River Landforms In Upper Course
CAPTURE
- This is also known as river piracy or river beheading. Its development is dependent on the different rate of back-cutting (headward erosion) into a divide.
- For instance, if one side of the divide is of greater gradient or receives more precipitation than the other, stream A will cut back more rapidly than stream B. Its greater erosive power will succeed in enlarging its basin at the expense of the weaker stream. Stream A may eventually break through the divide and capture or pirate stream B.
- The bend at which the piracy occurs is termed as the elbow of capture. The beheaded stream (Z) is called the misfit. The valley below the elbow is the wind gap and can be valuable as a road and rail route.
RAPIDS, CATARACTS AND WATERFALLS
- These are liable to occur at any part of the river course, but they are most numerous in the mountain course where changes of gradient are more abrupt and also more frequent.
- Due to the unequal resistance of hard and soft rocks traversed by a river, the outcrop of a band of hard rock may cause a river to ‘jump’ or ‘fall’ down stream. Thus rapids are formed.
- Similar falls of greater dimensions are also referred to as cataracts.
- When rivers plunge down in a sudden fall of some height, they are called waterfalls. Their great force usually wears out a plunge pool beneath.
- Large and deep holes at the base of waterfalls are called plunge pools.
POTHOLES
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Over the rocky beds of hill-streams more or less circular depressions called potholes form because of stream erosion aided by the abrasion of rock fragments.
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Once a small and shallow depression forms, pebbles and boulders get collected in those depressions and get rotated by flowing water and consequently the depressions grow in dimensions.