1. Water Quality Problems Related To Erosion and Sedimentation

Sediment is the largest pollutant by volume of surface water in the U.S. Sediment results from erosion from agricultural sources, logging operations, construction sites and land-disturbing activities in urban settings that leave the soil exposed to rainfall. The majority of soil erosion in the U.S. occurs on agricultural land—4 billion tons of soil are estimated to be lost from 395 million acres of cropland each year; an average soil loss of 10 tons/acre/year.

Erosion and sedimentation cause numerous problems in the U.S.

• Navigation. Sediment deposited in rivers or lakes decreases water depth, making navigation difficult or impossible. Dredging to remove sediment and maintain navigation is expensive and may release toxic chemicals into the environment. Sedimentation of navigation channels is a concern in many U.S. rivers, including the Mississippi River, the Columbia River, and the Delaware River, the Great Lakes, Chesapeake Bay, and harbors such as New York/New Jersey and Philadelphia. The U.S. annually spends more than $520 million to dredge soil sediments from navigable waterways.
  

• Fisheries/Aquatic habitat. Sediment in streams affects fish in several ways. Water clouded by suspended sediment impairs fish feeding and can lead to reduced survival. Suspended sediments irritate or clog gills and can cause death. Sediment can damage the eyes and scales of fish, making them more susceptible to infection and disease. Settling sediments may bury and suffocate fish eggs or make streambeds unsuitable for fish spawning or rearing. Sediment particles may also carry toxic chemicals that can cause abnormalities or death in the fish.
  

• Water supply. Almost 900 million tons of agricultural soils are deposited into U.S. reservoirs and aquatic systems each year. Heavy sediment loads can fill in water supply reservoirs and reduce their useable capacity. The presence of sediment in the water can wear out pumps and turbines, increasing maintenance costs. High sediment concentrations also increase treatment costs for drinking water; added costs to water treatment facilities to deal with sediment were estimated to be more than $120 million in 1995.
  

• Energy production. Dams and reservoirs trap sediment that would normally be carried downstream. Trapped sediment settles and decreases the size of the reservoir and reduces its use for power generation.
  

• Flood control. Heavy sedimentation reduces the storage capacity of flood control reservoirs and can lead to river flooding. Some of the 1993 flooding in the Midwestern U.S. was caused by increased sediment deposition in the Mississippi, Missouri, and other rivers.
  

• Phosphorus. Phosphorus is strongly attached to sediment particles. Transport of P from land to water by eroded sediment is a major source of P loading and a significant contributor to eutrophication of streams, lakes, and reservoirs.
  

• Toxic chemicals. Toxic chemicals such as metals or pesticides can become attached to sediment particles and then transported to and deposited in other areas. These pollutants may later be resuspended or redissolved in waters to affect animal and human health.

Erosion and sedimentation also cause direct problems in agricultural production. As we learned from the Dust Bowl era, soil movement and loss from agricultural land through water and wind erosion can cause loss of nutrients and productive capacity and deterioration of soil structure. Changes in soil properties such as increases in clay content, decreases in organic carbon content, rooting depth, and plant available water storage contribute to decline in crop yields with increasing soil loss through erosion. Reduction in crop yields due to soil erosion affects farm profitability and provides an added incentive to control soil erosion.

Additional land damage from wind erosion may include abrasion of crops and structures by flying soil particles, air pollution by particles in suspension, transport of nutrients and pesticides, and burial of structures and crops by drifting soil. Total annual costs of damages attributed to wind erosion were estimated at more than $9 billion in 1995.

Despite great emphasis on erosion control in the past several decades, sediment continues to be one of the most widespread pollutants affecting surface waters in the U.S. In a 1998 report, the Environmental Protection Agency estimated that more than 1.6 million acres of U.S. lakes and 84,000 miles of U.S. rivers were significantly affected by siltation, representing 21% of impaired lake acres and 31% of impaired river miles.

In 1995, the economic cost of all off-site environmental impacts of soil erosion on agricultural land in the U.S. was estimated to be more than $17 billion each year. An additional annual loss of $27 billion was attributed to reduced soil productivity on agricultural land. The combined cost of erosion in the U.S. was estimated to be more than $44 billion per year.

More Information

Learn more about the processes and factors affecting erosion and the movement of sediment into water: What is soil erosion?.

Last Updated: December 19, 2003 13:36