How do surface streams gradually disappear into underground caves?

Surface streams gradually disappear into underground caves through a process driven by the dissolution of soluble bedrock, such as limestone or dolomite.12

As a surface stream valley deepens, it eventually cuts into these carbonate rock layers. The stream begins to lose water into the bedrock through existing joints and fractures, which are gradually enlarged by the chemical action of water—often containing carbonic acid—into small cave passages.23

This process follows a progressive transition:
* Initial stage: The cave passages are small and can only divert a portion of the stream's flow, leaving the surface channel active.3
* Intermediate stage: As the underground conduits enlarge over time, they accommodate more water. During low-flow periods, the surface stream may run dry as the entire flow is diverted underground.3
* Advanced stage: The cave system eventually becomes large enough to carry even major flood flows, leaving the surface channel permanently dry. This abandoned surface feature is known as a dry valley.3

In some cases, surface streams flowing over insoluble rock will sink immediately upon reaching the border of a karst region, becoming tributaries to the underground drainage system.1