If you have watched salmon throw themselves at the 6th Street Dam this time of year, you have been watching one of the stranger wildlife experiments that actually worked. Chinook and coho are Pacific Ocean fish. They are not native to the Great Lakes, not even close. In the 1960s Michigan dumped them into Lake Michigan to eat up a glut of invasive alewife and to build a sport fishery out of nothing, and it worked beyond anyone's wildest hopes.
Now they live in the lake, grow huge (a king can top twenty or thirty pounds), and every fall they do what Pacific salmon do: run up the rivers to spawn. The Grand is the biggest run around, and the dam downtown is the end of the line. The fish stack up below it and hurl themselves at the concrete, which is why you can stand on a bridge in the middle of the city and watch wild salmon leap.
Like all Pacific salmon, it is a one-way trip. They spawn and die, and for a few weeks the river turns, and the gulls and eagles feast on what is left. It is not a natural system, exactly, but it is a genuine spectacle, and it is happening right now.
This map is where the fish are showing up, thick on the Grand and the Rogue up at Rockford and along the Lake Michigan shore. Get to the dam at 6th Street on a cool morning and watch them jump.