Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 49

2 Vegetation-Erosion Dynamics
Abstract
In a broad sense, all rivers develop from rills and gullies in the process of erosion. Erosion is classified
according to agents causing erosion into water erosion, wind erosion, gravity erosion, glacier erosion and
cultural erosion. Water erosion occurs in different forms: splash erosion, sheet erosion, rill erosion, gully
erosion and channel erosion. Grain erosion is defined as the phenomenon of breaking down of bare rocks
under the action of sun exposure and temperature changes, detachment of grains by wind, flow of grains
down the slope under the action of gravity, and accumulation of eroded material at the toe of the mountain
forming a deposit fan. Vegetation is the most important factor affecting the erosion process and development
of rills. Vegetation, fluvial-geomorphic processes and landforms are inextricably interconnected parts of
the landscape. Vegetation-erosion dynamics studies the laws of evolution of watershed vegetation under
the action of various ecological stresses. Vegetation and erosion are a pair of competing and mutually
interacting aspects of a watershed. For a watershed, vegetation and erosion may reach an equilibrium
state if the circumstances remain unchanged for a long period of time. However, the equilibrium may not be
stable. Ecological stresses, especially human activities, may disturb the balance and initiate a new cycle of
dynamic processes. Studies considering both geomorphology and vegetation in the watershed are uncommon
yet may provide important information regarding geomorphological and ecological processes.
Key words
Agents of erosion, Water erosion, Ecological stresses, Vegetation-erosion dynamics, Riparian vegetation
2.1 Erosion
2.1.1 Agents of Erosion
According to Columbia Encyclopedia (Columbia University, 2000): erosion is generally defined as the
processes by which the surface of the earth is constantly being worn away. In other words erosion means
the detachment and removal of solid particles from their original place. Erosion is distinguished from
weathering, which is defined as the process of chemical or physical breakdown of the minerals in the
rocks, although the two processes may occur concurrently (Halsey et al, 1998; Wikipedia, 2009). According
to preliminary statistics compiled for the world, the annual erosion of surface soil from river basins amounts
to 60 billion tons, of which 17 billion tons are discharged into the oceans. In the process, as much as
5 million to 7 million ha of farms are annually ruined. Eroded soil contains nitrogen, phosphorous, and
other nutrients that deposit in lakes and reservoirs contaminating the waters resulting in eutrophication
and other biological as well as chemical processes.
The principal erosion agents are gravity, running water, glaciers, and wind. Human activity is also an
agent of erosion. Erosion overlaps with detachment of solid particles from the rock and mass movement,
or, the transfer of solid material down slopes. Weathering is also included as an erosive process. Over the
surface of the planet, the average rate of erosion is about 0.02 mm/yr. In some places the rate is much
higher, and in others it is greatly lower (Columbia University, 2000).
Erosion may be classified according to the erosion agents as water erosion, gravitational erosion,
glacial erosion, and wind erosion (eolian erosion). Water is the substance most readily associated with
erosion. Water erosion is also the most important form of erosion for river management. Various types of
water erosion are discussed in the next section of this chapter.
Gravity erosion —Gravity plays an important role in all forms of erosion. Gravity erosion itself is the