The largest flood on the lower Mississippi River since 1973 was measured on April 22 in Vicksburg, Mississippi by the U.S. Geological Survey (USGS), in cooperation with the U.S. Army Corps of Engineers. The flood was caused by intense rainfall throughout the central plains and Ohio River valley in March and April that has now reached the lower Mississippi River basin. According to the National Weather Service, the Mississippi River is expected to remain above flood stage at Vicksburg, MS until May 20. Find current flood and high flow conditions across the country at the USGS WaterWatch website http://water.usgs.gov/waterwatch/flood.
Floods are important to water quality in the Gulf of Mexico because they carry large amounts of nutrients, which cause excess algal growth and hypoxia. Hypoxia, a zone of low dissolved oxygen which can stress and kill bottom-dwelling organisms, has been measured annually in the northern Gulf of Mexico since 1985. The timing and volume of freshwater inputs and the spring load of nitrogen and phosphorus contributed from the Mississippi River Basin to the continental shelf are important factors in determining the size of the hypoxia zone. Nutrients in the Mississippi River Basin come from a variety of sources, including soils, agricultural cropping, application of fertilizers and manure, urban sources, and atmospheric deposition. Monitoring and reporting of both streamflow and water quality are crucial to understanding and resolving the Gulf hypoxia issue. General information about hypoxia and USGS scientific contributions related to the issue can be found at http://toxics.usgs.gov/hypoxia/index.html.
Hypoxia means “low oxygen.” In estuaries, lakes, and coastal waters low oxygen usually means a concentration of less than 2 parts per million. In many cases hypoxic waters do not have enough oxygen to support fish and other aquatic animals. Hypoxia can be caused by the presence of excess nutrients in water. Excess nutrients can cause intensive growth of algae. The consequences of this enhanced growth are reduced sunlight penetrating the water, a decreased amount of oxygen dissolved in the water, and a loss of habitat for aquatic animals and plants. The decrease in dissolved oxygen is caused by the degradation of dead plant material (algae), which consumes available oxygen. The overall effect is called eutrophication. Nutrients can come from many sources, such as fertilizers applied to agricultural fields, golf courses, and suburban lawns; deposition of nitrogen from the atmosphere; erosion of soil containing nutrients; and sewage treatment plant discharges.