Dams are overall very helpful to us and are very ecofriendly compared to the rest of the methods of producing electricity. But when there are pros, there must always be some cons.

There are a few disadvantages to constructing a dam on a fast flowing/rapid water river. One reason is that as in many rivers, the Kelani River (Kitulgala river) has fish that migrate, of course with the dam built, there is no way for fish to migrate downstream and get to the bottom of the river, this means that the fish will have to adapt or change their breeding/spawning habits and location. But not all fish can adapt as fast and for most, the change might be too much and this would kill off a large amount of the population of fresh water fish in the river due to separation of spawning habitats.

Another disadvantage is that when there is a large dam blocking the stream, this would mean that sediments will get trapped above the dam and block it from flowing down stream. Sediment is critical for maintaining the riverbed. Sediment helps conserving habitats downstream. Some habitats include barrier islands, fertile floodplains and coastal wetlands, in this case it is the habitats under the dam including the riverbed. This will stop new islands from forming downstream and also floodplains will not form meaning if a flood happens it will have worse affects. But upstream will also be affected, with all the sediment getting trapped right behind the dam, the water levels will rapidly increase meaning that there are larger possibilities of the dam breaking and this increases the chances of a very large flash flood destroying everything udder the dam.

Another significant disadvantage of building a dam on a fast/free flowing river ecosystem/habitat, and then you build a dam, this means that the fast flowing water will artificially change into a still water river, almost becoming a reservoir. This means that the temperature, chemical composition, dissolved oxygen levels and all their properties will change, changing it to properties of a reservoir, and these properties do not fit, or are not suitable for a river ecosystem with fast flowing water. Many species will not be able to adapt fast enough to the changes that the dam has made and this will kill off most of the wildlife.