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Stream capture,river capture,river piracy orstream piracy is ageomorphological phenomenon occurring when a stream or riverdrainage system orwatershed 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
• Naturaldamming, such as by alandslide orice sheet
• Erosion, either
  • Headward erosion of onestream valley upwards into another, or
  • Lateral erosion of ameander through the higher ground dividing the adjacent streams.
  • Within an area ofkarst topography, where streams maysink, or flow underground (a sinking orlosing stream) and then reappear in a nearby stream valley
• Glacier retreat

The cause is not always clear.

The additional water flowing down the capturing stream may accelerateerosion and encourage the development of acanyon (gorge).

The now-dry valley of the original stream is known as awind gap.

• Barmah Choke: About 25,000 years ago, an uplift of the plains near Moama on theCadell Fault first dammed theMurray 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 theGoulburn River for 500 km (310 mi).
• Indus-Sutlej-Sarasvati-Yamuna: The Yamuna earlier flowed into theGhaggar-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.^[1]
• Barrier Range: It was theorised that the original course of the Murray River was to a mouth nearPort Pirie where a large delta is still visible protruding into the calm waters ofSpencer Gulf.^[2] It was suggested that an uplift of the land blocked the river near the southern end of theFlinders Ranges, and the river eventually found its way to a new mouth nearLake Alexandrina. This has since been disproven in favour of findings that ancientLake Bungunnia overflowed at Swan Reach and the current course is as a result of northward erosion.^[3]

• TheRiver Thames in southernEngland originally entered theNorth Sea nearIpswich. About 450,000 years ago, anice sheet expanding from the north pushed the course of the river southwards, forcing the Thames to cut a new mouth where the mouth of theRiver Blackwater, Essex now is, north of London. It later moved southwards again to its current position as a result of cutting through theChiltern Hills atGoring-on-Thames, an event which created theGoring Gap.

• TheTeays River, captured by theOhio River.
• TheRio Grande which before capture flowed into aclosed basin,Lake Cabeza de Vaca, but after capture flowed into theGulf of Mexico.
• The ancestralNiger River captured what is now the upper reaches of the Niger which once flowed into anendorheic basin to the east northeast ofTimbuktu.^[4]
• TheRiver Stour, Kent, largely captured by theRiver Beult/bɛlt/,River Teise/tiːz/ and others.
• TheRiver Wey, in southern England, the western arm of which is the former upper waters of theRiver Blackwater (River Loddon).
• TheRiver Rheidol inWales which has captured the headwaters of other streams and now runs for part of its length in a deep gorge.
• TheRiver Lyd inDevon, England.
• The Black River, inKings County, Nova Scotia, Canada, captured by theGaspereau River
• TheCasiquiare canal is adistributary of theOrinoco River that is currently in the process of capturing the upper reaches of the Orinoco.^[5]
• The Saint Mary's and Saint Joseph rivers were captured by theMaumee River in present-dayFort Wayne, having previously flowed into theWabash River.

• TheDonauversickerung (Danube Sink), currently developing inGermany, where a large portion of the upper part of theDanube river sinks into thelimestone bedrock, and resurfaces in theAachtopfspring, a tributary of the RiverRhine.

TheSlims River was previously fed bymeltwater from theKaskawulsh Glacier in theSaint Elias Mountains in theYukon and its waters flowed intoKluane Lake and on to theBering Sea. Because ofclimate change, the glacier has rapidly receded and the meltwater no longer feeds the Slims. The water instead now feeds the Kaskawulsh River which is a tributary to theAlsek River and drains into theGulf of Alaska.^[6][7]

River capture is a shaping force in thebiogeography or distribution of manyfreshwaterfish species.^[8][9]

Geological uplift in the southern South Island led to the divergence of freshwatergalaxiid populations isolated by river capture.^[10][11][12]

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The formerly massiveGreat Dividing Range runs the length of the eastern coastline of Australia and has isolated native freshwater fish populations east and west of the range for millions of years. In the last two million years erosion has reduced the Great Dividing Range to a critical point where west-to-east river capture events have been possible. A number of native fish species that originated in the Murray–Darling river system to the west are (or were) found naturally occurring in a number of coastal systems spanning almost the entire length of the range.

None of the river capture events that allowed native fish of the Murray-Darling system to cross into and colonise these East Coast river systems seem to have formed permanent linkages. The colonising Murray-Darling fish in these East Coast river systems have therefore become isolated from their parent species, and due to isolation, thefounder effect,genetic drift andnatural selection, have become separate species (seeallopatric speciation).

Examples include:

• Golden perch (Dawson–Fitzroy river system, centralQueensland).
• Eel-tailed catfish (several rivers, northernNew South Wales). However, note recent genetic research which now indicates eel-tailed catfish colonised east coast drainages in multiple colonisation events relatively recently (by evolutionary standards) and may subsequently have colonised the Murray–Darling system via an east-to-west river capture event, contrary to usual west-to-east capture events listed here.
• Macquarie perch (Hawkesbury-Nepean rivers,Shoalhaven River, southern New South Wales).
• River blackfish (multiple rivers,Victoria).
• Murray cod, whose eastern species/subspecies are:
  • Eastern freshwater cod (Clarence River system, northern New South Wales. It was also found in theRichmond River system in New South Wales but that population is now extinct.)
  • Brisbane River cod (Brisbane River system, southern Queensland. That population is now extinct, and its exact taxonomic status is not known.)
  • TheMary River cod (Mary River, southern/central Queensland.)
• Themountain galaxiasspecies complex (multiple rivers, southern Queensland, New South Wales, Victoria).

Olive perchlet (Ambassis agassizii),western carp gudgeon (Hypseleotris klungzingeri), pygmy perch (Nannoperca australis) andAustralian smelt (Retropinna semoni) also appear to have made crossings into coastal systems, the last two species seemingly many times as they are found in most or all coastal streams in south eastern Australia as well as theMurray-Darling system.

Unfortunately, with the exception ofeastern freshwater cod andMary River cod, it has not been widely recognised that these coastal populations of Murray–Darling native fish are separate species and their classifications have not been updated to reflect this. Many are threatened and two, theRichmond River cod and theBrisbane River cod, have become extinct.

• Lake capture
• Niger River
• Misfit stream

Wikimedia Commons has media related toStream capture.

• Rivers
• Biogeography
• Fluvial geomorphology
• Hydrology

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