This is a guest post by my travel partner, Todd Dwyer, head blogger for Dell's ReGeneration.org.

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Four times a day, without fail, New York City's East River will change directions. It's been doing that for ages and will continue to do so long after we are gone. The tides are a constant, powerful force, and the folks at Verdant Power are on Roosevelt Island experimenting with a way to draw energy from them without impeding their flow or harming the local wildlife.

The Roosevelt Island Tidal Energy Project gives Verdant Power the rare opportunity to test turbines that were developed by Dean Corren, their director of technology. Submerged beneath the surface of the river, these turbines turn passively to face the tides the same way a weather vane would. What makes these turbines special is that unlike dams and barrages, they will employ a kinetic hydropower method will have little to no effect on the local ecosystem.

So far, the tests have been very promising. They are drawing power successfully, and in the two years they have been monitoring the project, there has been no evidence of any harm to local fish or birds. While the flow of the tides are strongest and the turbines are getting the bulk of their work done, the fish aren't even around. They prefer to save their energy for when the tide is weaker. Of course, during that time, the turbines are not turning. Also, the fish there tend to spend their time near the banks of the river, and the turbines are in the depths near the center. In fact, Verdant Power has gathered so much new data about local fish and bird populations that biologists have benefited from the project.

I must admit, a lot of this stuff is over my head, but Jonathan Colby, Verdant Power's hydrodynamic engineer, knows more than a thing or two about going with the flow. Sarah and I caught up with him on a beautiful afternoon on Roosevelt Island:

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