Regions where rivers flow into, but no rivers flow out of, are known as what?

Regions where rivers flow into, but no rivers flow out of, are termed endorheic basins. These geographical features are characterized by closed drainage systems where water does not reach the ocean. Instead, any water in an endorheic basin can only exit through evaporation or infiltration, leading to distinctive ecological environments. Because they lack an outlet, endorheic basins often accumulate salts, resulting in high salinity levels in the water. Common examples of endorheic basins include the Great Salt Lake in Utah and the Caspian Sea.

Other options such as salt marshes refer to coastal wetlands rather than enclosed systems without drainage. Continental divides pertain to geographical boundaries where water flows to different oceans or seas, and isthmuses are narrow landforms connecting two larger landmasses, which do not relate to the concept of internal drainage.