Soil Erosion and Sediment Pollution
sediment is a part of the natural environment, human activities sometimes
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
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