It is not a dam in the sense of Columbia or Cowlitz dams. It is a setup quite like Snoqualmie Falls. When generating power the water goes in the pipe at the top of the falls and comes out at the bottom. When not generating, it all goes over the falls.

Like Salmo suggested, if you want electricity this is one of the more fish friendly ways to do it.

As long as people keep moving here, or having kids, the demand for electricity will rise. There is no method of generation currently in place that does not have some negative environmental impact.