A mega dam is like a normal dam but is build in a huge scale. The reasoning behind the mega dam is to generate even more electricity for the growing demands. The current biggest dam, the Three Gorges Dam where the walls of the dam have enough materials to build 63 Eiffel towers and the reservoir contains 39.3 cubic kilometer which the area of land covered by the water is the size of Singapore. But did you ever wonder why a dam that size was built and yet not everyone in china receives electricity from it.
Once the dam is up and running, electricity should come cheaper to everyone and be more easily assessable but that is not always the case. The people that got forcibly relocated due to the building of the dam rarely benefited once the dam is built. It is because building infrastructures to provide electricity to everyone is too expensive. They may spend millions or billions to build a dam but none was spared to improve the lively hood of the citizens. The real benefactor would be factories owners and neighboring countries where they can get a supply of cheap electricity.
With the area of big scale is needed to be filled with water, the surrounding needed to be cleared to the ground. Then what once was a flowing river will turn into a reservoir of still water. This will clearly affects the climate and biodiversity in that area. Animals and plants can go extinct from these actions. Though when compared with the needs of men, the needs of nature always fall short. This however affects mankind in return.
After the dam is built, it may not operate at full capacity. This might be due to the slow development of the reservoir water where there is not enough water being raised or due to the climate changes introduced when building the dam. With either of these reason being in effect and the dam not functioning at full capacity, the effort and sacrifice will be wasted. Even if they manage to reach full capacity at some point in the future, how long will they be able to keep at it while the current state of the reservoir is deteriorating.
While big has its advantages, so is being small. Mini dam can be constructed without destroying anything and affecting the biodiversity of the river. While it is not enough to power an entire nation, it is still more safer to the environment than building a mega dam.