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<Lakes and Reservoirs - Similarities, Differences and Importance>
Management of Lakes and Reservoirs
The Nature of Lake and Reservoir Water Problems
Because the drainage basin
is simultaneously (i) the source of water, (ii) the place where it is used, and
(iii) where human activities impact both water quantity and quality, the drainage
basin is the logical management unit for lakes and reservoirs. Activities that
generate pollutants (e.g., urbanization, industrialization, agricultural production)
are similar in both lake and reservoir drainage basins, whether from point or
nonpoint pollution sources. Point sources are ‘pipeline’ discharges of pollutants
to receiving waters, e.g. domestic sewage discharges or industrial waste effluents
from factories or plants. They are relatively easy to identify and isolate. In
contrast, nonpoint pollution results from storm runoff or snowmelt, which transports
polluting materials diffusely and over land in urban and agricultural areas to
rivers, lakes and reservoirs. Thus, non-point source pollution is closely tied
to precipitation and runoff events, and less predictable and more variable in
nature. Because of their diffuse nature, nonpoint pollutant sources are also more
difficult to identify and deal with.
Lakes and reservoirs are used for many, often competing human needs (Photo
17), including drinking, agricultural irrigation, industrial and cooling water
supply, sports or commercial fisheries (Photo 18), recreation and navigation.
Reservoirs are often constructed for the specific purposes of flood control and
power generation too (Photo 19). Accordingly, it is difficult to focus on a
single issue when considering the long term use and protection of these
important and finite water resources.
Of particular importance
in addressing lake and reservoir problems is the need to consider an “ecosystem
approach”. Here, rather than adopting the sectoral approach that focuses on a single
water use, the ecosystem approach considers both human water needs within the
larger context of the drainage basin and environmental water needs or ecological
requirements. This approach, therefore, is a prudent means of balancing the water
needs for economic development and environmental protection.
Virtually all lake and reservoir water problems are related either to issues
of (1) quantity – there is too little or too much water, or (2) quality –
the water is too degraded (polluted) for drinking water supply, agricultural
irrigation and/or industrial or other purposes. The problem of too little water
results either from limited precipitation or excessive water usage. In contrast,
the problem of too much water is typically manifested as floods. Solutions to
these problems, therefore, usually involve developing larger water supplies or
reducing current water uses, respectively.
Water quality problems typically involve water pollution issues. Major water
pollutants include a variety of organic and inorganic chemicals such as heavy
metals and industrial compounds. They can affect human health and/or interfere
with industrial or agricultural water use. If the level of a pollutant in the
water supply exceeds an acceptable level for a given water use (e.g., domestic
or industrial water supply), the water is considered unsafe or too degraded for
that use. Solutions to lake and reservoir pollution problems, therefore, usually
focus on reduction of pollution at the source and/or treatment of the polluted
water prior to use.
Considerations Unique to Reservoirs
Many processes in reservoirs
and downstream rivers are complex, long term and “non-traditional” from the perspective
of current understanding of lake and river limnology (Table 1). Some non-traditional
processes are because reservoirs represent an intermediate aquatic system, one
between a flowing river and a lake (Fig. 3), and our limnological understanding
of these processes is in its infancy. Nevertheless, each is of significant ecological
importance for reservoir management and downstream protection.
Because reservoirs are man-made water-bodies, they are more amenable to
artificial operation and regulation than lakes. As previously noted, operational
possibilities unique to reservoirs include the ability to (1) discharge known
volumes of water at predetermined times, and (2) selective discharge of water
from different water layers within the reservoir.
The presence of a reservoir in a drainage basin where no such water-body
previously existed significantly impacts on the watercourse, its flora and
fauna, and the human inhabitants in the drainage basin. These potential impacts
should be identified and thoroughly examined prior to reservoir construction, in
order to comprehensively assess the total value of the reservoir project.
Procedures to identify and properly evaluate potential environmental, social and
economic consequences of reservoir construction involve so-called ‘Environmental
Impact Assessment’ (EIA). Such an assessment is now obligatory by law in many
countries for all new dam construction.