EFFECTS OF NATURAL PROCESSES AND ANTHROPOGENIC ACTIVITIES ON SEDIMENT DELIVERY FROM TWO HIGHLY MANAGED AGRICULTURAL WATERSHEDS LOCATED IN MISSISSIPPI AND IOWA, 2006 TO 2008

Due to erosion and subsequent deposition, sediment is the leading pollutant of surface water in the world. The effects of differing management practices, climate, hydrology, soil types, and stream characteristics on sediment transport at four basin sizes each within watersheds in central Iowa and in northwestern Mississippi were compared from 2006 through 2008. Land use in both watersheds is primarily agricultural; agricultural management practices (AMPs) have been implemented to decrease erosion and sediment transport from fields, but both watersheds have been altered to facilitate drainage from the cultivated fields offsetting the AMPs. In northwestern Mississippi, the fields are drained largely by ditches and channelized streams; whereas, in central Iowa, fields are drained by an extensive network of subsurface drains leading to ditches. Average suspended sediment yields for the study period from the two smallest sub-basins in northwestern Mississippi ranged from 2 to 10 times higher than from the two smallest sub-basins in eastern Iowa. This is most likely due to the larger amount of overland flow, soils of higher erodibility, and rainfall with higher erosivity in Mississippi compared to Iowa. In 2007 and 2008, flooding in Iowa increased sediment loads from the Iowa River at Wapello, IA (32,375 km²) above the 12-year average by 49% and 230%, respectively. In contrast, a drought in Mississippi resulted in sediment loads from the Yazoo River below Steele Bayou, MS (34,589 km²) that were 60% less than the 12-year average in 2007 and 28% less than the average in 2008. However, suspended sediment yields at the two sites were almost identical, indicating that more discharge is needed in the Iowa River to move as much sediment as the Yazoo River at the same scale.