Satellite image of the Gulf of Mexico and coastline
Image Credit: USGS
Artfully captured as early Americana by Mark Twain, the largest river system in North America, the Mississippi, begins in northern Minnesota and winds 2,530 miles to empty into the Gulf of Mexico. After contributions from 31 states, it discharges, on average, 593,000 cubic feet of water per second into the gulf. That is 4,436,000 gallons per second, or as WolframAlpha helpfully adds, 6.7 times the volume of an Olympic-sized swimming pool every second. Lest we become pompous, this is only about 8 percent of the discharge of the Amazon River.
On the long journey from St. Paul Minnesota, the river accumulates chemicals and other things that are not water. Particularly harmful are nitrogen and phosphorus that are primarily agricultural runoff, although there are other sources. These two nutrients result in algae growth. The real problem is not while they are alive and producing oxygen, but instead, when they die, drop to the bottom of the gulf and decompose. The massive decomposition robs sealife of oxygen, creating a dead zone.
Dead Zone: The Source of the Gulf of Mexico's Hypoxia tells that in 2002, the dead zone was 8400 square miles. Most recently the dead zone has been about the size of Connecticut and Rhode Island combined - 6700 sequare miles. This year though, there has been a drought in the midwest, and the Mississippi has dumped a little more than 1/2 the normal amount of nutrients into the gulf. Hopefully that will reduce the dead zone to only the size of Rhode Island, sparing Gulf of Mexico aquatic life and the fishing industries.
If you are interested, USGS has produced a new online research tool for assessments of streams and rivers.