Waterfalls and Rapids
Waterfalls and rapids occur when there is a sudden change in gradient of the river as it flows downstream. Waterfalls are more dramatic than rapids and may be a result of:
- A resistant band of rock occurring across the course of the river
- The edge of a plateau
- The rejuvenation of the area, giving the river renewed erosional power as sea level falls
They are formed by a band of soft rock being eroded more quickly than the band of hard rock above it. This creates a drop a river falls into it, eroding the bottom of the river, creating a plunge pool, thus creating a waterfall. Erosion continues and the waterfall retreats backwards and a gorge is formed.
Potholes are cylindrical holes drilled into the rocky bed of a river by turbulent high-velocity water 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 in the rock by abrasion. Potholes are generally found in the upper or early middle course of a river.
Braiding occurs when the river is forced to split into several channels separated by islands. It is most likely to occur when a river has variable discharges. The banks are formed from sand and gravel and are generally unstable and easily eroded. As a consequence, the channel becomes very wide in relation to its depth. Braiding also occurs in environments in which there are rapidly fluctuating discharges:
- Semi-arid areas of low relief that receive rivers from mountainous areas
- Glacial streams with variable annual discharge. In spring, melt water causes river discharge and competence to increase; therefore the river can transport more particles. As the temperature drops and the river level falls, the load is deposited as islands of deposition in the channel
Meanders part 1
Meanders are sinuous bends in a river. Alternating bars of sediment on the bed causes riffles (shallow sections) and pools (deeper sections). This varied depth changes from side to side as the meanders change direction. The corkscrew like motion where the water goes back on itself is called a helicoidal flow. Can get incised meanders which are either entrenched (vertical) and ingrown (lateral) and are formed by rejuvenation.
Meander part 2
- As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders.
- The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
- The force of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy due to decreased friction.
- On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction.
- Over time the horseshoe become tighter, until the ends become very close together. As the river breaks through, eg during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cut-off from the main channel. The cut-off loop is called an oxbow lake.
Oxbow lakes are formed by both erosion and deposition. 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 narrows. During times of high discharge, such as floods, the river cuts through this neck and the new cut eventually becomes the main channel and the former channel is sealed off by deposition, thus forming an oxbow lake.
In its middle and lower courses, a river is at risk from flooding during times of high discharge. If it floods, the velocity of the river falls as it overflows the banks. This results in deposition. The coarsest material is deposited first and forms raised banks called levees, along the side of the channel. More floods increase the size of these banks as more deposition occurs.
Floodplains are created as a result of erosion and deposition. They are the flat areas either side of the river. They are composed of alluvium- river deposited slits and clays. Over time, a floodplain becomes wider and the depth of sediment accretion increases. The width of a floodplain is determined by the amount of meander migration and lateral erosion that has taken place.
A delta is a feature of deposition, located at the mouth of a river as it enters a sea or lake. Deposition occurs as the velocity and sediment- carrying capacity of the river decrease on entering the lake or sea and the bedload of material is dumped. Flocculation occurs as fresh water mixes with sea water and clay particles coagulate due to chemical reaction. Deltas form when the rate of deposition exceeds the rate of sediment removal. In order for a delta to form the following conditions are likely to be met:
- The sediment load is large
- The coastal area where the river empties its load has a small tidal range and weak currents. This means limited wave action and therefore there is little transportation of sediment after deposition
The three types of deltas are arcuate, bird’s foot and cuspate deltas.