Stream capture, river capture, river piracy or stream piracy is a geomorphological phenomenon occurring when a stream or river drainage system or watershed is diverted from its own bed, and flows down to the bed of a neighbouring stream. This can happen for several reasons, including:

River piracy, also known as stream capture, happens when melting glaciers divert the path of a river. Due to our warming planet, a major Alaskan river is under threat of being redirected in this way – stealing an essential source of water away from the ecosystems and people that rely on it.

The diversion of the part of the course of a river by another river is called stream diversion or stream capture or stream piracy. The river which captures the course of another river is called the capturing or captor stream while the part of the stream which has been divested of its course and water is called the captured stream.

River piracy—the diversion of the headwaters of one stream into another one—can dramatically change the routing of water and sediment, with a profound effect on landscape evolution.

The most common type of stream piracy is by a process that occurs when one stream (River A, Fig. 4a) erodes headward into the channel of another stream (River B, Fig. 4a) and diverts...