Abstract:The specific sediment yield (SSY) from watersheds is the result of the balance between natural, scale-dependent erosion and deposition processes, but can be greatly altered by human activities. In general, the SSY decreases along the course of a river as sediments are trapped in alluvial plains and other sinks. However, this relation between SSY and basin area can actually be an increasing one when there is a predominance of channel erosion relative to hillslope erosion. The US Geological Survey (USGS) conducted a study of suspended sediment in the Iowa River basin (IRB), Iowa, and the Yazoo River basin (YRB), Mississippi, from 2006 through 2008. Within each river basin, the SSY from four largely agricultural watersheds of various sizes (2.3 to 35,000 km 2 [0.9 to 13,513 mi 2 ]) was investigated. In the smallest watersheds, YRB sites had greater SSY compared to IRB sites due to higher rain erosivity, more erodible soils, more overland flow, and fluvial geomorphological differences. Watersheds in the YRB showed a steady decrease in SSY with increasing drainage basin area, whereas in the IRB, the maximum SSY occurred at the 30 to 500 km 2 (11.6 to 193 mi 2 ) scale. Subsurface tile drainage and limits to channel downcutting restrict the upstream migration of sediment sources in the IRB. Nevertheless, by comparing the SSY-basin size scaling relationships with estimated rates of field erosion under conservation and conventional tillage treatments reported in previous literature, we show evidence that the SSY-basin size relationship in both the IRB and YRB remain impacted by historical erosion rates that occurred prior to conservation efforts.Key words: agriculture-annual sediment loads-scale effect-sediment yield-soil management practices Watershed disturbance by changes in land use, soil management practices, dam construction, and dredging and straightening of channels has resulted in substantial changes in the sediment loads of rivers around the world (Walling 2008). In the United States, changes in sediment load by watershed disturbance have resulted in hydrologic, ecologic, and engineering effects both upstream and downstream (Walling 1999; Horowitz 2010). Changes in land use from forest and prairie to agricultural land during the 19th century accelerated the erosion process, transferring sediment from uplands to valleys and the fluvial system (Trimble 1999). Later, during the 20th century, reservoir construction and soil conservation programs implemented in agricultural areas reduced sediment transference from uplands to rivers and created new, artificial sediment sinks (Renwick et al. 2005). Horowitz (2010) identified the construction of dams on the Missouri River in the late 1950s, and the sediment trapped behind them, as the primary cause of reduced annual suspended sediment loads in the Mississippi River. Nonetheless, Meade and Moody (2010) reported that the MissouriMississippi dams trap only about 1 × 10 8 to 1.5 × 10 8 Mg (1.102 × 10 8 to 1.653 × 10 8 tn) of sediment per year, which is less than h...