Europe: The Rancid Rhine

The Rhine is one of the world's most scenic and storied waterways. It was a commercial route before Christ, and Julius Caesar first spanned it with a bridge in 55 B.C. Along its picturesque banks, flanked by medieval castles, are Drachenfels, the cliff where Siegfried slew his dragon, and the Lorelei rock, where a beautiful siren lured rivermen to their death on the treacherous shoals.

The Rhine is also one of the world's filthiest rivers. The crystalline waters that tumble from Alps near Reichenau, Switzerland, are choked with wastes by the time they pour into the North Sea, 820 miles...