ALTHOUGH many dams in the tropics have had negative health impacts on nearby populations, it is also possible that dams could have positive effects on local health. Such information was sought because it may be helpful in evaluating proposed dams, and in designing or operating them to avoid negative impacts. A recent field study was conducted to determine if the new 18GW hydro power project at Three Gorges dam on the Yangtze river in China will reduce the risk of establishing malaria and schistosomiasis around the 500km reservoir. Similarly an established hydro power dam in Africa on the upper Nile river was evaluated for its possible positive impact on a parasitic disease known as river blindness.

Health problems

Since World War II it has been recognised that the impoundment of tropical and subtropical rivers will create habitats for disease-bearing insects and snails. The Tennessee Valley Authority in the US recognised the risk from malaria mosquitoes and successfully included malaria prevention in the design and operation of its 17 reservoirs in the subtropical valley. Other diseases such as bilharzia or snail fever (schistosomiasis), rift valley fever and river blindness have appeared around numerous tropical reservoirs in Africa, Asia, the Americas and the Middle East.

There are ways to estimate the risk of insect and snail infestations around reservoirs from information on topography and water quality. The amount of habitat available around a reservoir for breeding of mosquitoes and snails can be measured in cubic metres of illuminated shore zone per metre of shoreline. The illuminated shore zone is the volume of shoreline habitat in which the bottom of the reservoir is illuminated by the sun and can thus support rooted vegetation (Figure 1). The depth of solar penetration depends on clarity of the surface waters. The rooted vegetation protects mosquito larvae from wave action and from predation by fish, and also provides food and cover for snails. Along the shore of a reservoir in which snails and mosquitoes flourish, the illuminated shore zone is usually greater than 1m3 of illuminated shore zone per metre of shoreline.

It is also possible to quantitate the mosquito habitat in the shore zone due to vegetation by calculating the length of intersection line that the vegetation creates as it emerges through the water surface and provides a meniscus at the plant-water-air intersection for attachment of mosquito larvae.

Asian schistosomiasis

In November 2004 Blue Nile Associates carried out a survey in boats along the entire 500km length of the new reservoir for China’s Three Gorges dam, including excursions up some of the Yangtze tributaries, to evaluate the suitability of the reservoir for the aquatic snails or malaria mosquitoes which spread Asian schistosomiasis and malaria in China.

Asian schistosomiasis, also known as snail fever, is transmitted by small amphibious snails to people working in water, primarily during rice irrigation. The Asian form of schistosomiasis is also found in the Mekong river valley, and has been a problem in the rice fields of Japan, the Philippines and Indonesia.

Ecological studies by parasitologists at Tongji Medical University in Wuhan, China found that the climate of the Three Gorges area is suitable for the amphibious snails. Thus if the snails found suitable sites to colonise they would reproduce during the warmer months of April to August.

Upstream of the Three Gorges reservoir, Chengdu and other communities in Sichuan have classically been important foci for schistosomiasis. Scientists at the Hubei Institute of Schistosomiasis Control determined that the snails attach to floating wood and other materials and can travel long distances. Thus, although schistosomiasis was not a problem in the rapidly flowing river passing through the Three Gorges prior to construction of the dam, the snails could float down from Chengdu and could theoretically reproduce in the Three Gorges if they found suitable sites along the shores of the quiet reservoir.

However the walls of the reservoir on the Yangtze river at the normal pool level of 175m are all very steep. Along the entire reservoir, even as far up as Chongqing, the walls are too steep for walking. Even the flattest slopes in mid-reservoir near Fengdu where limited terraced agriculture was practiced, are steep enough to make walking difficult; in one of the tributaries on the left bank near Wushan, the reservoir walls are nearly vertical. There would be no possibility of snail habitats along these steep shores, as the snails require flat and stable swampy areas to colonise, with rooted vegetation for protective cover and food.

If one were to calculate the amount of illuminated shore zone along even the flattest shoreline near Fengdu where the slope might be at a 45º angle (1:1 slope), given the murky conditions of the reservoir waters and thus sunlight penetration of only about 0.1m, the illuminated shore zone would be only 0.005, well below the requirement for a favourable mosquito or snail habitat (one or greater). In the steeper areas near the Three Gorges, the illuminated shore zone is less than 0.0001.

It is reassuring to determine from this survey that the new reservoir would not become a suitable habitat for the amphibious snails, and that new foci could not be established in the reservoir by snails floating down from Chengdu, because of the steep slopes of the reservoir walls at normal pool level. Thus, unlike those in reservoirs behind dams in West Africa such as Volta Lake in Ghana and Diama dam on the Senegal river, snails coming downstream into the Three Gorges reservoir would not be able to provoke epidemics of this parasitic disease on the reservoir shores.


The anopheline mosquitoes which transmit malaria require protected shorelines for breeding. A steep reservoir wall would not support vegetation to protect them from lethal pounding by waves, nor would it provide protection against fish predation.

An additional factor that makes mosquito breeding unlikely along the steep shoreline is the large amount of heavy boat traffic, especially the high-speed hydrofoil ferries. Wake from these fast boats caused waves to pound against the shoreline frequently, interrupting any chance for the mosquito eggs to mature.

In summary, primarily because of the steepness of the walls of the main reservoir and tributary streams, there is no risk of the establishment of snail fever or malaria in communities along the reservoir of the new Three Gorges hydro power project. However, this analysis does not apply to communities downstream of the dam, where there might be risk of additional disease due to the changes in seasonal flows.

Owen falls dam

Another important parasitic disease around tropical dams is river blindness, found in Africa, along the Red sea in Arabia, in Central America and in parts of South America. The rapids of the upper Nile river used to be a classic focus of this blinding parasite, spread by the bite of a species of blackfly which breeds in white-water habitats, and on dam spillways.

During the health impact assessment in 1991 for a new dam proposed at Bujagali Falls on the upper Nile river in Uganda, the history of river blindness can be traced to the Owen falls dam, slightly upstream of Bujagali Falls (Figure 2). This dam flooded out Owen Falls and also Ripon Falls near the outlet of Lake Victoria. The impact from the Owen Falls dam over the last 50 years gave a reliable indication of what would happen with the new dam proposed for Bujagali Falls.

River blindness

Unlike malaria and schistosomiasis, the transmission of river blindness is usually found along fast rivers or streams where white-water rapids and cascades are found. The species of blackflies which transmit this blinding parasite require well-aerated, high velocity flow to deposit their eggs, usually on rocks or overhanging vegetation. The larval stages are filter-feeders and need large flows passing their habitat to obtain sufficient food and oxygen for development. People along many rivers in Africa migrate out of the fertile river valleys because of the painful bites of the flies and the eventual blindness resulting from this parasite.

Prior to construction of Owen Falls dam, river blindness was endemic among the Buganda people who live downstream along the Nile river. Surveys in 1950 showed a prevalence of the parasite of 65%. It was then recognised that the large work-force needed to construct the dam would have to live near the river for several years, and thus would have to be protected from the disease. To protect the workers, weekly applications of DDT were made at the outlet of Lake Victoria, treating the entire flow during the construction phase, and eliminating the blackflies for at least 50km downstream. This massive application of DDT was successful, and the biting flies were not a problem for the construction workers.

However entomologists from Uganda noticed an interesting fact after the dam was completed in 1954 and the DDT applications were discontinued. Contrary to expectations, the blackflies did not return to their former habitats downstream of the dam. By 1974 the prevalence of the parasite had dropped to 0.2% among the populations along the river. Re-infestation must be continually occurring from surrounding sites, because the blackflies have long flight ranges, and can be transported hundreds of km by winds. If the blackflies could not re-establish their populations, there must have been a change in the basic habitat conditions in the river. In order to understand how this might happen, Blue Nile Associates investigated some of the hydraulic and hydro-ecologic changes that occurred as a result of the dam.

The dam had two major hydraulic effects on the river. The turbines extract power from the flow, thus resulting in lower velocities downstream. Also there is no significant seasonal variation in power production, thus the downstream flow pattern was steady, whereas prior to the dam it had been seasonal, affected by heavy rainfall and slight variations in the water level in Lake Victoria.

The major hydro-ecologic effect of Owen falls dam was the establishment of a stable and tranquil reservoir due to the complete submergence of Owen falls and Ripon falls upstream of the dam. These falls were undoubtedly favourite sites for blackfly breeding, and their permanent submergence eliminated the breeding completely.

Power extraction and lower velocities

The power capacity of Owen Falls dam before the extension was added in 1992 was 150MW, the major electrical power source for Uganda. Extracting 150MW from this flow would cause a decrease in velocity downstream. The preferred range of water velocity for breeding of the East African species of blackfly involved in River blindness is between 0.5m/sec and 3m/sec. At present the mean velocities downstream of Owen Falls dam are between 0.4m/sec and 0.5m/sec, slightly less than the required velocities. Based on simplified calculations of changes in water slope due to the dam, it is estimated that prior to construction of the dam the mean velocity was probably twice the present velocity, and thus highly suitable for blackfly breeding.

There are waterfalls and cascades in several locations along the Nile river from Owen falls downstream to Lake Kyoga. Water velocities over these cascades would remain very high due to the vertical fall, even with the extraction of 150MW of power at the dam and lower mean velocities in the river. The absence of blackfly breeding at these cascades would indicate that seasonal variations in flow formerly touched off the blackfly breeding. The elimination of these seasonal variations by the dam could have eliminated the seasonal breeding of the blackflies as well.

A third important effect of Owen Falls dam must have been the elimination of the breeding sites at Owen Falls and Ripon Falls, which were submerged by backwaters from the dam. If, by chance, these were the primary breeding sites for blackflies in the area, their elimination might have been enough to prevent a persistent population of blackflies.

Thus because it decreased the river velocities, eliminated seasonal breeding conditions in the Nile river, and eliminated two major breeding sites, the Owen falls undoubtedly contributed to the elimination of this blinding disease downstream. This is a clear example of a dam with a positive health impact. The changes in downstream velocity conditions should prove useful guidelines for developing operational methods to control blackflies in proposed and also in existing dams, near blackfly habitats.


This evaluation of two examples of positive health impacts, from the dams at Three Gorges on the Yangtze river and at Owen Falls on the upper Nile river, showed that an evaluation of shore slopes around proposed reservoirs, and also an evaluation of power and velocity changes in flow downstream of proposed dams should be useful for estimating the health impact of dams proposed for tropical rivers, and also for developing engineering control measures for the operation of existing dams.

Author Info:

Dr. William R. Jobin, Sc.D., Director Blue Nile Associates, 25558 Road N.6 Cortez, Colorado, 81321, US. Tel: +1 970 565 8331. Email: