Dams are massive barriers built across rivers and streams in order to impound water. Usually, they are built across valleys.
They are usually constructed because of the numerous benefits derivable from them.
Dams have been used to generate electricity in many parts of the world. The Hoover dam on the Colorado river supplies much of the electricity of Las Vegas in the USA while the Kainji and Shiroro dams in Nigeria generate a large volume of electricity which is supplied to a number of cities and villages. The provision of electricity has enhanced productivity in many industries hence improving earning power and thus purchasing power. This enables him to afford basic necessities of life like good nutrition and shelter which in turn promote good health. Electricity itself improves health care provision in the various health care facilities since many modern diagnostic and therapeutic equipments used in hospitals are also dependent on electricity for proper functioning.
Dams are also used for irrigation. By constructing a dam across a river, a large reservoir of water can be held back and later released at any time of the year to feed adjacent farms with resultant increase in food production which enhances nutrition and well-being.
Detention dams are dams which either stop or slow down the flow of water in a river. These help to control flooding. Thus farmlands are protected from flooding and destruction. The increased food production impacts positively on health.
Water is a basic necessity of life. Provision of portable water reduces the morbidity and mortality associated with water-borne diseases. Some dams have been used as sources of portable water supply. For example, the Ikpoba dam in Benin-City, Nigeria supplies 70% of the water supplied to to the city and its environs. Dams can also serve recreational purposes which also promotes good health. They can also be used for debris and silt collection for improved agricultural yield.
In spite of these benefits, dam construction also inadvertently creates some problems.
The downstream areas are deprived of nutrient-rich silt resulting in poor agricultural yield in these areas.
There may be species extinction. Dams have harmful effects on fish and marine mammals. Majority of dams do not include proper bypass systems for these animals, interfering with their life cycles and sometimes forcing species to extinction.
Though dams can be used to control flooding, they can also cause flooding with destruction of vast expanse of farmlands and animals with negative implications on health and nutrition.
Dams can also lead to spread of diseases. The containment of water in the reservoir and the irrigation channels promotes the build-up of water snails which are vectors of schistosoma. There may also be build-up of mosquitoes, since the stagnant water favours the breeding of mosquito larvae and pupae. This will increase the incidence of malaria. The larvae and pupae of the insect vector of onchocerciasis, the black fly thrive in highly oxygenated water attached to submerged vegetation and stones. The upstream area which is fast-flowing and well-oxygenated is thus a favourable breeding area. This may result in increased incidence of onchocerciasis.
Since dams have beneficial and deleterious effects on human existence it is necessary to do a proper evaluation before embarking on dam construction.