Multi-purpose river valley projects nearly always involve construction of a large dam. That provides flood control, electrical generation, and irrigation water for farms. The disadvantages are often ignored by developing countries. Communities along the river are flooded and must relocate. Excellent fields, orchards and forests may be lost. Fish lose natural habitat such as gravelly areas to lay eggs -- this is especially problematic in the American West. Migratory fish are destroyed because they can't navigate past the dam. The still water becomes warmer that moving water and this can increase unwanted flora (algae, moss, weeds) and unhealthy bacteria. Silt carried by the river deposits in the new lake and slowly fills it up. The sand and silt is essential for healthy beaches -- it is supposed to reach the sea, but ends up stuck behind a dam.