Based on the 3-cent postage rate, this postcard must be circa 1958 to 1963. The description on the back reads:
The Aerial Bridge in raised position for an ore boat passing into Duluth Harbor. When the span is lowered traffic may move without interruption between Minnesota Point and downtown Duluth. Through this canal pass about 4500 boats in a 7-1/2 month season, carrying a total tonnage of about 17 million tons. (Average for five years.)
