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<Lakes and Reservoirs - Similarities, Differences and Importance>

 Reservoir Development in the Future

Reservoirs represent an important component of the social and economic structure of both developed and developing countries. Reservoir construction is largely completed in North America and Europe, but continues in the developing world, and most new reservoirs in the future will be located in Asia, Africa and Latin America. In some cases it has been found that the environmental disbenefits of dams outweighs their economic benefit. Thus, in the United States, the federal government has refused to renew the operating licenses of some reservoirs in several locations throughout the USA.

Particular interest is now being directed to ongoing or planned construction of large dams. In some cases, these have incited serious public and international concern, as in the case of the Sardar Sarovar Dam in India (Photos 20 and 21). On the one hand, proponents of large dams say that they bolster local economies, improve energy supply and flood control, and help manage the world’s water resources more effectively. The opponents of large dams say that dams cause significant damage to the environment and the local culture, and produce little overall economic gain.

Table 1. Comparison of the general characteristics of lakes and reservoirs on a global scale

Lakes Reservoirs

Especially abundant in glaciated areas; orogenic areas are characterized by deep, ancient lakes; riverine and coastal plains are characterized by shallow lakes and lagoons Located worldwide in most landscapes, including tropical forests,  tundra and arid plains; often abundant in areas with a scarcity of natural lakes
Generally circular water basin Elongated and dendritic water basin
Drainage: surface area ratio usually  <10:1 Drainage: surface area ratio usually >10:1
Stable shoreline (except for shallow, lakes in semi-arid zones) Shoreline can change because of ability to artificially regulate water level
Water level fluctuation generally small (except for shallow lakes in semi-arid zones) Water level fluctuation can be great
Long water flushing time in deeper lakes Water flushing time often short for their depth
Rate of sediment deposition in water basin is usually slow under natural conditions Rate of sediment deposition often rapid
Variable nutrient loading Usually large nutrient loading their depth
Slow ecosystem succession Ecosystem succession often rapid
Stable flora and fauna (often includes endemic species under undisturbed conditions) Variable flora and fauna
Water outlet is at surface Water outlet is variable, but often at some depth in water column
Water inflow typically from multiple, small tributaries Water inflow typically from one or more large rivers

These conflicting points of view require major attention be given to balancing the beneficial and adverse environmental and socio-economic impacts to be expected with the construction of large dams (Table 2). This attention must begin early in the planning stage to insure that it is properly considered by all relevant parties and interests prior to initiation of construction activities. The Sanmenxia Dam on the Yellow River, China (Photo 24), provides an example of problems that were not sufficiently considered prior to dam construction. Finished in 1960, the goals of the reservoir construction were to prevent floods, provide water for irrigation, and produce hydroelectric power. However, significant silt loads in the Yellow River were not adequately considered in the planning sstage. The reservoir water basin was largely filled with silt only four years after construction, and the reservoir was subsequently taken out of operation. 

Another example is the construction of the Aswan High Dam (Photo 25) which impounded the Nile River. The dam has now been in operation for about 30 years, and based on a comprehensive assessment report of 1989, both positive and negative impacts have resulted from this reservoir project. The economic positive impacts include (i) an improvement of summer crop rotations and guaranteed availability of irrigation water for agricultural production, (ii) expanded rice cultivation (iii), conversion of about one million acres from seasonal to perennial irrigation, (iv) an expansion of about 1.2 million acres of new land due to increased water availability, (v) protection from high floods and droughts, (vi) generation of significant quantities of hydroelectric power, (vii) improved navigation possibilities, and (viii) increased tourism. The negative environmental and social impacts include (i) declining water-levels at Nile River barrages downstream of the dam, (ii) rising water-levels upstream of the Delta Barrage, (iii) increased riverbank erosion and river meandering, (iv) production of river channel scour holes downstream of existing river barrages, (v) decreased water quality due to increased industrial and agricultural discharges, (vi) increased reservoir siltation, (vii) increased reservoir eutrophication, (viii) increased water evaporation, (ix) increased coastal erosion at the mouth of the Nile River, (x) decreased human health due to increased incidence of schistosomiasis and spread of water-related vectors, and (xi) inundation of historical monuments.

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