(i) The river channels of the upper course has more energy gradiant then the channels of lower course.
(ii) As the river flows in the lower course it corrodes sideways and makes it wider.

(i) Limestone is made up of Calcium Carbonate.
(ii) It dissolves in water easily.
(iii) So during rainfall the water at the joints and fractures of the limestone seeps easily underground.
(iv) So the underground water is more common than the surface run off.

(i) As the desert are with little or no rainfall, wind is the only gradational agent.
(ii) Mechanical weathering is the dominant activity as there are limited plants.

Deposition occurs when the speed of wind is reduced by the presence of obstacles like bushes, forests and rock structures. The sediments carried by wind get deposited on both the wind ward and leeward sides of these obstacles Some of the depositional landforms are sand dunes, barchans and loess.
Sand Dune: In deserts, during sandstorms, wind carries loads of sand. When the speed of ·wind decreases, huge amount of sand gets deposited. These mounds or hills of sand are called sand.dunes. There are different types of sand dunes
Barchan: Barchans are isolated, crescent shaped sand dunes. They have gentle slopes on the ·windward side and steep slopes on the leeward.side
Transverse Dunes: Transverse dunes are asymmetrical in shape. They are formed by alternate slow and fast winds that blow from the same direction.
Longitudinal Dunes: Longitudinal dunes are long narrow ridges of sand, which extend in a direction parallel to the prevailing winds. These dunes are called Seifs in Sahara.
Loess: The term loess refers to the deposits of fine silt and porous sand over a vast region.
Extensive loess deposits 'are found in Northern and Western China, the Pampas of Argentina, in Ukraine and in the Mississippi Valley of the United States.

GLACIERS:
(i) A Glacier is a large mass of ice that moves slowly over the land from its place is called River of ice.
(ii) A large mass of ice creates pressure at its bottom and generates heat due to this it melts and starts to move.
TYPES OF GLACIER:
(i) Glaciers are broadly divided into two types based on the place of occurrance such as continental glacier and valley glacier.
CONTINENTAL GLACIER:
(i) This sheets of ice that covers vast areas of a continent is called continental Glacier.
VALLEY GLACIERS:
(i) The glacier which takes its origin from a snow covered mountain range is known Valley Glacier.

Most erosion takes place due to the process of solution. When rain water mixes with carbondi- oxide and enters into a limestone region, it dissolves and destroys much of the limestone.
As a result, landforms such as Terra rossa, Lappies, sinkholes, swallow holes, dolines, uvalas, poljes, caves and caverns are formed. 
Terra Rossa (Italian term for Red soil): Deposition of red clay soil on the surface of the Earth is due to the dissolution of limestone content in rocks. The redness of the soil is due to the presence of iron oxide.
Lappies: When the joints oflimestone rocks are corrugated by groundwater, long furrows are formed and these are called LAPPIES.
Sinkhole: A funnel shaped depressions formed due to dissolution of limestone rock is called sinkholes. Their average depth ranges between three and nine meters.
Caves and Caverns: Caves and caverns are subterranean features of karst topography. Caves are hollows that are formed by the dissolution of limestone rocks when carbon di oxide in air turns into carbonic acid after its reaction with water. They vary in size and shape. Caverns are the caves with irregular floors. e.g., Guptadham caves in Western Bihar.
All types of deposits in the caves and caverns are collectively called speleothems which
(i) includes travertines,
(ii) tufa,
(iii) dripstones.
Swallow Holes, Uvalas, Dolines, Poljis are other erossional Features of karst regions predominant in other parts of the world.

Weathering is the disintegration and decomposition of materials of the earth's crust by their exposure to atmosphere.
There are three types of weathering,
(a) Physical weathering,
(b) Chemical weathering
(c) Biological weathering
Physical weathering: It is the breakdown of rocks without changing their chemical composition, through the action of physical forces. The constant freezing and thawing of rocks during the night and day leads to the expansion and contraction of rocks. Cracks are formed and disintegration occurs eventually. Exfoliation, block disintegration, granular disintegration etc., are the different types of weathering.
Chemical weathering: Disintegration and decomposition of rocks due to chemical reactions is called Chemical Weathering. This is predominantly high in the hot and humid regions such as the equatorial, tropical and sub tropical zones. Chemical weathering takes place through the processes of oxidation, carbonation, solution, and hydration. The agents of Chemical weathering are Oxygen, Carbon-dioxide and Hydrogen.
Biological weathering: Biological weathering occurs due to the penetration and expansion of plant roots, earthworms, burrowing animals (rabbits, rats) and some human activities