How did the controller know that the barrels would be empty?

I just wonder how the controller in the city on the water knew that he would come with a load of empty barrels? When they float on barrels down the river they meet up with a guy with a boat. He loads the empty barrels they floated on, on the boat. He says that he knows where the barrels comes from. But when they are across the water, how do the controller know that the barrels should be empty? It is just a coincidence that those barrels even came down the river that day. How do he even know about the barrels, without a cellphone or anything? smoke signals?