Forests are storage mechanisms for water, both in the soil and in the organisms which live in them. Tree roots are essential to hold soils, which in turn are able to absorb vast amounts of rain water. With deforestation, surface runoff increases and flooding and silting of dams, rivers, and of river mouths often ensues. Another consequence of deforestation is that more water remains in the soil, as few deep roots remain to take it up, so that there is excess groundwater which enters streams, lakes and rivers, raising water levels. Serious flooding occurs regularly in areas where forests have been cut, in tropical as well as temperate areas. Deforestation in mountainous areas leads to swollen rivers, as water runs off the bare hillsides. These rivers overflow their banks, causing destruction to human habitation as well as to agriculture. Deforestation, coupled with the seasonal alternation of monsoon and dry season in Asia, has led to great changes in water flow from rivers, and, frequently, to flood or drought. This has encouraged the construction of more dams for flood control (which further alters the water flow). In Thailand, unrestrained logging had reached such levels by 1988 that the mountainous northern part of the country was engulfed in massive floods, with villages buried in mud slides and logs from clear-cut hills. In the Philippines, a similar disaster struck in 1991 when a typhoon triggered flash floods in areas which had been clear-cut by logging. Bangladesh is notorious for the recurrent floods which occur on lowlands cleared of trees by a growing population needy of fuelwood (Fornos, 1991; Sponsel, Bailey and Headland, 1996).

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