Theory question 307
Rivers and River Landforms(a) List three features that can be found in the middle course of a river. (b) Describe the characteristics and mode of formation of pothole.
Show the model answer
Model answer
(a) Meanders; river cliffs and slip-off slopes; bluffs; small flood plains; open V-shaped valley; confluences; tributaries.
(b) Characteristics: round, cylindrical holes in the rocky bed of a river; found in the upper course; sizes from a few cm to several metres; often contain pebbles. Formation: pebbles and boulders are trapped in hollows of the river bed; the swirling current (eddies) spins them round; they grind the rock by abrasion (drilling action); the hollow is deepened and widened into a pothole; worn pebbles are replaced by new ones.
Explanation
Describe the drilling action of pebbles.