Large Dams Damage Health 171
Dams destroy the natural flow of the river. They cause either an increase or
decrease in water flow, depending on the dam. The natural cycle of flood and
drought may be disrupted, affecting the entire river and damaging huge areas
of land.
New insect breeding grounds
Mosquitoes breed in the shallow, sunny waters of irrigation canals, and at the
edges of reservoirs. Regularly raising and lowering the level of the reservoir
can kill young mosquitoes. But the people who manage dams do not usually
consider this important.
Black flies that spread river blindness lay their eggs in fast flowing water,
like the water that flows out of a dam. The still waters in dam and irrigation
projects are breeding grounds for snails that carry blood flukes (see page 56).
Erosion of riverbanks and floodplains
When a dam blocks a river, bits of soil and rock carried by the water (silt)
settle on the river bottom and in the reservoir instead of on riverbanks. When
water is let out of the reservoir, the water has no silt in it. Because silt is part
of what makes land rich for farming, downstream lands become poor. And
because water released from the dam collects silt as it moves, it further erodes
the land as it digs deeper into the riverbed.
Dams take water from rural communities.
Water once used by rural farmers and
people who fish is used instead to generate
electricity for cities and industry.
Crop yields decline. Dams flood
productive farmlands and block silt
from flowing downstream to fertilize
crops. Water released from dams can
wash out riverbank gardens.
Dams release polluted
water. Poor quality water
causes illness for people
and animals downstream.
A Community Guide to Environmental Health 2012