Theory:

Biological Weathering
Living organisms are the agents of Biological weathering. Here, weathering occurs due to the penetration and expansion of plant roots, earthworms, burrowing animals (rabbits, rats) and some human activities.
 
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Gradation
Gradation is the process of removing unevenness or levelling of highlands through erosion and filling up of lowlands through deposition. Agents of erosion are rivers, winds, groundwater, glaciers, and sea waves. These agents, in due course of time, on their travel path, create various gradational relief features.
Gradation takes place via degradation and aggradation.
 
Gradation or denudation is the process of wearing away the earth causes a general lowering and levelling out of the surface.
Aggradation refers to an increase in the level of land by various natural agents due to the deposition of sediments.
Degradation refers to the lowering of a landform through the erosional activities of various natural agents.
Gradation = Weathering + Erosion + Transportation + Deposition
Weathering: Gradual disintegration of rocks by atmospheric or weather forces
Erosion: Active wearing away of the earth's surface by moving agents like running water, wind, ice and waves
Transportation: The removal of the eroded debris to new positions
Deposition: Dumping of the debris in certain parts of the earth, where it may accumulate to form new rocks
Agent of Gradation: Running water (River) 
Among all other agents of gradation, the work of running water is the most extensive. Rivers generally originate on higher landforms like mountains, plateaus and hills that receives water from various sources like rain, glaciers, springs, lakes, etc. The origin of the river is called its source. It runs through various features like valleys, plateaus, plains and finally drains into the sea or any water body. The place where it joins the sea is known as its mouth.
Depending on the speed and landforms through which they travel etc., the river is divided into three courses. Let's see about the various courses of rivers in the following unit.