Off-site effects of soil erosion

In addition to its on-site effects, the soil that is detached by accelerated water or wind erosion may be transported considerable distances. This gives rise to 'off-site problems'.

Yangtse at Three Gorges
China's Yangtze River at the Three Gorges, in Hubei province. Note the sediment-rich water. (Source of photo: unknown)

Water erosion’s main off-site effect is the movement of sediment and agricultural pollutants into watercourses. This can lead to the silting-up of dams, disruption of the ecosystems of lakes, and contamination of drinking water. In some cases, increased downstream flooding may also occur due to the reduced capacity of eroded soil to absorb water.

Movement of sediment and associated agricultural pollutants into watercourses is the major off-site impact resulting from erosion. This leads to sedimentation in watercourses and dams, disruption of the ecosystems of lakes, and contamination of drinking water. Rates of erosion do not have to be high for significant quantities of agricultural pollutants to be transported off-site. This is a shorter-term impact than loss of soil quality; in the more affluent areas of the world it can be the main driver for present-day soil conservation policy initiatives. A more minor off-site effect can occur in situations where eroded soil has a decreased capacity to absorb water: increased runoff may lead to downstream flooding and local damage to property.

Yangtse Delta with sediment plume
A satellite view of the delta of the Yangtze River as it discharges into the East China Sea. The sediment plume is clearly visible. (Image: NOAA)

Another major off-site impact results from the agricultural chemicals that often move with eroded sediment. These chemicals move into, and pollute, downstream watercourses and water bodies.Where inputs of agricultural chemicals are high - as in the more affluent nations - costs of removing such pollutants from drinking water can be considerable.

Therefore the on-site impacts of soil erosion are a present-day problem for many of the developing nations. Such on-site impacts will be a problem only in the long term future for developed areas: as such they are outside the relatively short time horizon within which their policy makers work.

In the short term however, erosion's off-site effects can be a notable problem for developed nations. Off-site impacts may therefore be the major driver for policy changes in such countries.

Dave Favis-Mortlock, March 2005

For more information on erosion's off-site effects: