Apart from what g_pepper has correctly pointed out regarding size/power of hydro plants...
If we build a lake at the top, 10 meters deep and 1 kilometer on a side
With the right terrain, this is pretty trivial, all you need is a relatively small dam wall closing off a small ravine between mountains... here is a nice example:
http://www.iwb.ch/media/de/picdb/2012/366/nant_de_drance_stausee_vieux.jpg
http://www.iwb.ch/media/de/picdb/2012/367/nant_de_drance_stauseen_vieu.jpg
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.