Further Reading

Abrahams, A. D., and A. J. Parsons. Geomorphology of Desert Environments. Norwell, Mass.: Kluwer Academic Publishers for Chapman and Hall, 1994. Bagnold, R. A. The Physics of Blown Sand and Desert

Dunes. London: Methuen, 1941. Blackwell, Major James. Thunder in the Desert: The Strategy and Tactics of the Persian Gulf War. New York: Bantam, 1991.

Walker, A. S. Deserts: Geology and Resources. Denver: U.S. Department of the Interior, U.S. Geological Survey Publication, 1996.

erosion Erosion encompasses a group of processes that cause Earth material to be loosened, dissolved, abraded, or worn away and moved from one place to another. These processes include weathering, dissolution, corrosion, and transportation. There are two main categories of weathering: physical and chemical processes. Physical processes break down bedrock by mechanical action of agents such as moving water, wind, freeze-thaw cycles, glacial action, forces of crystallization of ice and other minerals, and biological interactions with bedrock such as penetration by roots. Chemical weathering includes the chemical breakdown of bedrock in aqueous solutions. Erosion occurs when the products of weathering are loosened and transported from their origin to another place, most typically by water, wind, or glaciers.

Water is an extremely effective erosional agent, including when it falls as rain and runs across the

Lavaka from severe soil erosion in Ankarafantsika Nature Reserve, Madagascar (© Pete Oxford/Minden Pictures)

surface in finger-sized tracks called rivulets, and when it runs in organized streams and rivers. Water begins to erode as soon as raindrops hit a surface— the raindrop impact moves particles of rock and soil, breaking it free from the surface and setting it in motion. During heavy rains, the runoff is divided into overland flow and stream flow. overland flow is the movement of runoff in broad sheets. overland flow usually occurs through short distances before it concentrates into discrete channels as streamflow. Erosion performed by overland flow is known as sheet erosion. streamflow is the flow of surface water in a well-defined channel. Vegetative cover strongly influences the erosive power of overland flow by water. Plants that offer thicker ground cover and have extensive root systems prevent erosion much more than thin plants and crops that leave exposed barren soil between rows of crops. Ground cover between that found in a true desert and in a savanna grassland tends to erode the fastest, while tropical rain forests offer the best land cover to protect from erosion. The leaves and branches break the force of the falling raindrops and the roots form an interlocking network that holds soil in place.

under normal flow regimes streams attain a kind of equilibrium, eroding material from one bank and depositing it onto another. small floods may add material to overbank and floodplain areas, typically depositing layers of silt and mud over wide areas. During high-volume floods streams may become highly erosive, even removing entire floodplains that may have taken centuries to accumulate. The most severely erosive floods are found in confined channels with high flow, such as where mountain canyons have formed downstream of many small tributaries that have experienced a large rainfall event. other severely erosive floods have resulted from dam failures, and in the geological past from the release of large volumes of water from ice-dammed lakes about 12,000 years ago. The erosive power of these floodwaters dramatically increases when they reach a velocity known as supercritical flow, at which time they are able to cut through alluvium like butter and even erode bedrock channels. supercritical flow cannot be sustained for long periods, as the effect of increasing the channel size causes the flow to self-regulate and become subcritical.

Cavitation in streams can also cause severe erosion. Cavitation occurs when the stream's velocity is so high that the vapor pressure of water is exceeded and bubbles begin to form on rigid surfaces. These bubbles alternately form, then collapse with tre mendous pressure, and form an extremely effective erosive agent. Cavitation is visible on some dam spillways, where bubbles form during floods and highdischarge events, but it is different from the more common and significantly less erosive phenomenon of air entrapment by turbulence, which accounts for most air bubbles observed in white-water streams.

Wind is an important but less effective erosional agent than water, particularly in desert or dry environments with exposed soil-poor regolith. Glaciers are powerful agents of erosion, and are thought to have removed hundreds of feet (meters) from the continental surfaces during the last ice ages. These moving masses of ice carve deep valleys into mountain ranges and transport eroded sediments on, within, and in front of glaciers in meltwater stream systems. Glaciers with layers of water along their bases, known as warm-based glaciers, are more effective erosional agents than cold-based glaciers that have no liquid water near their bases. Cold-based glaciers are known from Antarctica.

Mass wasting is considered an erosional process in most definitions, whereas others recognize that mass wasting significantly denudes the surface but classify these sudden events separately. These rapid processes include the transportation of material from one place to another, so they are included here with erosional processes. Most mass wasting processes are related to landslides, debris flows, and rock slides and can significantly reduce the elevation of a region, typically occurring in cycles with intervals ranging from tens to tens of thousands of years.

Humans are drastically altering the planet's landscape, leading to enhanced rates of erosion. Cutting down forests has caused severe soil erosion in Madagascar, South America, the United States, and many other parts of the world. Many other changes are difficult to quantify. urbanization reduces erosion in some places but enhances it elsewhere. Damming of rivers decreases the local gradient slowing erosion in upland areas, but prevents replenishment of the land in downstream areas. Agriculture and the construction of levees have changed the balance of floodplains. Although difficult to quantify, estimates suggest that human activities in the past couple of centuries have increased erosion rates on average from five times to 100 times previous levels.

See also deserts; glacier, glacial systems; mass wasting; weathering.

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