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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:
- Tectonic earth movements, where the slope of the land changes, and the stream is tipped out of its former course
- Natural damming, such as by a landslide or ice sheet
- Erosion, either
- Headward erosion of one stream valley upwards into another, or
- Lateral erosion of a meander through the higher ground dividing the adjacent streams.
- Within an area of karst topography, where streams may sink, or flow underground (a sinking or losing stream) and then reappear in a nearby stream valley
- Glacier retreat
The additional water flowing down the capturing stream may accelerate erosion and encourage the development of a canyon (gorge).
The now-dry valley of the original stream is known as a wind gap.