I think what I would do, is place a board on a hinge below the opening, then with a hook and eye, one could partially block the opening, leaving a 1 -2 inch gap at the top under the eaves. That would keep a lot of the elements out, but still give that great ventilation.
Thing is, heat rises, warm moist air rises, and will move right out with that ventilation. I would hesitate to block it, especially with the humidity in Sioux Falls. East River tends to be much more humid than over here in the west.