CONTENTS-
Abstract
Acknowledgement
References

Management of lotic waters into lentic ecosystem has become a necessity to mankind's endeavour to have better quality of life. Since a few decades conversion of natural ecosystem into manmade or engineered ecosystem has increased tremendously as the agrarian and industrialized societies need water for their multifarious activities.   It is estimated that up to 1980, about 4000 large reservoirs (ICSU-SCOPE 1972) and about 800 000 small reservoirs (WWF 1999) worldwide are serving mankind.

Manmade water bodies reflect both the terrestrial and aquatic environments as their construction involves superimposition of lentic over the lotic ecosystem which in its turn over the terrestrial. The establishment of manmade reservoirs, mostly multipurpose creates waterlogged areas, sometimes on unstable substrate with special plants and animal communities turning the river into wetland system.

With the current developmental phase every ecosystem is changing fast and exert immense influence on the flora and fauna, geographical and geological structures, hydrological features may be flood related, environmental related which in itself may be natural or artificial, technology related or co-inhabitant related. The construction of the reservoir may be for flood control, hydropower generation, irrigation or industrial supply or as coolant water recipient body.

Modification of running waters into standing water bodies creates water logged areas around which human settlements increase using this system as a source of livelihood based on either agriculture, horticulture, pisciculture or other diverse livelihood activities. This results in anthropogenic interference    around the ecosystem. The water quality depends on the geomorphic nature of the catchment areas, activities surrounding the catchment areas, point and non point source of contamination and anthropogenic activities. Many of the chemicals chemisorbs at the bottom sediment and slowly interact with the overlying water mass.

Classification of the reservoirs has been attempted on the basis of various factors such as morphometry, hydrology, geomorphology, depth, biota, and ownership. Majority of them are multipurpose and multifunctional. Retention time of the water in the reservoir is one of the important aspects for various physical, biological and chemical functions.

When reservoirs are made, it turns into a fresh water ecosystem generally virgin except for the main track of the river basin. The surrounding terrain as well as the original biota especially soil microorganisms and litter play a significant role in the further evolution and succession of the aquatic system.

The reservoir initiates a new set of socio-economic scenario to the population along the area thus moulding a new way of life. The impact of this man made system and the need to sustain it for the socio economic situations of the stakeholders are being discussed with suitable example of Hemavathy reservoir, which is constructed along the confluence of the Hemavathy and Yagachi, the two tributaries of River Cauvery.

Role of drainage basin of the    reservoir play a prime role in the ecology, economy and aging process of the water body. We have ample examples where the major reservoirs have become defunct so for as the primary objective for which they are built. Mainly this is due to the sedimentation process over the years. Sedimentation of the reservoir primarily affects depth, water holding capacity, biological features and life for phytophils and lithophils. Habitat- animal interaction and compatibility is lost in many cases pushing some of the species into endangered list. Productivity features are lost and water quality becomes poor. Diagnostic study of the reservoir may reveal the health of the ecosystem, status of ecosystem functioning and environmental use of this natural resource.

Reservoir sedimentation may take place due to many factors. The natural filling up is due to bank erosion, may be due to wind force or other external agencies, turbid water inflow from the catchment areas and tributaries, land use pattern along the watershed areas favoring topsoil removal etc. are other factors.

Many restoration techniques are available which need to be employed depending on the causative factors and end use of the ecosystem and societal values of the ecosystem. There are many possible methods to enhance the environmental values so that different user agencies could put to maximum use of the ecosystem resources.

The socio-economic status of the communities residing around the reservoir depends on the natural resources it harbors and the sustainable yield it delivers. The reservoir site is mostly rural conglomeration without any industrial contaminants. The population is dependent on the agrarian and fishery activities. Hence any step in increasing productivity of the water body is a boon to the community. Many of our reservoirs are biological deserts except for the wild fish population. Proper understanding of the ecological characteristics and ecosystem functioning are of paramount importance before embarking any massive resource enhancement and fishery development activities. Perhaps the most significant threat and stress to these water bodies is the domestic sewage from the urban catchment areas which could be easily prevented provided proper planning along the catchments areas are undertaken with people's involvement and community participation.

The author gratefully acknowledges AICTE for awarding Emeritus Fellowship.

Madhyastha.M.N and S. Murugan, 1995 Icthyofauna of the sector of Cauvery River in the head region. In:   Advances in fish and wild life Ecology and Biology. Ed” Bansi Lal Kaul. Vol I, 155-158.

Murugan, S.1995. Ecological studies of Yagachi and Hemavathy rivers with special reference to the biology of Glossogobius giuris in the Hemavathy reservoir, Karnataka. Ph.D. Thesis submitted to Mangalore University.

EMERITUS FELLOW,
National Institute of Technology,
Surathkal, Mangalore 575025. India.
E-mail: vinyas@vasnet.co.in