Deforestation increases damaging erosion and destroys a river's watershed.
Silt is trapped behind dams, reducing fertility downstream as well as the capacity and life-span of the dam. Downstream silt must be replaced by expensive chemical fertilizer. Dams always carry the danger of collapse due to earthquakes, flooding or sabotage. Casualties from dam collapse will be much higher than those from normal flooding. Silt is trapped behind dams, reducing fertility downstream as well as the capacity and life-span of the dam.