Watershed sediment source identification: tools, approaches, and case studies

Many of us who have watched a stream turn turbid during a rainstorm have often wondered—where is all that sediment coming from? Geomorphologists, engineers, and environmental scientists have thought about this question for decades and have placed understanding sediment sources under the general framework of sediment budgets. This framework which examines not only sediment sources but the storage, transport, and delivery of sediment has been used to understand surficial processes, as well as address management concerns as they relate to stream turbidity and surface-water quality. Traditional tools used in sediment budgets have included field measurements of erosion, storage and transport rates, photogrammetric analysis, and modeling. The relationship between soil erosion and sediment yield at the watershed outlet has been long identified as a major research need (Walling 1983) and is still a poorly understood or misunderstood component of fluvial sediment transport (Kinnell 2004). In recent decades, sediment fingerprinting approaches using the geochemical and physical properties of sediment to determine sediment sources have become increasingly popular. This special issue on Watershed Sediment Source Identification, for the Journal of Soils and Sediments was produced as a result of a special session convened at the Association of American Geographers (AAG) 2012 annual meeting in New York City, NY, USA. The session was entitled “Watershed Sediment Source Identification” and brought together leading experts in the field of sediment sourcing. As a result of the AAG session, the Journal of Soils and Sediments offered to have the session and a few other papers on the same topic published as a special issue in the journal. This special issue reflects contributions from 29 researchers discussing sediment sourcing results from studies in Africa, Asia, Europe, and North America where settings range from urban to agricultural to forested. Sediment source assessment is not only important to our understanding of sediment dynamics in fluvial systems but is increasingly becoming an important management tool. The papers in this special issue reflect a range of studies that address both topics. Studies emphasize traditional field approaches to determine sediment yields and sources (Davis and Sims 2013; Zhu 2013) as well as the use of the sediment fingerprinting approach to determine significant sediment sources (Dutton et al. 2013; Gellis and Noe 2013; Huisman et al. 2013; Koiter et al. 2013; Mckinley et al. 2013; Voli et al. 2013). Several papers highlight site-specific methods and approaches to streamline the sediment fingerprinting approach into a practical management tool. Other papers relate findings from sediment fingerprinting to hydrologic conditions, and caution users on over-interpreting the sediment fingerprinting results. Walling (2013), in the opening paper, provides a history on the sediment fingerprinting approach and illustrates how publications on the subject have increased exponentially since the 1970s. A review of these publications indicate that many studies have used sediment fingerprinting to determine sediment sources as a means to improve our understanding of erosion and sediment delivery processes. Other papers use sediment fingerprinting as a management tool. Walling (2013) lists key advances in the sediment fingerprinting approach over the last 30 years that include the use of multiple or composite fingerprints, statistical tests and models, size and organic correction factors, increased source and target assessments, and improved estimates of uncertainty. Given these advances in the sediment A. C. Gellis (*) U.S. Geological Survey, 5522 Research Park Drive, Baltimore, MD 21228, USA e-mail: agellis@usgs.gov