Aim: To develop a knowledge and understanding of Rivers in

the landscape

By the end of the lesson I should know:

•New terminology for important parts of the river.

•The course of a river

Rivers in the landscape

• Where a river starts is called its SOURCE. This occurs in an upland area. The surrounding water/rain that feeds the river is known as the rivers CATCHMENT AREA. The river flows downstream by GRAVITY, the rivers that feed into it is known as its TRIBUTARIES. When the river finally ends and opens into the sea is known as the MOUTH.

CatchmentArea

Source

TributaryStreams

Mouth

The amount of water in a river is called its VOLUME. A rivers volume and speed gives it ENERGY to carry and erode soil and rocks. The soil and rocks a river carries are called its LOAD.

A river had three courses. Upper, Middle and Lower river course.

Upper Course Source

InterlockingSpurs

Middle Course

RiverBeach

RiverCliff

Lower Course

Ox BowLake

Meander

Delta

Upper CourseInterlocking Spurs

Waterfall

Gorge

Waterfalls – Stage 1

• Needs: River with a lot of energy to erode. Hard and soft rock.

Force of water starts toWear away the softerRock.

Hard Rock

Soft Rock

Waterfalls – Stage 2

• As the softer rock is not as durable it wears away quickly leaving the hard rock to start to stick out, forming the beginning of a waterfall Hard

Rock SoftRock

Harder rock wears away slowerSo therefore it protrudes out.

Waterfalls – Stage 3

• Eventually the harder rock will begin to crack and break off, this means the waterfall will move back into the hill and start forming again

Cracks appear in harderrock

V shaped Valley• In the upper course

of a river, water flows quickly through a narrow channel with a steep gradient; as it does so it cuts downwards. This vertical erosion results in a number of distinctive landforms including the steep sloping v-shaped valley through which the river flows in its upper course.

V Shaped Valley• 1. Vertical erosion (in the form of

corrosion, hydraulic action and corrasion) in the river channel results in the formation of a steep sided valley2. Over time the sides of this valley are weakened by weathering processes and continued vertical erosion at the base of the valley3. Gradually mass movement of materials occurs down the valley sides, creating the distinctive v-shape.4. This material is then gradually transported away by the river when there is enough energy to do so

V Shaped Valley

• As the river flows through the valley it is forced to swing from side to side around more resistant rock outcrops (spurs). As there is little energy for lateral erosion, the river continues to cut down vertically flowing between spurs of higher land creating interlocking spurs.

Middle Course

Middle River Course

• Here the river channel has become much wider and deeper as the channel has been eroded and the river has been fed by many tributaries upstream. Consequently, despite the more gentle gradient the velocity of flow may be as fast as in the uplands. As well as changes in the river channel, its surrounding valley has also become wider and flatter in cross-section with a more extensive floodplain

Meander

• Meanders form due to the greater volume of water carried by the river in lowland areas which results in lateral (sideways) erosion being more dominant than vertical erosion, causing the channel to cut into its banks forming meanders

Meander – Stage 1• Water flows fastest on the

outer bend of the river where the channel is deeper and there is less friction. This is due to water being flung towards the outer bend as it flows around the meander, this causes greater erosion which deepens the channel, in turn the reduction in friction and increase in energy results in greater erosion. This lateral erosion results in undercutting of the river bank and the formation of a steep sided river cliff.

Meander – Stage 2• On 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 beach of material builds up on the inner bend; this is called a slip-off slope

http://geobytesgcse.blogspot.com/2006/11/middle-course-of-river-meanders-ox-bow.html

• Remember - a meander is asymmetrical in cross-section (see diagram). It is deeper on the outer bend (due to greater erosion) and shallower on the inside bend (an area of deposition).

Formation of an Ox-Bow Lake

• As the outer banks of a meander continue to be eroded the neck of the meander becomes narrow and narrower.

• Eventually due to the narrowing of the neck, the two outer bends meet and the river cuts through the neck of the meander. The water now takes its shortest route rather than flowing around the bend.

• Deposition gradually seals off the old meander bend forming a new straighter river channel.

• Due to deposition the old meander bend is left isolated from the main channel as an ox-bow lake.

• Over time this feature may fill up with sediment and may gradually dry up (except for periods of heavy rain). When the water dries up, the feature left behind is known as a meander scar.

Lower Course of the River

• As a river continues its journey towards the sea, the valley cross section continues to become wider and flatter with an extensive floodplain either side of the channel. The river erodes laterally and deposition also becomes important. By the time it reaches the lower course the river is wider and deeper and may contain a large amount of suspended sediment.

Floodplain

• When the river floods over the surrounding land it loses energy and deposition of its suspended load occurs. Regular flooding results in the building up of layers of nutrient rich alluvium which forms a flat and fertile floodplain.

• When the river water bursts its bank, the shallower depth of water flowing over the surface results in frictional drag and a consequent reduction in velocity (speed) of flow. This results in the loss of energy and therefore deposition occurs. The heaviest materials are deposited first as these require the most energy to be transported and therefore build up around the sides of the river forming raised banks known as Levées. Finer material such as silt and fine clays continuing to flow further over the floodplain before they are deposited.