Environmental Engineering Reference
In-Depth Information
Figure 2.
The areal extent of bottom water hypoxia in mid-summer on the Louisiana/Texas
continental shelf from 1985-2004 (updated from [14]).
primarily due to salinity differences, and the stratification intensifies in summer
with thermal warming of surface waters.
Mississippi River Watershed.
The Mississippi River watershed drains 3.2
million km
2
encompassing 41 percent of the lower 48 United States. The
Mississippi River has been modified for navigation and flood control with nar-
rowing of the channel and levees extending along the river from Cairo, Illinois
to the Gulf of Mexico. Fifty-six percent of the wetlands in the Mississippi River
basin have been lost to agriculture, navigation, reservoirs and leveeing. Land
use in the watershed that supports 27 percent of the U.S.'s population (about
70 million people) is predominantly agriculture [17], the conversion of which
began in the early 1800s as settlers migrated west across the North American
continent [46]. Artificial subsurface drainage in much of the croplands expe-
dites the transport of nitrate from the soil to surface waters. This management
practice coupled with the increase in fertilizer applications can only increase the
flux of nitrate from agricultural fields to the streams. In addition to landscape
changes, anthropogenic inputs of nitrogen and phosphorus have increased from
agriculture, point sources, and atmospheric deposition.
