River landforms

Question 1

How is a V-shaped valley formed?

Answer 1

1. Rivers begin high up in the mountains so they flow quickly downhill eroding the landscape vertically.
2. The river cuts a deep notch down into the landscape using hydraulic action, corrasion and corrosion.
3. As the river erodes downwards the sides of the valley are exposed to weathering which loosens the rocks (some of which will fall into the river) and steepens the valley sides.
4. The rocks which have fallen into the river help the process of abrasion and this leads to further erosion.
5. The river transports the rocks downstream and the channel becomes wider and deeper creating a V-shaped valley between interlocking spurs.

Question 2

How do rapids form?

Answer 2

1. Alternating bands of hard and soft rock
2. The soft rock erodes faster than the hard rock.
3. This cause the river bed to drop and create turbulent water

Question 3

How does a waterfall form?

Answer 3

1. The river flows over bands of softer and harder rocks.
2. Softer rock is more quickly eroded.
3. The river undercuts the harder rock leaving an overhang.
4. The river forms a plunge pool below the waterfall using hydraulic action, abrasion and corrosion.
5. Overhanging rock is unsupported and falls into the plunge pool, abrasion is increased because of the material, attrition also takes place as the fallen rocks erode.
6. The waterfall is moved upstream.
7. This process continues and a gorge is cut back into the hillside.

Question 4

How does a meander form?

Answer 4

1. Water twists and turns around stones and other obstructions resulting in areas of slower and faster water movement.
2. The river starts to flow from side to side in a winding course but still in a relatively straight channel- helical flow.
3. Water moving faster has more energy to erode (hydraulic action, corrosion). This occurs on the outside of the bend and forms a river cliff.
4. The river erodes the outside bends through abrasion, corrosion and hydraulic action.
5. Water moves slowly on the inside of the bend and the river deposits some load, forming a river beach/slip-off slope.
6. Continuous erosion on the outer bank and deposition on the inner bank forms a meander in the river.
7. The meander will migrate downstream and change shape over time.

Question 5

How does an ox-bow lake form?

Answer 5

1. The river is meandering across the valley.
2. The river is eroding laterally (from side to side).
3. The river flows faster on the outside bends and erodes them.
4. The river flows slowly on the inside bends and deposits material so its course is changing.
5. Continual erosion and deposition narrows the neck of the meander.
6. Often during a flood the river will cut through the neck.
7. The river continues on its straighter path and the meander is abandoned.
8. New deposition seals off the ends and the cut-off becomes an oxbow lake that will eventually dry up.

Question 6

How does a levee form?

Answer 6

1. The river is on its floodplain with a very low gradient.
2. The river is mainly depositing (silt and sand).
3. When the river floods a lot of silt is deposited on the river banks and much less on the floodplain.
4. Frequent flooding continues this build up.
5. When the river is flowing normally it deposits in the river bed.
6. This raises the river above the floodplain.
7. Usually the built-up levées protect the plain from flooding.

Question 7

How does a floodplain form?

Answer 7

1. The Flood Plain is the area of alluvial deposits found beside the river in its lower course (often used for agriculture).
2. As meanders move slowly down the course of the river they erode away the valley to create a wide valley floor, and they deposit layers of alluvial material on the slip off slopes.

Question 8

How do deltas form?

Answer 8

1. Deltas occur where a river that carries a large amount of sediment meets a lake or the sea.
2. This meeting causes the river to lose energy and drop the sediment it is carrying.
3. An example is the Nile Delta in Egypt.

Question 9

What are the 2 different types of delta?

Answer 9

1. Bird’s foot delta: the flow of the river is stronger than the sea, so the distributaries carry the sediment out into the sea. eg. the Mississippi delta
2. Arcuate delta: shaped like a fan. When the sea has a strong current, and the sediment is kept close to the coast. eg. the Zambezi delta

Question 10

Niagara Falls

Answer 10

1. Length of brink: 2600 ft
2. Height: 167 ft
3. Volume of water: 6 million cubic feet per minute in peak flow
4. Hard rock of resistant Limestone. Underneath layers of softer Shale and Sandstone
5. The falls have a Headward Erosion rate of 1 metre per year.
6. Niagara has a deep gorge, over 11 km in length.
7. Two power stations (HEP) run off the tremendous power of the River Niagara’s flow.
8. The Horseshoe Falls are Canadian but 1/3 of the Falls actually lies in US territory
9. The falls lie 120km south-east of Toronto, Canada. They are 27km north-northwest of Buffalo, New York.
10. Niagara Falls consists of 3 major waterfalls, The Horseshoe, The American Falls and the smaller Bridal Vail Falls