It's just a guess, but it seems possible that photo was taken near the beginning of the American Federation of Grain Millers strike, which lasted for twelve weeks starting in July of 1979 and led to a pile up of ships awaiting cargo. It matches up quite well with a photo on Minnesota Reflections from nearly the same perspective but taken in the fall near the end of the strike. The caption on that photo notes that you can clearly see the orange bottoms of the empty, cargoless vessels, just as you can in the postcard photo.