Hydroelectric power has always been a pretty "green" way to generate electricty, simply interrupting the natural hydrological cycle to borrow some of its potential energy to convert to electrical power. Green, that is apart from the thousands of square miles of land that get flooded to manufacture artificial lakes behind hydroelectric power-station dams. But that's where the nation's first commercial hydrokinetic power station earns its eco-points: It's simply submerged in a river without needing a dam.
The massive turbine has been placed in the Mississippi River in Minnesota by Hydro Green Energy. It has a 35-kilowatt generating capacity, and simply spins in the natural flow of water--somewhat like an underwater analog of a wind turbine. As such, it has a fairly low environmental footprint--it does nothing to particularly disturb the river's natural course, requires no damming and not much in the way of superstructural hardware. Of course the spinning blades do present a potential risk to plant and animal life in the river, but compared to a hydroelectric dam its environmental impact is small. And compared to traditional "dirty" methods of generating electricity, the Mississippi power station appears practically saintly.
A similar project has just gone live in the UK, where the world's first commercial tidal turbine power generator has been installed in Strangford Lough, Northern Ireland. Seagen looks even more like a traditional wind turbine, and has twin blades that can be lowered into the river. In this case, instead of a simple downhill-flow of a river, the system captures energy from the back-and-forth surging of the tides, but the principle is exactly the same. When it's up to full capacity it'll generate 1.2 megawatts of power--enough for 1,000 nearby homes--and it's been deemed that it's slow-spinning blades aren't a threat to local fish life.
Following the apparent success of that project, Scottish Power has made a planning request to install 20 similar devices, each measuring some 100-feet across, off the coast of Scotland to generate enough power for 40,000 homes.
In the US the irony of the Mississippi hydrokinetic power station is that due to regulatory short-sightedness it, and other systems like it, have to go through the same review and approval process as large hydroelectric projects. And as such, until red tape is minimized, they'll most likely find use downstream of dams where the regulatory process has already been carried out.
With projects like these, and the world's first commercial wave-power generator--recently installed  off the coast of Portugal, and farming enough wave-power for 1,500 homes--it really looks like hydro-power's time has come.
And perhaps the main thing that would really boost its adoption is willingness to simplify existing legislation and regulation.