Lakes, ponds and tanks, which act as storage of fresh water, mainly from rainfall, are also facing the problem of water pollution due to excessive withdrawal of water, city sewage and unhealthy cultural practices. An interesting study of pollution in Nainital Lake has been made by S.M. Das of the Deptt. of Science and Technology who found that the lake water was grossly polluted and highly silted with organic waste matter and toxic effluents as a result of which fish mortality had been as high as 80,000 to 100,000 per annum (Indian Express, 17.8.1982).
The bottom of the lake by deposition of different materials had shallowed by about 7.6 m and the depth of the lake water had been reduced from 27m to 19.8 m during the last 80 years. Similar problems are also faced by many lakes of the country such as Wular and Dal lakes of Kashmir, Chilka lake of Orissa, Kolleru and Polecat lakes of Andhra Pradesh, Powai lake of Maharashtra, and Ooty Lake of Tamil Nadu etc.
Due to growing urban and agricultural activities many ponds and tanks have lost their existence. This has not only affected natural drainage, aggravated the problem of floods and water logging but has deprived us from the storage of natural water which could have been used during lean season.