How will reservoirs in dams flood areas such as farmlands, wilderness areas and inundate towns and villages?

I just want to know how would reservoirs in dams inundate towns and villages?How will reservoirs in dams flood areas such as farmlands, wilderness areas and inundate towns and villages?
the inundation happens when they build the dam. basically what is happening is that they build a huge wall at th bottom of a valley which has a small river running through it. The wall stops the river flowing so the valley fills up with water (making a reservoir). whatever was in the valley before - villages, wilderness, animals, will get flooded. Have a look at info on the three gorges dam in china for examples. once the dam is built, there's no more inundation (unless there's a breach or something where the dam breaks)