Soil Erosion and Sediment Pollution

Although sediment is a part of the natural environment, human activities sometimes increase the amount that ends up in streams. These sediments are usually fine grained sands, silts and clays that can cover up coarser sediments and the spaces between rocks and cobbles that provide habit for aquatic life.

Excess eroded sediment degrades habitat in the following ways:

• Suspended sediment decreases the penetration of light into the water. This affects fish feeding and schooling practices, and can lead to reduced survival.
• Sediment reduces the amount of light penetrating the water, depriving the plants of light needed for photosynthesis.
• Sediment particles absorb warmth from the sun and thus increase water temperature. This can stress some species of fish.
• Settling sediment can bury and suffocate fish eggs and bury the gravel nests they rest in.
• Suspended sediment in high concentrations can dislodge plants, invertebrates, and insects in the stream bed. This affects the food source of fish, and can result in smaller and fewer fish.

The stream-bottom sediments on the left provide spaces for fish to lay eggs and for invertebrates to live and hide. Excess erosion has deposited fine grained sediments on the stream bottom to the right. There are no spaces available for fish spawning or for invertebrate habitat.

• Excess sediment from eroding soils contains organic matter that contributes to oxygen depletion in the water as it is decomposed.
• Eroding soils also contribute the nutrients nitrogen, and especially phosphorus. In low nutrient streams and recovering waters these can contribute to algal growth and oxygen depletion.
• Suspended sediment in high concentrations irritates the gills of fish, and can cause death.
• Sediment can destroy the protective mucous covering the eyes and scales of fish, making them more susceptible to infection and disease.
• Sediment may carry toxic agricultural and industrial compounds such as heavy metals and pesticides. If these are released in the habitat they can cause abnormalities or death in the fish.
• Sediment loads in waterways often result in further increased erosion and instability of streambanks, causing stream channels to become wider and shallower, which leads to warmer water temperature.

Schematic adapted from "Turbidty: A Water Quality Measure", Water Action Volunteers, Monitoring Factsheet Series, UW-Extension, Environmental Resources Center. It is a generic, un-calibrated impact assessment model based on Newcombe, C. P., and J. O. T. Jensen. 1996. Channel suspended sediment and fisheries: a synthesis for quantitative assessment of risk and impact. North American Journal of Fisheries Management. 16: 693-727.

Where does excess sediment come from?

Sediment is a natural part of the ecosystem. Streams transport sediment washed in from the watershed and deposit it on natural bars or into the larger rivers and lakes that they empty into. Problems occur, however, when activities such as road construction, building construction, landscaping, logging, or poorly managed farming remove the protective vegetative covering from soils. Loose sediments are then free to wash into the streams with surface water runoff during rain storms. The dirt and sand that builds up on city streets is also a source of sediment, as this gets washed into streams through the stormwater system.

The streams, themselves, can also be a source of excess sediment. As stream flows increase, the increased amount of water leads to streambank erosion. The sediment that was a part of the streambank then enters the stream, further increasing the suspended sediment concentrations and loads. Streamflows increase when forests are removed, wetlands filled or impervious surface area is increased.

How can sediment pollution be controlled?

In order to minimize the amount of sediment free to wash into streams during construction and landscaping activities, a sediment control plan must be created and implemented before there is a problem. First-off, minimizing the amount of land disturbed can significantly reduce the amount of erosion, and reduces the area where seiment needs to be controlled. Sediment management techniques include installing silt fences, structural modifications, diversion ditches, sediment traps and basins. In order to be effective, these techniques must be properly installed and, of equal importance, maintained over the duration of the project. Once construction activities are complete, mulch and vegetation should be applied to bare surfaces as soon as possible to anchor the soil in place.

How is measure suspended sediment measured?

Total suspended sediment concentration (TSS) can be estimated in several ways ranging from simple tranparency tubes to complex automated sensors. Quantitative methods vary but essentially involve a gravimetric procedure where a known volume of water is filtered to concentrate the sediment on to a filter, drying, then weighing. More can be found in Unit 3, Modules 9 and 11 within the water science curriculum.