Fortunately, this isn't true, or at least hasn't been for 15+ years.
> ... a lot of grain produced throughout the Midwest is transported ... via the waterways of the Ohio and Mississippi Rivers
Only because the price is lower. Grain shipment is extremely price-sensitive! [1]:
> Prior to 2003, containers were mainly restricted to specialty crops, which would not fill a hold in a ship, and feed ingredients like corn gluten meal, bone and meat meal The containerization of grain in the US began to pick up significantly in 2004 because of the spread that emerged between backhaul container rates and bulk shipping. The rates for bulk shipping increased because of the strong demand for scrap metal.
> The North American consumer demand for imported Asia Pacific manufactured goods also surged between 2003 and 2008. Consequently, freight rates for containers on the westbound traffic lanes fell as the volume of empty containers returning to Asia became excessive. During this period, grain could be shipped in containers from Chicago at $35/40 per ton, while bulk rates for grain at the Gulf of Mexico were $60/70 per ton. This provided the incentive for commercial bulk grain shippers to begin arbitraging the freight and moving grain exports in containers.