In the geography of rivers, streams, and glaciers, a debouch, or debouche, is a place where runoff from a small, confined space emerges into a larger, broader space. The term is of French origin and means to cause to emerge. The term also has a military usage.
In fluvial geomorphology a debouch is a place where runoff from a small, confined space emerges into a larger, broader space. Common examples are when a stream runs into a river or when a river runs into an ocean. Debouching can generate massive amounts of sediment transport. When a narrow stream travels down a mountain pass into a basin, for example, an alluvial fan will form from the mass deposit of the sediment. The four largest rivers (the Amazon, the Ganges-Brahmaputra, the Yangtze and the Yellow) are responsible for 20% of the global discharge of sediment in to the oceans by debouches.
In fluvial geography, a debouch is a place where a body of water pours forth from a narrow opening. Some examples are: where a river or stream emerges from a narrow constraining landform, such as a defile, into open country or a wider space; a creek joins a river; or a stream flows into a lake.
In military usage of debouch: as a noun, a fortification at the end of a defile is sometimes known as a debouch; and as a verb, soldiers emerging from a narrow space and spreading out are also said to "debouch." 
- Oxford English Dictionary debouch and as a verb and a noun.
- Ma, Yanxia(2009). Continental Shelf Sediment Transport and Depositional Processes on an Energetic, Active Margin: the Waiapu River Shelf, New Zealand. pp 2, 19
- Mitchell, Martha S. River Rules: The Nature of Streams. pp 5
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