Fish, Tides, and Turbines - Natural History, December 2013/January 2014 

An exerpt: Modern tidal turbines are in many ways closer to wind turbines than traditional hydroelectric stations. They extract energy from the horizontal flow of water instead of from its vertical drop. To distinguish them, these systems are referred to as Marine Hydrokinetic (MHK) turbines. Over the last decade companies like Verdant and ORPC have been working with the Federal Energy Regulation Commission (FERC) to develop guidelines for this technology and the environmental studies surrounding such projects. A decade ago it was not clear whether these turbines could survive the harsh marine environment. It also was not clear whether the environment could survive the constant churning of the turbines.