Geomorphic Effects of Mosses in a Low-Order Stream in Fairfax County, Virginia

Local conditions play an important role in determining the effects of riparian vegetation on stream bank stability. In environments where trees have limited success in stabilizing stream banks, non-vascular plants, especially mosses, may provide alternative protection. Although mosses lack root structure, they are well adapted to saturated conditions and a thick tree canopy. This project evaluated the rate of bank erosion and effectiveness of mosses as a factor in reducing bank erosion in Thompson Creek, Virginia. The Thompson Creek watershed is dominated by non-cohesive sandy strata and a thick canopy cover. The thick canopy cover provides shade to stream banks, while the sandy non-cohesive bank materials make those banks susceptible to erosion. Field surveys were conducted to record stream bank erosion and changes in moss patches over time. A variety of techniques, including repeat photography, erosion pins, visual observations, and measurements of channel morphology, were used in field surveys. The exposure of erosion pins ranged from 0.5 cm to > 11 cm over a period of 19 months. In general, erosion rates appear to be high in Thompson Creek. The dominant processes of bank erosion are undercutting and bank collapse. However, the majority of moss-covered sites remained intact during the study period. The results of field investigations revealed that mosses have a potential role in stabilizing stream banks.