A stream channel is a creek, river, or lake conveying water out of a defined area on the surface. It is also sometimes called outflow point.
The English term stream channel is used as a common term for the bed of a river. It is also a hydrologic scientific term, with some importance for karst geology. It is an important concept for the understanding of karst evolution.
An example: a mountain ridge composed of limestone or dolomite rocks, surrounded by lowlands, is karstified. The lowlands are drained by surface streams and the ground water table is slightly below the surface. In some areas it is even difficult to build houses with cellars, as the ground water is less than four meters below the ground.
Around the karst area, at the rim to the lowlands are numerous karst springs. The karst area, or more exactly the fissures, clefts and caves in the limestone are filled with ground water. We know the level of the ground water for all springs, as it is identical with the water surface of the spring itself. If the ground water table was higher it would spring higher up the hill.
Unconfined groundwater, also called phreatic groundwater or free groundwater, is able to flow free inside the rock. This is typical for karst, as there are numerous caves and crevices allowing the water to flow freely. The result is water moving under gravity in the direction of the slope of the water table.
Now we have a drain of the karst area, the spring, where the rain water leaves the area. The recharge is rain water on the complete area, or better on the catchment area of the respective spring. The ground water needs a slope towards the spring, or if we look at it the other way round, the water table rises from the spring into the rock.
A very important effect on the karst is the lowering on the stream channel. This erosional process in the foreland of the karst area is not connected to the geologic process in the karst. But it has enormous influence on the karst development. The erosion of the stream channel lowers the spring and thus the whole ground water table.
When the stream channel goes down continually, after some time the ground water leaves the area of the river cave which devloped in the phreatic zone around the ground water table. The water table rises and falls depending on the amount of water, but when the water table gets deep enough, the cave is never reached by the ground water, not even during floods. Then this cave is called fossil or dry cave.
At the same time, below the former river cave, a new level is formed by the solution of limestone in the phreatic zone. The continual lowerage of the outflow point is responsible for the formation of different cave levels. Some cave systems show three, four or even more levels.