River Landforms and Management Strategies

Erosional Landforms

100

Name the landforms formed by erosional processes

Waterfall, Gorges, Valley

200

Describe how a gorge is formed.

1) When a river flows through an area with resistant rocks by its sides, it can only erode its channel vertically. 2) Over time, a deep, narrow and steep-sided valley known as a gorge is formed.

300

Describe the formation of a waterfall due to the erosion of rocks of different resistance.

1) River flows across rocks of different resistance. 2) River erodes the less resistant rock more quickly and causes a change in the gradient of the river course. 3) Over time, the river plunges from a great height to the hit the river bed below with great force. A waterfall is formed as a result. 4) Repeated pounding of the river bed may leave a depression at the base of the waterfall known as a plunge pool.

Depositional Landforms

100

Name the landforms formed by deposition

Floodplains and levees

200

Draw a labelled diagram of a floodplain and its levees.

Must include: floodplain, levee made of coarse sediments, fine sediments, river

300

Why are the levees made of coarse sediments?

When the river overflows its banks, the river deposits the materials it is carrying on the land. Coarse sediments are deposited closer to the river edges forming levees as they weigh more.

Combination Landforms

100

Name the landforms formed by a combination of processes (eg. erosion, transportation and deposition)

Meanders, Oxbow Lakes, Deltas and distributaries

200

Draw a labelled diagram of the cross-section of a meander.

Must include: River cliff created on outer bank (concave slope), outer bank is undercut by erosion, eroded material; slip-off slope created on inner bank (convex slope), deposition of sediments

300

Which part of the river are deltas and distributaries found?

Lower course of the river, at the mouth of the river where it enters a sea or a lake.

400

How is an oxbow lake formed from a meander? You may use labelled diagrams to help you explain.

1) With constant erosion along the outer banks of the two meanders, they move closer to each other forming a loop. 2) The loop becomes more distinct and is separated by a narrow neck of land. 3) As deposition and erosion continues to take place along the inner and outer banks, the two meanders finally meet. 4) Sediments deposited will eventually dam up the water in the cut-off, forming an oxbow lake.

Management strategies

100

Name the channelisation methods.

River re-alignment, re-sectioning, bank protection and planting of vegetation

200

Describe planting of vegetation and 2 benefits.

Vegetation is planted along the river bank so that the roots will hold the soil together firmly and protect river banks from erosion. Benefits are: Vegetation reduces rate of surface run-off through interception; there is less erosion and less eroded material is washed into the river; roots of trees and plants hold soil together firmly and stabilises the river bank; effective long term solution; requires less maintenance than other methods hence cheaper.

300

Briefly describe the method of bank protection.

1) The method of bank protection includes building of artificial levees and dyles. They are walls of sand, stone or concrete built along river banks that often overflow their banks. It is to increase holding capacity of the river to hold water. 2) Concrete structures like gabions and revetments are built along the river banks to divert flow of water from river banks to centre of the channel. It protects the river banks from erosion, reduce amount of sediments that flow into the river.The capacity of the river increases and less flooding occurs.

400

Describe 2 disadvantages of river re-alignment.

1) Meanders may re-develop over time. 2) The speed of river increases after channelisation and volume of water flowing downstream is increased. The channel in the area downstream may not be able to contain the increased flow of water and flooding downstream may occur. 3) More sediments are carried downstream or washed into the sea or ocean. Increase in sediments in oceans reduce amount of sunlight reaching the seabed and affects growth of marine life.

500

What is re-sectioning and briefly describ 2 benefits.

Re-sectioning is the widening and deepening of the river channel. The river bank and beds may be smoothened by replacing soil with cement and granite. 1) It increases the channel's capacity to hold more water and 2) increases speed of the river that allows water to flow away quickly hence risk of flooding is reduced.