ZIMSEC O Level Geography Notes: Meanders and Ox-bow lakes
- Meanders are pronounced bends in a river’s course
- They are formed when a river twists and turn in wide bends.
- They are common on the floodplain but can develop in any part of the river’s course.
- Meandering is a common behaviour of fluids that avoid a straight path to flow in a twisting and turning path.
- It is believed that meandering is a thermodynamics behaviour that maximizes velocity and reduces friction.
- Other experts have theorized that Meanders start when friction with the channel bed and banks causes turbulence in the water flow.
- This results helicoidal flow.
- This is a corkscrew like movement of the water as it spirals downstream from bank to bank as shown in the diagram above.
- This often occurs during floods and results in the formation of meanders and their associated features such as pools and riffles.
Pools and riffles
- Pool-this is a deep section in a meander where a lot of erosion takes place where the river’s energy builds up due to reduced friction and the water has higher velocity.
- Riffle-this is a shallow section in a river where there is deposition due to reduced capacity in a river resulting from energy dissipation (reduction) in a river due to increased friction and a reduction in a river’s velocity.
- The spacing of the pools and riffles are fairly regular in a river channel about six to five times the width of the channel.
- Helicoidal flow is responsible for the erosion on the outside bends and then depositing it into the inside bends of meanders.
Point bars and meander cross section
- Water flows fastest on the outer bend (concave bank) of the river where the channel is deeper and there is less friction.
- It erodes this bank laterally by attrition and hydraulic action.
- There also vertical erosion which deepens the channel, which reduces friction and increases in energy results in further erosion.
- The lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff these cliffs are also known as bluffs.
- the inner bend water is slow flowing, due to it being a low energy zone, deposition occurs resulting in a shallower channel.
- This increased friction further reduces the velocity (thus further reducing energy), encouraging further deposition.
- Over time a small river beach or runoff slope builds up on the inner bend.
- The greater erosion of the concave bank occurs just downstream of the axis of the meander bend, because the course of the maximum velocity zone in the channel does not reflect the meander shape.
- This causes meander to migrate down the valley.
- The lateral erosion of the meanders and their migration widen the flood plain.
- A point bar is a depositional feature made of alluvium that accumulates on the inside bend of streams and rivers below the slip-off slope.
- They are crescent-shaped and located on the inside of a stream bend of meanders.
- They show the former positions of a meander during its downstream migration.
- The term is sometimes used synonymously with slip-0ff slopes although the term slip-off slope is used to refer to the cross section and the term point bar is used to refer to the aerial view.
It is important to note again that meanders to not remain stationary but migrate downstream resulting in the widening of the floodplain