A river is a natural flowing watercourse, usually freshwater, moving toward another body of water. While most rivers eventually reach the global ocean, this is not always the case. A significant portion of the Earth’s land is drained by river systems that terminate far from any coast. These internally draining systems contrast with standard hydrology because the water leaves the basin through means other than surface outflow.
The Standard Flow: Rivers That Reach the Sea
The most common system is the exorheic basin, characterized by an open drainage system where water ultimately flows into the sea or ocean. These basins cover approximately 80% of the Earth’s land surface and include the largest rivers in the world. The entire area of land where surface water collects and is directed toward a single outlet is known as the drainage basin, or watershed. The boundary separating drainage basins is called the drainage divide, often formed by mountain ridges.
The Nile River drains northeastern Africa before discharging into the Mediterranean Sea. The Amazon River basin, the largest by discharge, collects water from South America and empties into the Atlantic Ocean. In these external drainage systems, the water balance is maintained primarily through surface outflow. The river volume is sufficient to overcome losses from evaporation and infiltration over the entire course, allowing the water to successfully reach the ocean.
Rivers That Do Not Reach the Ocean
Rivers that never reach the sea flow within a closed hydrological system known as an endorheic basin. These internal drainage systems cover about 18% of the global land area, with the water terminating at a point on the continent itself. This termination occurs through three primary mechanisms, preventing the water from joining the world’s exorheic network.
Terminal Lakes
One common destination is a terminal lake, a large body of water with no natural outlet to the ocean. The Volga River flows into the Caspian Sea, the world’s largest terminal lake, where water leaves the system only through evaporation. Rivers feeding the Great Salt Lake in Utah are similarly trapped within a closed basin, resulting in high salinity that characterizes such landlocked water bodies.
Evaporation and Absorption
Another mechanism involves rivers that disappear due to the arid conditions of the landscape. The Okavango River in southern Africa spreads out into the Okavango Delta, where its water is lost entirely to evaporation and plant absorption across the vast inland swamp. This process of the river vanishing into a dry region is common in desert and steppe environments.
Subsurface Flow
A third termination point is the subsurface, where river water is lost to underground flow. In areas with highly permeable ground, such as limestone or sandy soil, the water infiltrates the surface and feeds underground aquifers or cave systems. The Luni River in India, for example, flows only a short distance before its water is absorbed by the porous terrain of the Thar Desert, ceasing to exist as a surface flow.
Geographical and Climatic Conditions for Inland Drainage
The existence of endorheic basins is determined by a specific combination of geography and climate. Topography plays a primary role, as rivers are prevented from reaching the ocean by surrounding mountain ranges or elevated plateaus that form the drainage divide. These geological features create a depression or closed basin, physically blocking the water’s path to the external drainage network.
The Great Basin in the western United States, for example, is defined by numerous mountain ranges that trap all precipitation and river flow within its boundaries. The formation of these basins is often linked to tectonic activity, where crustal movements create low-lying areas isolated from oceanic drainage systems. This geological isolation is a prerequisite for internal drainage.
Climate determines the water’s final fate within the basin. Endorheic regions are found in arid or semi-arid climates, where the rate of evaporation significantly exceeds the rate of precipitation and river inflow. The Tarim Basin in China experiences high evaporation and low rainfall, ensuring its rivers cannot maintain the volume required to breach the surrounding mountains.
In the Lake Chad basin in central Africa, the high seasonal evaporation rate is the primary control on the lake’s size and the overall hydrological balance of the system. This climatic condition ensures that water collected in the basin is removed vertically by the atmosphere rather than horizontally by surface outflow. The combination of a closed topographical container and a dry climate guarantees that the river’s journey ends inland.