> We've pretty much already dammed every river that's capable of generating reasonable amounts of hydro power
This is patently false.
In pretty much any area you look with some hydro development now, there's about 50% untapped. That includes here in Canada, where only about 55% of the conventional resources are used, and if they expended to all the large ones only, it would provide enough electricity to power everything we already have an all our cars. With some left over to sell to the US as well.
Here, for instance, is a recent-ish report for my area. Ontario currently has 8 GW of installed capacity. The study found about 6 GW of untapped easy capacity, 1 GW of which required internal changes only (new turbines and distribution systems). Another 14 GW exists but is not currently economic. If fully developed, it would be more than enough power to run the entire province.
This ignores the emerging fields like hydrodynamics. It's not entirely clear how large this is, but estimates I've seen place it at about the same as existing sources. Unlike conventional systems, these take up no space and have basically no effect on the river.