ABSTRACT
In general weed problems are more pronounced in lakes with nutrient-rich water; those with acidic, nutrient-poor waters suffer least. Downstream of a dam water-tables may well fall; upstream of a dam they are more likely to rise fed by leakage from the reservoir, irrigation channels and irrigated land and because the river is backed-up. Raised water-tables, altered diets, better access to medical care and improved communications all have long-term health impacts. The shallows of reservoirs, canals and water bodies in irrigated fields are potential breeding sites for mosquitoes which can transmit malaria, filarial infections and arboviral diseases like dengue haemorrhagic fever, yellow fever and viral encephalitis. The blackfly vector may breed where weirs, spillways or fast-flowing water in channels oxygenates the water; careful design of dam spillways and irrigation supply canals can do much to reduce such opportunities.
