Figure 1. U.S. Geological Survey hydrologic regions (outlined in black, Seaber and others, 1994) of the Mississippi River Basin (MRB) (shown in blue) in the United States. MRB hydrologic regions are numbered: 05, Ohio; 06 , Tennessee; 07, Upper Mississippi; 08, Lower Mississippi; 10, Missouri; and 11, Arkansas-White-Red. Soil organic carbon (SOC) storage and inventory estimates for the hydrologic regions are given in table 3. See table 4 for the SOC storage estimates for counties (mapped areas of counties shown in gray) in these hydrologic regions.
