President Obama declared a disaster for Michigan this week because of heavy flooding in April. Parts of Illinois also experienced heavy flooding because of spring rains.
Scientists working with the National Oceanic and Atmospheric Administration at the University of Michigan, Louisiana State University and the Louisiana Universities Marine Consortium said heavy rains in the Midwest washed nutrients into the Mississippi watershed and into the Gulf of Mexico.
A high nutrient content in waters creates a condition known as hypoxia, a condition of very low oxygen levels. This in turn leads to a so-called dead zone where oxygen levels are insufficient to support most marine life.
NOAA said it expects the dead zone in the Gulf of Mexico will measure 7,286- to 8,561 square miles, one of the top 10 largest predictions ever.
Acting NOAA Administrator Kathryn Sullivan said ecological forecasts give industries like fisheries a good idea of what to expect from season to season.
"Monitoring the health and vitality of our nation's oceans, waterways, and watersheds is critical as we work to preserve and protect coastal ecosystems," she said in a statement Tuesday.
The highest level ever recorded for was 8,481 square miles in 2002.