Tile drainage in the Mississippi Basin is one of the great advances of the 19th and 20th centuries, allowing highly productive agriculture in what was once land too wet to farm. In fact, installation of new tile systems continues every year, because it leads to increased crop yields. But a recent study shows that the most heavily tile-drained areas of North America are also the largest contributing source of nitrate to the Gulf of Mexico, leading to seasonal hypoxia. In summer 2010 this dead zone in the Gulf spanned over 7,000 square miles.

Scientists from the University of Illinois (U of I) and Cornell University compiled information on each county in the Mississippi River basin including crop acreage and yields, fertilizer inputs, atmospheric deposition and number of people and livestock to calculate all nitrogen (N) inputs and outputs from 1997 to 2006.  For 153 watersheds in the basin, they also used measurements of nitrate concentration and flow in streams, which allowed them to develop a statistical model that explained 83% of the variation in springtime nitrate flow in the monitored streams. The greatest nitrate loss to streams corresponded to the highly productive, tile-drained Corn Belt from southwest Minnesota across Iowa, Illinois, Indiana and Ohio.

This area of the basin has extensive row cropping of fertilized corn and soybeans, a flat landscape with tile drainage and channelized ditches and streams to facilitate drainage.