- Created by: jack
- Created on: 11-05-11 19:55
Waterfalls and rapids
Occur when this is a sudden change in the gradient of the river as it flows downstream. Waterfalls are more dramatic features than rapids.
The river falls over a rock edge into a deep plunge pool at the foot of the fall, where the layers of weak rock are excavated more quickly then the overlying resistant rock. The force of the swirling water around the rocks and boulders enlarges and deepens the plunge pool by hydraulic action. This undercuts the resistant rock above. Eventually the overhang collapses and the waterfall retreats upstream, leaving a gorge ahead of it.
Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high-velocity waterr loaded with pebbles. The pebbles become trapped in slight hollows and vertical eddies in the water are strong enough to allow the sediment to grind a hole into the rock by abrasion. attrition rounds and smooths the pebbles caught in the hole and helps reduce the size of the bedload.
Occur when the river is forced to spilt into serveral channels separated by islands. It is a feature of rivers that are supplied with large loads of sand and gravel. It is likely to occur when a river has variable discharges. The banks formed from sand and gravel are generally unstable and easily eroded. As a consequence, the channel becomes wider in relationship to depth.
Meanders are sinuous bends in a river. In a low flow conditiond straight channels are seen to have alternating bars of sediment on their beds and the moving water is forced to weave around these bars. This creates alternating shallow sections and deeper sections. The swing of the flow that has been induced by the riffles direct the maximum velocity towards one of the banks, and results in erosion by undercutting on that side. Depostion takes places on the inside of the bend.
Are features of both erosion and deposition. An oxbow lake is a horseshoe-shaped lake separated from and adjacent river. The water is stagnant, and in time the lake gradually silts up, becoming a cresent-shaped stretch of marsh. An oxbow lake is formed by the increasing sinuosity of a river meander. Erosion is greatest on the outer bank, and with deposition on the inner bank, the neck of the meander becomes progressively narrower during times of high discharge.