The government had taken control of all shipping in the United States during the war and after. Food, resources and manufacturing moved in the great lakes was a humongous part of the after. Food shipments after the war prevented mass starvation.
It was the greatest conversion of a capitalist economy into a control economy ever. It killed a lot of Naziz and Japanese. Great lakes shipping was a big part of the war effort. Gramps was a cog in the machine.