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PHYSICO-CHEMICAL LIMNOLOGY OF A STRETCH OF NARMADA RIVER WITH A MAINSTREAM IMPOUNDMMENT. K. Sankaran Unni, Department of Botany, Govt. P.G. College, Chhindwara, M.P. 480001, India. As the World Bank International Monetary Fund and other organizations are assisting the controversial Narmada Valley Projects, environmental organizations are agitating for modification and alternative strategies. The ecology of the Upper Narmada basin has been investigated to understand river water quality and the effect of impoundment on the physico-chemical limnology of the river. From 24 degrees Celsius in the peak summertime, the water temperature increased to 32 degrees Celsius as the river descended from 1121 m above mean sea level to 445 m at Mandla. Narmada registered uniform mean pH of 7.9 and 7.3 mg 1-1 of chloride in the upstream and showed a decline after Bergi Dam. The BOD varied between 3.0 to 3.84 and COD between 14 to 29 mg 1-1. Domestic sewage at the upstream stretch at Amarkantak, Dindori, Mandla and other stations did not change the water quality. Bergi Dam had remarkable impact on the water temperature regime. Specific conductivity declined from 292 to 212 u mhos downstream after the impoundment. Carbonate, bicarbonate, phosphate, nitrate, calcium and magnesium declined downstream causing nutrient depletion after impounding, similar to other deep storage reservoirs. IMPACTS OF RIVER REGULATION ON FLOODPLAIN VEGETATION: A CASE STUDY OF RIVER YAMUNA AT DELHI (INDIA). Malavika Sah and Brij Gopal, School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067. River Yamuna, .the largest tributary of the river Ganges, is extensively regulated in its middle stretch. Soon after the river descends on to the plains, two canals take off withdrawing most of the water. Later, about 300 km downstream, at Delhi, it is regulated by three barrages at Wazirabad (drinking water supply), I.T.O. (for thermal power plant) and Okhla (diversion to Agra canal). Along its 25 km stretch through the city, artificial levees restrict the lateral spread to only about 200 to 500 m. Thus, the riparian areas immediately upstream of Wazirabad and Okhla barrages remain under water throughout the year with relatively small water level changes whereas the major section of the river through the city and downstream of Okhla remain nearly dry except during the rainy season. The large year to year variability in the amount and timing of rainfall in the upper catchment area of the river is responsible for further regulation of water flows. The river carries a heavy sediment load and the regulated discharge of water from .the barrages has also contributed to large accumulation of sediments particularly behind Okhla barrage, converting these shallow reservoirs into marshes. A detailed survey shows that in recent years, the natural riparian vegetation has changed considerably both in species composition and abundance of common species. The changes in the duration, frequency and amplitude of flooding have caused a decline of the woody Tamarax dioica end herbaceous Phra~inites %tar/ra whereas Typha angustata, Typha elephantina, Ipomoea fistulosa and species of Saccharu® have become dominant. Among other species which have spread over the past Pew years taking advantage of large areas of shallow stagnant water, are waterhyacinth, Salvsnia ~olesta and Alternathera philoxeroides. The riparian fringes subjected to frequent erosion are dominated by species of Rumex, Polygonum and other annual herbs. Preliminary observations suggest also the changes in the invertebrate fauna associated with the aquatic plants. Since the larger part of the floodplain has already been reclaimed and brought under human habitation, both native and migratory waterfowl have started concentrating in the impoundment behind Okhla barrage. The significance of the changes in the floodplain vegetation to the nature and magnitude of interactions between the river and its floodplain are briefly discussed. 14