Most dam's are already using the flow of the river to produce hydroelectric power. Its one of the few constant supplies of clean energy we have at the moment.
And in answer to the battery question. Compared to the cost of the coal that we burn that is essentially turned into power that is currently wasted (power during the night and early morning), batteries are going to save money. If all of the power that is harnessed by burning the coal can be stored and used, the amount of coal required will be reduced drastically. This in turn will result in lower power costs around the board. Batteries can make ANY power source a base load power source because the fluctuations can be leveled out by the batteries delivering a constant stream of power. With the batteries my dad is working on, they will be able to store 100MWH in a 1/2 acre piece of land. Thats a LOT of power that can be stored! Enough to power your house for more than 10 years in just one charge.
Expensive? Sure but the amount of money able to be saved will make up for that cost in a short amount of time while at the same time reducing the carbon footprint we leave.
These batteries are the key!