Soil erosion and floods

Soil erosion is one of the most serious threats to the sustainability of agriculture, silviculture, and forestry in Amazonia. Although many of the region's soils are deep, extending down several thousand metres in some cases, fertility is usually concentrated in the first few centimetres of topsoil. The need to protect the soil is a major reason that perennial crops, silviculture, and properly managed pastures are among the more viable options for rural development. Soil erosion is a contributing factor in the decision of many farmers to abandon their fields and clear a fresh plot from the forest.

Soil erosion can lead to larger-scale environmental problems by aggrading river beds. Increased run-off from cleared land deposits silt and sand on river beds, thereby provoking more severe floods. A rising and lowering of water levels along streams and rivers in Amazonia is part of the normal seasonal pulse of wet and dry seasons (Sternberg 1975). Some unusually heavy floods along the Amazon in the mid-1970s raised the spectre that deforestation in the foothills of the Andes was having a tangible impact downstream (Fowler and Mooney 1990: 106; Gentry and Lopez-Parodi 1980; Smith 1981a: 122). But statistical analyses of flood peak levels do not reveal any trend to more intense flooding along the Amazon (Richey, Nobre, and Deser 1989; Sternberg 1987b). The greatest flood ever recorded along the Amazon was in 1953, well before major development projects were unleashed in Amazonia. The Amazon River reached unusually high levels again in 1993, although deforestation rates have abated since the 1980s.

Similar findings have been made along the Ganges, where flooding in Bangladesh is not a result of deforestation in the Himalayas, as is commonly thought (Ives and Messerli 1989). An important variable in runoff from deforested lands is the infiltration rate of the soil (Newson and Calder 1989). Some compaction of the soil is likely after the forest is cut, but how adversely infiltration is affected over the long term depends on land management and soil type.

Destruction of forests along streams and some river banks is surely affecting water quality and flow on a local scale. But the vast scale of Amazonia's forests appears to be masking the impact of deforestation on smaller watersheds. Landscape changes are not currently radical enough to affect Amazon hydrology on a large scale (Bayley 1989). Also, the variability of rainfall in different parts of the Amazon basin could lead to premature conclusions that floods are more pronounced along the Amazon.