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Identifying the Primary Sources of Sediment in an Anthropogenically Altered Watershed: Year 2
YEAR: 2011
INVESTIGATORS: Thanos Papanicolaou, Marian Muste, Douglas Schnoebelen, Larry Weber, Christopher Wilson

The sediment in transport within a stream is a complex mixture of eroded upland surface soils, collapsed channel bank material, and resuspended bed sediments. The proportion of sediment contributed from each primary source (i.e., uplands, banks, and bed) is affected by the relative rates of the dominant erosion processes occurring in a watershed. As we still deal with the daunting problem of water quality degradation by high sediment loads, the following question remains: What are the contributions from each of the primary sources during a storm event and how do these contributions vary in space from the headwaters to the mouth of a watershed? Upland erosion rates in Iowa watersheds are relatively high for the U.S. (e.g., Cruse et al., 2006). However, channel sources in some streams during extreme events can contribute up to 80% of the stream load (e.g., Wilson et al., 2008). The key objective of this research is to identify the major source(s) of sediment to the stream load of an anthropogenically altered watershed in SE Iowa, the Clear Creek, IA watershed (CCW). This information will provide better understanding of the relative roles of upland and channel erosion processes in the delivery of sediment to local streams. Identifying the major source(s) of sediment to the stream load will also allow for better focusing of remediation efforts to abate another predominant water quality concern in Iowa, namely high sediment-associated P loads. Transport and mixing of sediment-associated P in rivers often constitutes a high percentage of (23-61%) of the total annual P load (e.g., Lick, 2009). We propose to use established tracing techniques, such as the naturally occurring radionuclides, 7Be and 210Pbxs, for identifying the proportions of sediment derived from each primary source (i.e., uplands, banks, and bed) under different magnitude hydrologic events, including floods. In particular, 7Be has shown the unique ability to differentiate upland soils from stream bank and bed sediments (e.g., Wilson et al., 2008). 7Be is produced continuously in the atmosphere but delivered to the earth surface in high concentrations mainly during precipitation events. Moreover, 7Be does not remain long in the soil before decaying due to its relatively short half-life of only 53 days. Thus, there is a strong relationship between a single erosion event and high signatures of 7Be in the eroded upland soils. To meet our research goal, a comprehensive study is designed that takes advantage of existing monitoring and database infrastructure in the CCW. First, we will construct sediment rating curves (i.e. flow discharge vs. sediment flux curves) using existing in-situ, measuring instrumentation for flow discharges and sediment fluxes [Task 1]. We will partition the sediment sources corresponding to each event by analyzing the radionuclide activity of collected grab samples through gamma spectroscopy in the laboratory. The activities of the grab samples will be compared against the radionuclide activities of sediments extracted from the primary sources, namely, the uplands, channel banks, and stream bed. An established unmixing model (Fox and Papanicolaou, 2007) will be used to isolate the fraction of each primary source contributions in terms of mass (or volume) [Task 2]. The data from this study will be incorporated in the Clear Creek Digital Watershed and used for model verification and model refinement [Task 3]. These rare data will allow us to quantify for the first time the sediment fluxes from different sources under different hydrologic conditions and will lead to the refinement and extension of sediment rating curves applicable to most Midwestern agricultural watersheds. If we can quantify the dominant sediment source(s) in streams, we can identify the areas, which need BMPs to control sediment-bound P, and design our BMPs more efficiently. The results will be disseminated in EOS and the journal of Soil Water Conservation (JSWC). This study will support a Masters level student and laboratory sediment analysis.