THE EROSIONAL WORK OF WATER

The erosional work of streams/rivers carves and shapes the landscape through which they flow.

There are 3 functions of rivers:-

a.Erosion

b.Transportation

c.Deposition

A river may erode in 4 ways

1.Abrasion/ corrosion:-

•Load carried by a river will grind against its bed and sides. This process slowly wears the bed and sides away.

2. Attrition

When thrown against the sides and bed of rivers, the load gets broken into smaller pieces.

EROSION

Water Erosion

• Rivers, streams, and runoff

3. Hydraulic action

The work of turbulence in the water.

Running water causes friction in the joints of rocks in a stream channel Joints may be enlarged

Loosened fragments of rocks get swept away.4. Solution/Corrosion

Certain minerals in rocks like limestone can be dissolved in water.

Rocks are then eroded.

Potholes: 1)Circular depressions on the river bed2)Formed by corrosion (abrasion)3)Most effective in flood conditions4)Pebbles which are trapped in hollows on the river bed are swirled about in turbulent/ fast flowing water Plunge pools:1)A large depression at the foot of a waterfall2)This depression is deepened by hydraulic action of the plunging water3)Condition: likely to form when the water plunges into less resistant rocks4)It may be further enlarged by rock debris swirling about by turbulent water at the base

Rapids:1)A series of very short and fast falls2)Condition: an area of alternating bands of resistant and less resistant rock Waterfalls: -1)Vertical flows of fast moving water flowing from great heights2)Formed by 2 ways: -    Due to unequal resistance of rocks  or faulting i)Unequal resistance- less resistant rocks are eroded more rapidly than resistant rocksii)As a result, there is a change in gradientiii)By faulting- displacement of rocks results in a difference in height between 2 rocks, water plunges downwards

ROCK WEATHERING

Relationship of velocity and sediment size to erosion

B. Transportation (4 ways)

1.Traction

Larger and heavier rocks/gravels are dragged or rolled along the bed.

2. Saltation (saltim: by leaps/jumps)

Smaller and lighter rock fragments and sand hop and bounce along the river bed.

At times, the distinction between traction and saltation may be difficult to determine

3. Suspension

Some of the load like silt and clay (fine-grained) will float along.

They may only be deposited when stream velocity reaches near 0.Turbulence in the water is crucial in holding a load of sediments.

4. Solution

Some minerals are transported in dissolved form.

Especially chemical solution derived from minerals like limestone or dolomite.

Water

• Water weathers rock by dissolving it

C. Deposition

A river will drop its load when:

a.Volume decreases

b.Speed decreases

A river’s volume decreases when

•Dry season

•Dry region with high evaporation

•Presence of permeable rocks

•Receding flood waters

 

Floodplains and Levees -->   A floodplain is a low-lying plain on both sides of a river that has repeatedly overflowed its banks and flooded the surrounding areas. When the floods subside, alluvium is deposited on the floodplain. The larger materials, being heavier, are deposited at the river banks while the finer materials are carried and deposited further away from the river. The larger materials at the river banks build upinto embankment called levees. Meanders1)Meanders are loop-like bends in a river. The water flows round the meander in a spiral manner. This causes erosion to take place on the outer bank and deposition on the inner bank.

2)Gradually, a steep river cliff is formed on the outer bank, making it concave in shape. On the inner bank, deposition of alluvial materials produces a gentle slip-off slope and the bank takes on a convex shape. 

OXBOW LAKES  

An oxbow lake is a crescent -shaped lake formed on a river when a meander has been cut through and abandoned.When a river meanders in very big loops, the outer bank is so rapidly eroded that the river cuts through the narrow neck of the meander. The river then flows straight through the channel. When deposition seals off the cut-off from the river channel, an oxbow lake is formed. It may silt up and eventually dry up  

Weathered rock

C. Deposition

A river’s speed decreases when

•It enters a lake

•It enters a calm sea

•It enters a gently sloping plain

The work of a river depends on its energy

Energy a function of

a.Volume of water

b.Speed of water flow (dependent on gradient)

Type of Deposition

• There is two types of Deposition

Alluvial fan:A triangular deposit of sediment that forms when water rushing down a slope loses its energy and abruptly slows at the bottom, depositing its sediment load.

Delta:A triangular deposit of sediment that forms when a stream or a river slows as it empties into an ocean, gulf, or lake.

Stream picture

SUBMMITTED BYKAUSHAL JANGID

KRISHNAKALI GHOSHPUSHPRAJ BARAIKAVINASH KAUSHIK

ROHIT GUPTAANAND SINGH

AJITESH CHATTERJEEANKIT BELE