Lower & Middle River Course 18th April 2009

LOWER & MIDDLE RIVER COURSE

• Middle and lower courses of rivers have a higher discharge than the upper course because water from the rest of the drainage basin has drained into the river in its middle and lower course. • The river has a greater discharge and so has more energy to transport material. Material that is transported by a river is called its load. • The river is now flowing over flatter land and so the dominant direction of erosion is lateral. • Deposition is also an important process and occurs when the velocity of the river decreases or if the discharge falls due to a dry spell of weather.

Transportation Four processes: 1. Traction : Boulders and pebbles are rolled along the river bed at times of high discharge. 2. Saltation : Sand sized particles are bounced along the river bed by the flow of water. 3. Suspension : Fine clay and sand particles are carried along within the water, even at low discharges 4. Solution : Some minerals dissolve in water such as calcium carbonate. This requires very little energy.

Transportation

Suspension Solution

Traction

Saltation

LANDFORMS: Landforms found can include: (I) Meanders & Ox bow lakes (created by deposition & erosion) (II) Floodplains and Leveés, & Delta (created by deposition) (III) Braided channel (Created by deposition)

Characteristics of a Meander

Plan view of a meander

F a s t e s t f lo w

Meanders are bends in the course of a river channel. A

Cross-section through a meander A-B F a s tes t F lo w O u te r B e n d

Helicoidal flow

R iv er C liff (high velocity in deep water)

In ne r B e n d

Low velocity In shallow water

S lip O f f S lo p e

(point bar) U n d e rc u tting

(lateral erosion especially by Hydraulic action & corrasion)

(sand & shingle) A re a of d ep o s it io n

B

Formation: •

Appears as a river approaches middle course & gradient of channel becomes less steep.

•

Meanders are a result of helicoidal flow, in which fastest current spiral downstream in a corkscrew fashion.

•

This result is erosion on outside bend of meander to form a river cliff & deposition on the inside bend forming slip-off slope. (Point bar)

•

The material eroded from outer bank of a meander will spiral downstream & deposited on the inner bank building up to form point bar deposits.

Click this link to see animation on meander http://www.cleo.net.uk/consultants_resources/_files/meander4.swf

Riffle & pools Riffles: deposition of a coarse material that create areas of shallow water. (water velocity increases as it passes over riffled surface) Pools: areas of deeper water between riffles. ( flows more sluggishly out of pools) Pools and riffles developed in section along river channel which create different gradient of channel. Coarse pebbles create steeper gradient than eroded pools.

River cross-sections in a meander

Meander migration Meanders change their location over time hence the term ‘migrate’, move in 2 directions: (i) Sideways (ii) Downstream

(i) Migrate sideways • Over time a meander widens & the neck narrows. • This is due to erosion (lateral) on the outside of the meander forming a river cliff.

(ii) Migrate downstream • The meanders migrate downstream & the river cliffs ‘join up’ to form a line of river bluffs. • The point bar deposits, which are added to by silt deposited during flooding, build up the thickness of floodplain.

Ox-bow lake A crescent shape lake formed in a meander.

FORMATION 1. As meander moves downstream, one side meander catch up river channel downstream. 2. Eventually river may break through neck of meander cause major river diversion.

3. The river abandons the original meander channel in favour shorter steep new route. 4. Formation of cut-off occur at times high energy of the river, such as at peak discharge (bankfull condition/flood).

5. Reduced velocity at the entrance to former meander, especially when floodwater subside, results in deposition which seals off the meander to leave an ox-bow lake. 6. The water in ox-bow lake becomes calm resulting deposition of sediment and over time water in lake may dissapear through infiltration and evaporation to leave a meander scar.

Floodplains & levees

Floodplain & levees LEVEES: Levees are high banks of silt close to the river channel which are formed by repeated river flooding. It is common in lower course of a river where there is floodplain. Floodplain: It is flat area of land either side of river forming valley floor. They are composed of alluvium deposited by river and form fertile soil.

Formation of levees & floodplain. • When river floods, water overflows its banks & valley floor is shallower, velocity falls & result in deposition of bed. • Coarser material deposited first building up natural embankments along the channel called levees. • In time of low flow such as during a dry season river bed raised by deposition (silt and sand) aggraded (build up) river bed • Finer material such as sand and clays are deposited further from river to alluvium on floodplain

Delta • Deltas are areas of land at the mouth of a river jutting out into sea. • They are flat areas of land crossed by many stream channels called distributaries. • The distributaries are often flanked by levees. • The levees joined together by spits and bars sealing off shallow areas of water forming lagoons.

• Lagoons are gradually filled up with silt and sand to form marshes and eventually dry land colonised by vegetation. • This form a fertile land for farming and settlement, e.g. Deltas of the Ganges and Nile.

Conditions for deposition to occur: 1. River must carry large load, e.g. Mississippi river carries 450 m tonnes of sediment into its delta distributaries every year. 2. The material must be deposited faster than it’s removed by action of tides, waves and currents. 3. Most deltas occur in calm seas with a gently sloping sea bed.

4. The river meets sea which acts as a break slowing the velocity and encouraging deposition. 5. The salt in seawater on meeting river generates an electrical charge that causes particles to coagulate or stick together so increase their weight and encouraging deposition. This process is called flocculation. 6. The river floods frequently in lower course depositing alluvium in delta, build up levees & creating new distributaries.

Types of delta (A) Cuspate (tooth’s delta) : where material brought down by a river is spread out evenly on either side of its channel. E.g. Tiber

Bird’s foot delta. (C ) Bird’s foot: where the river has many distributaries bounded by sediment and which extent out to sea like the claws of a bird’s foot.e.g. the Mississippi.

Arcuate delta (B) Arcuate (fan-shaped delta) : having rounded, convex outer margin e.g. Nile.

Braided channel A braided stream has islands or eyots of deposited material within the channel. Braiding occurs in stream where load contains high proportion of coarser sands and gravel. Braiding is a characteristic of streams and rivers with very variable discharges common in Semi-arid environments, or glacier-fed streams.

Plan view of a braided channel

(i) In semi-arid environments High discharge: Torrential downpours lead to overland flow creating stream with high velocities and large loads. Low discharge: Rapid evaporation and infiltration following storm rapidly reduce volume and velocities resulting deposition of load.

(ii) Temperate climate: High discharge: Streams and river fed by glaciers have high discharges when there is rapid melting of ice during the day in Summer. Low discharge: At night and in winter.

However when velocity falls the stream’s competency and capacity are reduced. The large load is deposited forming the eyots and causing stream to divide into a series of smaller channels.