dendroica:

Currently, near record low river levels requires that the Army Corps of Engineers work non-stop to keep the Mississippi River passable for cargo ships and transport by dredging out 60,000 cubic yards of sediment each day.

The river conditions are forcing companies to decrease the number of barges on the river and the ones that are traveling up and down the river have decreased their loads. Even with these efforts, 60 vessels have run aground in the lower Mississippi since May. Plus, while the Corps has kept the main navigation channel open, harbors along the river have been closed. Four have closed so far and if the drought continues, eight more are likely to be closed- leaving 12 out of 19 harbors closed. These difficulties will increase transportation costs that will likely be passed on to consumers.

Besides impacting the 500 million tons of cargo that travels up and down the river annually, the low volume of water coming down the river is putting local water supplies at risk. The corps is building a dam of sediment to prevent a wedge of salt water creeping up the Mississippi from entering into local water supplies drawn from the river. The difficulties stemming from reduced river levels highlights how climate change will have impacts far beyond warmer temperatures.

(via Another Economic Impact of Climate Change: Drought is Draining the Mississippi River - The Demos Blog - PolicyShop)

The same is true for Danube in Europe: smaller vessels, smaller loads.