Basically, dams are built on the upstream of the river, people built a huge wall to store the water inside it days by days. On the upstream of the water, they are forests ( 90%), so if they want to build such a large construction, theyhave to cut off the wall and it is the first negative affect of dams. Many trees are cut off in order to build dams, for examle, in my country-Vietnam, so as to build a hundred of dams, thousand of trees were cut down. And, obviously, it ruins a thousand of wild animal's habitants.

        Secondly, when they store a huge amount of water on the upstrem of the river, it causes some bad impact to the ecosystem in the river, especially is the downstram. Because the water cannot move to the downstream, they cannot bring the alluvial to the downstream. At the same time, water creatures face with problems when they want to move to the downstram of the river.

         So why people keep using the dams? Why don't they use the solar system electricity instead of using the dams? Actually the cost of those techniques are the same. I hope there will be a fortuantely changes in the future where worldwide nations destroy all their dams and start using other way to produce electricity.