Importance of Erosion and Sedimentation to Water Quality

When soils are well managed and maintained, they are efficient receivers of rainwater. However, if soil is disturbed and improperly managed, rain and snow/ice melt may run off the surface, carrying soil particles with it. This process, called soil erosion, has been a major cause of soil degradation in North Carolina for many years. Damage to water quality occurs when this eroded soil enters surface waters.

Sedimentation occurs when water carrying eroded soil particles slows long enough to allow the particles to settle out. The smaller the particle, the longer it stays in suspension. Larger, heavier particles such as gravel and sand settle out sooner than smaller, lighter particles such as clay. Clay may stay in suspension for very long periods, contributing significantly to water turbidity.

Sediment comes from many sources: agricultural fields, woodlands, highway road banks, construction sites, and mining operations. By volume, sediment is the largest water pollutant in North Carolina. It affects water quality physically, chemically, and biologically. Damage from sediment is expensive, both economically and environmentally. Sedimentation destroys fish spawning beds, reduces useful storage volume in reservoirs, clogs streams, and makes costly filtration necessary for municipal water supplies. Suspended sediment can reduce aquatic plant life and alter a stream’s ecology. Because the environmental damage from sediment is often additive, the ultimate effects and costs may not be evident for years. The consequences of off-site sedimentation can be severe, both for those immediately affected and for those who must cope with subsequent problems.

Sediment often carries organic matter, animal or industrial wastes, nutrients, and chemicals. The most troublesome nutrient element is phosphorus. In freshwater ecosystems developed under very low phosphorus conditions, large additions of phosphorus can stimulate the production of algae blooms. As the algae die, organisms in the aquatic system decompose the algae to use as a food source. In the process, they also use significant amounts of oxygen. If the oxygen level is initially low, the decomposition process can further reduce it to a point that “fish kills” can occur. Phosphorus may come from such sources as fertilizers, organic matter, and animal manure. Phosphorus is very immobile in most soils and concentrates in the top few inches of soil, making it very susceptible to erosion and likely to be present in sediment.

Sediment can also carry pesticides—such as herbicides and insecticides—that may be toxic to aquatic plants and animals. The varying chemical properties of pesticides—for example, their solubility, toxicity, and chemical breakdown rate—determine the potential damage to water quality.

Consider the following:

  1. Erosion and sedimentation is the most widespread water pollution problem across North Carolina (erosion_facts2007.pdf).
  2. In western NC, most of the streams classified as “impaired” are impaired by sediment. (
  3. Sediment destroys wildlife habitat, smothers eggs and the macroinvertebrates that live in our streams, and kills fish and other aquatic species.
  4. High turbidity in our streams has a major adverse impact upon trout and other sensitive species.
  5. Sediment can contain and carry harmful chemicals and pollutants that are applied to land.
  6. Soil erosion removes the most valuable topsoil needed to grow plants and food.
  7. Sediment fills lakes, ponds, and streams, increasing maintenance and drinking water treatment costs.
  8. Land disturbing activities, such as road and building construction, can accelerate erosion and cause severe stream sedimentation.
  9. Lakeshore erosion and sedimentation impacts both the aquatic habitat and interferes with boating and docks in Lake James.
  10. The NC Sedimentation Pollution Control Act requires anyone involved in land disturbing activities to take preventative measures to reduce soil erosion and prevent sedimentation.