In the last few days there have been a number of articles like this one in the Peoria Journal Star about the slowdown of barge traffic along the Mississippi River due to the Midwest drought.
As the article mentions, we faced a similar situation in 1988. Stan Changnon, Illinois State Water Survey, wrote about the costs and other issues related to this in an article in the Bulletin of the American Meteorological Society.
Basically, in 1988 the barge industry lost about $1 billion due to low flows on the Mississippi as well as the Missouri and Illinois Rivers. The winner turned out to be the railroad industry as shippers scrambled to find alternative transportation for grain and raw materials.
Here is the graph of water levels on the Mississippi River at Chester, IL (south of St. Louis) since 2011. As you may recall, record rains in spring of 2011 caused much flooding on the lower Mississippi River. In fact, they had to blow some levies to save Cairo, Illinois. After facing heights of up to 40 feet in May 2011, levels fell throughout the rest of 2011 before leveling out in the winter and spring 2012. Since May of this year they dropped again to a current gauge height of about 1 foot. All these heights are in reference to the bottom of the gauge, not the bottom of the river. There is still water in the river – just not much.
You can find more of these at the USGS streamflow web site.