Fire Effects on Stream Sediments

The deposition of sediment in streams and lakes ranks as one of the top two water quality impairments in the United States (USEPA, 2000). Increased sedimentation can reduce aquatic habitat distinction and cause shifts in the composition of aquatic organisms (Berkman and Rabeni 1987). Extensively burned areas may contribute significant sediment loads by increasing overland flow and by leaving bare soil susceptible to direct rainfall impact. Not only does sedimentation cause problems for navigation and increase costs associated with water filtration, but many nutrient and chemical species bind to sediment that also contaminate the receiving waters. For example, phosphorus is a nutrient that readily binds to sediment and is commonly highly correlated with sediment loads in streams (see Fire Effects on Stream Nutrients).

Few studies in the South have concluded that burning alone causes significant sedimentation (Table:Fire effects on water quality). Field et al. (2000) concluded that sediment loss from prescribed burn plots in east Texas was within the range of undisturbed forests in the South. Similarly, Van Lear and Danielovich 1988 reported non-significant soil movement on steep slopes in the Southern Appalachians following burns under proper fuel conditions. Swift et al. (1993) also observed soil conditions that remained resistant to erosion over a range of burn intensities in the Nantahala Mountains of North Carolina. Van Lear and Kapeluck (1989), however, suggested that Piedmont sites may be more susceptible to high intensity burning than Southern Appalachian sites. A thin forest floor and root mat in the Piedmont is the primary the reason for the lower tolerance to fire. Ursic (1970) observed sediment yield increases of 400 lb ac^-1 in northern Mississippi watersheds that were both burned and treated with herbicides.

Steep topography generally increases the risk of soil movement during rainfall events. To reduce impacts of burning, state Best Management Practices (BMPs) should be followed. Table: Streamside Management Zone (SMZ) width recommendations shows the BMP guidelines for harvesting within streamside management zones. These guidelines could be used as a basis to define the widths of the streamside management zone (SMZ) where prescribed fires should be avoided. BMP guidelines for most states suggest that prescribed burning should be avoided in SMZs whenever possible. When necessary, fire lines should be constructed following the contour of the land and re-vegetated after the burn. Water bars should be installed where needed to prevent erosion. Special precautions should be taken when burning near riparian zones.

For more management guidelines on reducing the impact of fire on water quality, see Management Guidelines for Reducing Fire Effects on Soil and Water.