Stream capture

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Stream capture by headward erosion, leaving a wind gap

Stream capture, river capture, 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
The Maumee River basin. The Maumee, flowing north-east, has broken into part of another river's basin, capturing west-flowing streams and forcing them to reverse direction on entering it.

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[edit]

Tectonic uplift[edit]

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 500km.
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.

Glacial damming[edit]

The River Thames as it passes through the Goring Gap

Headward erosion[edit]

Karst[edit]

Effect on freshwater life[edit]

River capture is a shaping force in the biogeography or distribution of many freshwater fish species.

Australian freshwater fish[edit]

Australia provides a particularly fascinating series of examples of freshwater fish species and distributions resulting from river capture events. 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:

Olive perchlet (Ambassis agassizii), western carp gudgeon (Hypseleotris klungzingeri), pygmy perch (Nannoperca australis) and Australian 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 the Murray-Darling system.

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.

References[edit]

  1. ^ Tom L. McKnight; Darrel Hess (2005). "16, "The Fluvial Processes"". Physical Geography: A Landscape Appreciation (8th Edition ed.). Upper Saddle River, New Jersey: Pearson, Prentice Hall. p. 462. ISBN 0-13-145139-1. 

See also[edit]