If evolution is true, won't all species adapt or be replaced? "Only the strong will survive"? It's not like the humans are stealing the water from the marine life. The dam is saving these species, while changing the evolving landscape. Why not let nature take it's course? Screw the mussels, with or without that wildlife area, if water dries up in the north, so be it.
Just became some process occurs "naturally" doesn't mean we should allow it to go ahead, if it will harm us in the future.