Geology

Large-scale Excavation

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Science  11 Jul 2003:
Vol. 301, Issue 5630, pp. 145
DOI: 10.1126/science.301.5630.145a

Humans have become the greatest global geomorphic agents. Some modifications of the terrain are intentional—for example, building roads and dams, or removing mountaintops in mining. Nevertheless, these and many other projects are often accompanied by unforeseen and undesirable consequences, such as an increase in the number of floods in urban areas, that multiplies the impact of the original landscaping. Widespread deforestation, particularly in mountainous areas, is known to accelerate erosion, although the extent of the enhancement and the precise role of vegetation have been incompletely documented.

Hewawasam et al. have studied erosion in a river valley in Sri Lanka, where most of the mountains have been deforested in favor of agriculture. Using the sediment loads in rivers and cosmosgenic nuclide levels, which mark the amount of time that a mineral is exposed on the Earth's surface (and hence bombarded by cosmic rays), they show that soil is being lost at a rate of up to several thousand metric tons per square kilometer per year. This rate is almost 100 times the natural background rate in still-forested regions; over some areas, 1 m of soil disappears in just 250 years. — BH

Geology 31, 597 (2003).

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