The Mississippi as seen from Ed Drager’s tug boat is a river in retreat: A giant beached barge is stranded where the water dropped, with sand bars springing into view. The floating barge office where the tug boat captain reports for duty is tilted like a funhouse. One side now rests on the exposed shore. “I’ve never seen the river this low,” Drager said. “It’s weird.”

The worst drought in half a century has brought water levels in the Mississippi close to historic lows and could shut down all shipping in a matter of weeks — unless Barack Obama takes extraordinary measures.