Common Causes of Streambank Failure

Environmental Training Resource
August 16, 2012 — 1,609 views  
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Streambank failure can be a serious problem in natural ecosystems, as the flood of sediment in streams, rivers and other waterways is one of the most common pollutants among U.S. bodies of water. According to the Agricultural Research Service at the U.S. Department of Agriculture (USDA), trapped sediment can reduce the useful lifespan of dams and reservoirs, exacerbate flooding and cause harm to aquatic plants and animals.

"The problem is that the primary source of sediment in many...streams and rivers is from streambank failure, not from field runoff," said Glenn Wilson, a hydrologist at the Agricultural Research Service. "Up to 80 to 90 percent of sediment in these streams can be due to bank collapse."

Soil composition

There are several different factors that ultimately contribute to and cause streambank failure. One of the most important considerations is closely related to the composition of the streambank material, according to the USDA. Bedrock is generally regarded as being quite stable, but can cause erosion in the opposite bank if material is soft. Cohesive banks are more resistant to surface erosion because they aren't as permeable as other soils. However, this same low permeability can render the soil more susceptible to failure during a rapid drawdown of water levels.

Meanwhile, cohesion-less banks - heterogeneous mixtures of silts, sands and gravels - can fall victim to gravitational forces, bank moisture and characteristics within the particles. Finally, stratified or interbedded banks are generally the most common type due to natural layering processes. Cohesion-less layers are interbedded with cohesive soils to create a more erosion-resistant composition and maximum streambank stabilization.

Change is bad for streambanks

Another factor that can contribute heavily to streambank failure and erosion is any possible changes in watershed and reach levels around the bank, according to the Science Education Resource Center at Carleton College. Changes at the watershed level could be caused by any number of human-related factors, including excessive logging, mining and road building. Meanwhile, changes at the reach level are usually due to vegetation change or loss, animal use, channel excavation, an abrupt change in the course of the stream and climate change.

One final factor that can weaken streambanks tremendously is the removal of mature plants from the surrounding areas. On the flip side, adding young riparian plants can be an effective way of reinforcing the structure and has emerged as one of the most popular methods of streambank protection.

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