Drainage Basins

Usually rivers and their tributaries form a fluvial system (a system of rivers), and flow across a surface called drainage basin.
We all know rivers that flow into the sea or oceans.
But, there are also, some rivers that flow into a lake, went underground or simply evaporate.. These types of rivers are typical in desertic zones or in areas far from the ocean fall and surrounded by mountain ranges. The scientific name of this type of drainage basins is Endorrheic basins or closed drainage basins
Here you have a map showing the different ocean drainage. In dark there are the endorrheic basins.
These closed (or endorrheic) drainage basins are responsible for the biggest lake in the world: the CASPIAN SEA, where the Volga River flows into.