The construction of a dam drastically changes the volume and seasonality of flow of water in the river downstream. Secondly, dams block the flow and deposition of nutrient-rich silt to the downstream area. These changes lead to serious adverse impacts on the downstream river, reducing fish migration and breeding, increasing the concentration of pollution in relation to freshwater, changing the composition of flora and fauna, and affecting land fertility adjoining the river ( White 1978). Such impacts have been documented in several dams the world over, e.g. the Aswan Dam in Egypt, and the Amu Daria Dam in the former USSR ( Goldsmith and Hildyard 1984).
While most dam projects are supposed to specify how much water they will release downstream, the SSP has made no such provision. The Gujarat Government did ask the NWDT to set aside 0.716 MAF every year for downstream releases (in addition to the water assigned to Gujarat), but the Tribunal declined to do so, indicating that Gujarat had to release downstream water from its assigned share. Currently, project plans call for Gujarat's entire share of water to be diverted into the irrigation system ( SSNNL 1989). However, a study on downstream impacts commissioned by the project authorities has indicated the need for regular downstream releases throughout the year ( Wallingford 1993). As far as we are aware, the project authorities are yet to suggest any concrete measures for this.
In the case of SSP, all studies to date suggest that fisheries are likely to suffer in the stretch below the dam ( MSU 1983; NCA 1993). In fact, the latest study has stated that "The eventual decline of hilsa (Hilsa ilisha) and giant freshwater prawn (Macrobrachiun rosenbergii) seems highly probable, certainly after Stage 2 (i.e. on full development of irrigation) ( Wallingford 1993) (words in parentheses added). After full development of irrigation, virtually no water will be released downstream except during the monsoons. The loss of this fishery would represent an annual loss of Rs. 40 to 80 crores to the fishermen (Wallingford 1993), and probably an additional Rs. 40 to 80 crores to the fishing industry, since retail costs are at least double that paid to fishermen. The hilsa in fact has already suffered heavy declines due to large dams in several parts of India ( Jhingran 1991 ). The Narmada'is also one of the important areas for India's best "sport" fish, the mahseer, which is also threatened with decline due to the dam. The possible impacts of the dam on all non-commercial aquatic fauna have been completely neglected in all the studies done to date. In fact, detailed studies on the ecology of even the hilsa and the giant freshwater prawn have not been done to date, though a general study on ecology of the lower Narmada has been commissioned.
Aquatic life is also likely to suffer due to the fact that there will be less water to dilute the pollution already being discharged into the river. In the Mahi river in Gujarat (about 40 km north of the Narmada), pollution is carried upstream twice a month (tidal effect), since the Mahi dam has substantially reduced river flow ( CD Patel 1993). The pollution load is also going to increase as irrigation from the SSP along the north bank and from Karjan dam along the south bank will cause greater use of pesticides and artificial fertilisers, though the Wallingford study (1993) says that at current levels of use, toxic effects are not a problem. In addition, the power and other outputs of the dam will encourage urban and industrial