It doesn't look nearly that amazing in person. I see what you mean. I remember driving around it, but from that perspective it looks like it is right up against the water and completely different. It's all green roof and little concrete, but the actual structure is all concrete and little green roof. Or maybe I went there when it was just built, and it has filled in a lot over the past few years.
I wonder if there's some confusion about which building. National Great Rivers appears to have two buildings in close proximity: the Museum (right at the dam) and the Research and Education Center. The museum is a much more conventional building, but the education building seems to be all field stone and grass and no exposed concrete at all on the inland side. Also, the ed building seems quite a lot newer.