Rapid flow through the sediments of a headwater stream in the southern Appalachians

1. The flow of water through the sediment layer (underflow) of streams can influence nutrient uptake dynamics and the supply of materials to microbes, meiofauna and macroinvertebrates living within stream sediments. We examined the extent of underflow in Hugh White Creek, a headwater stream in the southern Appalachian Mountains and compared underflow at different depths and at different sites within the stream. 2. Initially rhodamine dye was used to trace the flow of water through the sediments; however, the dye was strongly absorbed by the sediments in Hugh White Creek. Thus rhodamine was not suitable as a tracer of water flow. Chloride reacted conservatively in laboratory experiments and was used as a tracer. 3. The tracer infiltrated the sediments within 5 min to depths of 10 cm at all six sites. Chloride infiltration tended to decrease with depth of sediments at all sites although there was no consistent statistical pattern in chloride concentration with depth for the different sites. Equilibrium between the water column and sediments was reached rapidly, within minutes for the sites with coarse sediments and within a few hours for sites with finer sediments. Minimum rates of chloride infiltration into the sediments ranged between 1.0 cm min−1 for the sites with cobble substrate to 0.2 cm min−1 for sites with fine sand sediments associated with debris dams. These data suggest that underflow may be a major component in the functioning of Appalachian mountain streams.