News Releases from the Southern Research Station

Dating Stream Sediments

Asheville, NC -- Government agencies responsible for water quality, as well as anyone whose drinking water, recreational activities, or economic livelihood depends on stream health, show increasing interest in determining the source of stream sediment. At issue is whether sediment at a particular location results from recent terrestrial erosion or reworking of sediment already in the drainage system. Jonathan D. Phillips, University of Kentucky, and Daniel A. Marion, USDA Forest Service, have developed new and more affordable field and laboratory techniques to address this question. In Residence Times of Alluvium in an East Texas Stream as Indicated by Sediment Color, published August 1 in the journal Catena, Phillips and Marion use sediment residence times to discriminate between "new" sediment caused by recent erosion outside a stream and "old" sediment that was eroded in the past and stored within a stream.

The Environmental Protection Agency is charged with developing watershed goals for areas identified as being in an impaired state. The Total Maximum Daily Load (TMDL) sets the allowable amount of sediment moving through a stream. Southern Research Station hydrologist Dan Marion says, "If monitoring reveals a TMDL has been exceeded, the next question becomes, 'Whose sediment is it?'" One path to finding the answer could be determining the sediment's age. Phillips and Marion show that soil color can indicate the length of time sediment has been stored at a sampled site. Hypothetically, if these methods reveal soil characteristics specific to new sediment, then the focus of regulatory agencies could shift to determining the cause of erosion.

Phillips and Marion caution that their methods apply only where soils contain a large amount of iron oxides (a characteristic of many soils in the South). They also state that soil color indicators are most useful when combined with other methods. The depth of sediment covering the root collar of trees and root growth patterns can reveal physical history before and after flooding. Buried soil horizons, the presence of invasive tree species, and the age of buried trees can provide additional evidence to more precisely date sediment or demonstrate changes in sedimentation rates.

Marion conducts research studies in Oxford, MS, although he is a member of the Managing Upland Forest Ecosystems in the Midsouth research unit, located in Monticello, AR. Marion began working with Jonathan Phillip, a geographer, when Phillips was with Texas A&M University. Phillips' ideas about geochemical dating and soil characteristics formed the basis for their research on stream sedimentation in Loco Bayou, a tributary of the Angelina River. The research methods described in Residence Times of Alluvium in an East Texas Stream as Indicated by Sediment Color are based on inexpensive equipment and techniques familiar to many environmental scientists. Other methods, such as radionuclides, require highly specialized expertise and equipment and expensive laboratory facilities. Thus, the method represents substantial potential cost savings. Phillips and Marion's next study on stream sediment dating will focus on more quantitative and rigorous analysis of iron reduction in soils so as to better refine sediment residence times.