SEGMENT-SCALE PATTERNS AND HYDRAULICS OF TRACE-METAL CONCENTRATIONS IN FINE-GRAIN SEDIMENTS OF A COBBLE AND BOULDER BED MOUNTAIN STREAM, SOUTHEAST ALASKA

This study documents and analyzes spatial patterns of variability in 1 2 trace metals in sand-size and finer sediments over a 2 km segment of a boulder and cobble bed mountain stream. The analysis focuses on con centrations rather than the total metal mass budget, because it is typ ically variations in metal concentrations that are important for asses sing potential environmental impacts. There are no consistent downstre am variations in metal concentrations, which indicates that there are no significant geochemical changes or external sources of metals in th e study segment. In fact, much of the spatial variability within the s tream segment appears to be random, although a dimensionless shear str ess model predicts the majority of locations where peak concentrations of metals contained in heavy minerals occur. The dimensionless shear stress model, however, only predicts the potential for enrichment and many sites where potential enrichment is predicted contain low concent rations of metals. Peak concentrations do not appear to be associated with metal-rich sediment waves moving through the system, but instead are probably the result of local hydraulic variability. Sampling of bo ulder and cobble streams should be stratified by hydraulic environment s in order to capture the full range of minimum and maximum metal conc entrations within the stream system.