There are three big problems with hydro power, Clocker:
1) It comes nowhere near to dealing with the power demands of typical Western civilisations. Bar a couple of weirdo countries with more snowfields and fiords than they have people (zorry JoJo), hydro is only capable of providing a small single-digit percentage of the total energy demand. even if we damn every single watercourse down to the size of a two-inch pipe.
2) Hydro schemes are incredibly destructive of riverine environments. There is a whole rich world down there under the water, enormously diverse and complex, which we never see. And another source of richness and critically important biodiversity in the above-surface riverine fauna and flora. Hydro schemes, as a rule, are terribly destructive of both. Over on Storage Review a while ago, in the Vegetarian thread, Tannin listed a huge number of vertabrate extinctions in the USA: dig it out and look at all the species killed off forever by hydro schenes in the USA.
3) Hydro schemes, in the long-term, cost a phenomenal amount of money. I'm not talking about the capital expenditure to build the thing here (although that is certainly not insignificant), rather I'm talking about the total-cost-of-project economics in the long term, after the loan to build the dam is paid off.
On the plus side, you get some cheap electric power, some extra water storage, and maybe a minor tourist attraction. On the minus side are the long-term economic effects. You lose the tourist attraction of the wild river valley (which, averaged out, more than makes up for the dubious attractions of the artificial lake), you lose the enormous economic benefit of the river system's natural silt-flow, which (before you dam it) spreads fresh, fertile topsoil over the entire floodplain, and makes riverine floodplains the most productive soil per-acre on earth. Then you lose the benefit of the inland fisheries (which in wet climates, like that of the USA or Europe can be substantial), and the enormous economic resource of the offshore fisheries, which, being denied the benefits of the clean nutrient that used to flow down the river in flood times, gradually decline. Consider, for example, the death of the Mediteranian sardine industry, which used to employ thousand upon thousands of people and feed millions before the damming of the Nile at Aswan. Now, it is dead. The Nile delta fisheries are defunct. It's the same wherever you go: the Murray River in South Australia, the Snowy in Victoria, and - yes - the Missippi or however you zpell it. The Mississipi now just flows in a relative trickle, without the cleansing of the annual floods, and the water it delivers to the Gulf is choked with nitrates (run-off from over-irrigated, over-fertilised farms). As a result, the Gulf of Mexico fishing industry has been decimated.
Finally, most of the water from the hydro schemes is used for irrigation. In many places, this brings short-term riches. The oranges of the Australian Riverina and of California are evidence of this. But as time goes by, the constant provision of extra water to grow crops does two things: it raises the underground water-table (because it's constant, not just a once or twice a year thing), and it leaches salts and minerals out of the topsoil. These are washed down into the groundwater, and when the groundwater gets high enough (as it almost inevitably does), the land turns into desert. You can't grow anything on it, no matter how much water you add. The only way to fix it is to stop irrigating, revegetate groundwater recharge areas on the surrounding hills, let the river flow free again to coat the sterile soil in life-giving silt once a year, and wait. It doesen't take long by geological standards. A thousand yours or so is plenty.
Like so much else you stupid humans do, hydro schemes are a clasic example of blinkered short-term thinking, of hocking the future to pay for present gains (much of which is wasted), of spending your capital instead of investing it wisely and living off the interest.
