REGIONAL-SCALE SEDIMENT BUDGETS FOR THE UNITED STATES EAST OF THE ROCKIES AND IMPLICATIONS FOR RIVER MANAGEMENT

River systems in much of the United States have received very large quantities of sediment derived from agriculturally-accelerated water erosion over the past 2-3 centuries. Watershed-scale studies in several locations and regional-scale budgets have shown that ~90% of this anthropogenic sediment was trapped in terrestrial settings rather than exported to the coastal zone, with streams functioning as net sediment sinks. Here we assemble regional-scale (2-digit Hydrologic Unit Code) data on current and historic agricultural land use, recent soil erosion rates, and estimates of total sedimentation in artificial impoundments. We use available data on dams to estimate historic sedimentation for ~43,000 reservoirs and ~2.6 million small ponds, and provide multiple estimates of erosion rates on a decadal time scale back to 1700. Although estimates of the absolute magnitude of historic erosion are speculative at best, available evidence indicates that total agricultural erosion peaked in the early 20th century and has declined dramatically since then. Simultaneously, sedimentation in artificial impoundments has risen dramatically as the number of such features on the landscape has grown. In most of the United States east of the Rockies total sedimentation in artificial impoundments began to exceed agricultural erosion in the last two decades of the 20th century. If erosion rates on non-agricultural land are low then on average stream systems, particularly in headwater areas, should be functioning as net sediment sources. The greatest excesses of impoundment sedimentation over agricultural erosion are in the South Atlantic-Gulf, Arkansas-White-Red, and Upper Mississippi Regions. This condition should contribute to widespread sediment starvation that is likely a factor in stream instability. Stream restoration efforts in agricultural and formerly agricultural regions should be conceived within this context, and plan for a prolonged period of declining sediment availability.