|MERTEN, GUSTAVO - Us Geological Survey (USGS)|
|WELCH, HEATHER - Us Geological Survey (USGS)|
Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/22/2015
Publication Date: 5/9/2016
Citation: Merten, G.H., Welch, H.L., Tomer, M.D. 2016. Effects of hydrology, watershed size, and agricultural practices on sediment yields in two river basins in Iowa and Mississippi. Journal of Soil and Water Conservation. 71(3):267-278. doi: 10.2489/jswc.71.3.267.
Interpretive Summary: The sediment yield from a watershed depends on a balance between natural processes of erosion and deposition, but this balance can be altered by human activities particularly agriculture. Sediment usually decreases along a river course as sediments are trapped in alluvial plains and other sinks, but sediment can increase with basin area where channel erosion becomes dominant. This study evaluated changes in sediment with watershed size in Iowa (Iowa River) and Mississippi (Yazoo River). In the smallest watersheds, sites in the Yazoo discharged more sediment relative to Iowa River sites, largely due to differences in rainfall intensities, soil erodibility, and seasonal variances in climate. As basin size increased, watersheds in the Yazoo basin had steady decreases in sediment yield. However, in the Iowa River basin the greatest sediment yields occurred in intermediate-sized watersheds (i.e., 12 to 200 square miles). Subsurface tile drainage and limits to channel incision decreased the amounts of sediment that could be sourced from Iowa River uplands, leading to channel widening in intermediate basins. Despite these differences, when the sediment-basin size relationships were compared with reported rates of field erosion under conservation and conventional tillage treatments from research in these basins, it was evident that sediment loads in both the Iowa and Yazoo basins remained impacted by historical soil losses that occurred prior to conservation efforts. These results are of interest to conservationists and hydrologists seeking to understand long-term changes in sediment loss as affected by watershed scale, and the apparent disconnect between agricultural conservation efforts and sediment loading in Mississippi tributary rivers.
Technical 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 increase when there is a predominance of channel erosion relative to hillslope erosion. The U.S. Geological Survey (USGS) conducted a study of suspended sediment in the Iowa River Basin (IRB), Iowa, and the Yazoo River Basin, Mississippi YRB), from 2006 through 2008. Within each river basin, the SSY from four largely agricultural watersheds of various sizes (2.3 to 35,000 km2 [0.9 to 13,513 mi2]) was investigated. In the smallest watersheds, YRB sites had greater SSY compared to IRB sites due to higher rain erosivity, more erodible soils, more surface runoff, and fluvial geomorphological differences. Watersheds in the YRS showed a steady decrease in SSY with increasing drainage basin area, whereas in the IRB, the maximum SSY occurred at the 30-500 km2 (11.6 to 193 mi2) 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.