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 instead down 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. Capture mechanisms Tectonic uplift Barmah Choke: About 25,000 years ago, an uplift of the plains near Moama first dammed the Murray River and then forced it to take a new course. The new course dug its way through the so-called Barmah Choke and captured the lower course of the Goulburn River for . Barrier Range: The original course of the Murray River was to a mouth near Port Pirie where a large delta is still visible protruding into the calm waters of Spencer Gulf. An uplift of the land blocked the river near the southern end of the Flinders Ranges, and the river eventually found its way to a new mouth near Lake Alexandrina. Indus-Sutlej-Sarasvati-Yamuna: The Yamuna earlier flowed into the Ghaggar-Hakra River (identified with the Sarasvati River) and later changed its course due to plate tectonics. The Sutlej River flowed into the current channel of the Ghaggar-Hakra River until the 13th century after which it was captured by the Indus River due to plate tectonics. Glacial damming The River Thames in southern England originally entered the North Sea near Ipswich. About 450,000 years ago, an ice sheet expanding from the north pushed the course of the river southwards, forcing the Thames to cut a new mouth where the mouth of the River Blackwater, Essex now is, north of London. It later moved southwards again to its current position as a result of cutting through the Chiltern Hills at