I would like to see where you get your statistics since barge traffic on the Mississippi is very strong. Trucks get it to the elevators and the barges get it to the ocean. A bump in the road occurred during the drought of 2013, but barge traffic seemed to recover.
Yes, trucks do deliver more than anything, but that is the nature of roads, UPS, FedEx, and the post office. The Mississippi is a large conduit of bulk items. It is also cheaper to ship than truck or rail ( https://www.cavs.msstate.edu/publications/docs/2013/03/11509... ) with rail being cheaper than truck.
From a little farther in the same article "The water transportation system, including coastal and inland-waterway barge service, is critically important for the transportation of heavy, bulky grains, clays, gravels, etc."
It goes on to talk about adding capacity to the waterways. Plus, you seem to be skipping the multimode column.
> The larger the economy gets, the more goods must be shipped via other modes. At the end of the day, only so much can fit on the Mississippi.
The report you cite is basically is talking about the need for more capacity of our roads. The Mississippi River barge transport is not going away (and expanding) because it is cheap with good infrastructure.
It's from the Bureau of Transportation Statistics. It shows the amount of freight (by value, tons, and ton-miles) for each transportation method.
Water (not just the mississippi, but all inland waterways and costal barges) is 6% by tons, 1% by value, and 9% by ton-miles.
The larger the economy gets, the more goods must be shipped via other modes. At the end of the day, only so much can fit on the Mississippi.