The Mississippi River is the primary river, and second-longest river, of the largest drainage basin in the United States.  From its traditional source of Lake Itasca in northern Minnesota, it flows generally south for 2,340 miles to the Mississippi River Delta in the Gulf of Mexico.

The Gulf of Mexico is the largest area of hypoxia in the United States. In the summer of 2002, the hypoxic zone measured larger than ever before. In fact, it covered an area greater than the size of Massachusetts. Hypoxia in the Gulf of Mexico has raised considerable concern throughout the United States because many people in coastal states make their livelihood fishing in the Gulf and many Americans vacation on the Gulf. It is also an indication of the quality of the waters that feed the Gulf, including the Mississippi River Basin. The nitrogen concentration in Mississippi River Basin water has doubled since the 1950s.