can be several feet to more than 100 feet deep. These entrances are often associated with sinkholes or crevices on bluffs. As the name suggests, wet pit caves have permanent water, in the form of underground streams or lakes.
Nutrients enter the cave in the form of organic materials (anything living or once living) that fall into the cave. These caves are rare. Bats are not usually associated with them.