Stream capture

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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 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
  • 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

Capture mechanisms

Tectonic uplift

• Barmah Choke: About 25,000 years ago, an uplift of the plains near Moama on the Cadell Fault 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 500 km (310 mi).
• 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.^[1]

• Lake Bungunnia overflowed at Swan Reach and the current course is as a result of northward erosion. ^[3]

• 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 Goring-on-Thames, an event which created the Goring Gap.

Headward erosion

• The Teays River, captured by the Ohio River.
• The Rio Grande which before capture flowed into a closed basin, Lake Cabeza de Vaca, but after capture flowed into the Gulf of Mexico.
• The ancestral Niger River captured what is now the upper reaches of the Niger which once flowed into an endorheic basin to the east northeast of Timbuktu.^[4]
• The River Stour, Kent, largely captured by the River Beult /bɛlt/, River Teise /tiːz/ and others.
• The River Wey, in southern England, the western arm of which is the former upper waters of the River Blackwater (River Loddon).
• The
  River Rheidol in Wales
  which has captured the headwaters of other streams and now runs for part of its length in a deep gorge.
• The
  River Lyd in Devon
  , England.
• The Black River, in Kings County, Nova Scotia, Canada, captured by the Gaspereau River
• The
  Orinoco River that is currently in the process of capturing the upper reaches of the Orinoco.^[5]

• The Donauversickerung (
  spring, a tributary of the River Rhine
  .

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

Australian freshwater fish

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The formerly massive Great 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, the founder effect, genetic drift and natural selection, have become separate species (see allopatric speciation).

Examples include:

• Fitzroy river system, central Queensland
  ).
• Eel-tailed catfish (several rivers, northern New 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).
• Victoria
  ).
• Murray cod, whose eastern species/subspecies are:
  • Eastern freshwater cod (Clarence River system, northern New South Wales. It was also found in the Richmond 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.)
  • The Mary River cod (Mary River, southern/central Queensland.)
• The
  mountain galaxias species complex
  (multiple rivers, southern Queensland, New South Wales, Victoria).

Unfortunately, with the exception of eastern freshwater cod and Mary 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, the Richmond River cod and the Brisbane River cod, have become extinct.

See also

• Lake capture
• Niger River
• Misfit stream

References

Wikimedia Commons has media related to Stream capture.