ABSTRACT
Erosion is a process that has operated since the earth was created. Erosion of soil by water has likewise been a process that has been an entirely natural phenomenon ever since soils first appeared. However, during the last few millions of years that humans have inhabited the earth, their activities have caused spasms of accelerated soil erosion associated with land cover and land use changes. In classical times, it was noted that slopes in areas such as Greece, Turkey, and the Levant had been destabilized by deforestation and overgrazing. Undoubtedly, such actions as the deliberate setting of fire, adoption of pastoralism and agriculture, deforestation, urbanization, and use of machinery to move and disturb the soil have all contributed to accelerating rates. Appreciation of the nature, causes, and consequences of soil erosion has a long history, and during the 20th century, there were some notable studies of the phenomenon[1-3] stimulated by such events as The Dust Bowl in the United States and the menace of donga (gully) formation in Africa.[4]
However, it has never been easy to separate the role of climatic fluctuations in causing soil erosion from the role of human activities, and this has spawned longrunning debates about the origin of phases of slope erosion, valley incision, and valley sedimentology in areas such as the Mediterranean basin[5,6] and the bottomlands of the western United States (see, for example, the discussion of arroyos[7]). Difficulties of determining how rates have been changed by human activities have also been bedeviled by an absence of direct long-term monitoring data. However, erosion leads to sedimentation, and so the study of rates of sediment accumulation in lakes, swamps, estuaries, reservoirs, and river floodplains provides a means of obtaining long-term data from which erosion rates can be inferred.
