Erosion
What is erosion?
Erosion is the action of wind, water and ice that dislodges, dissolves or removes surface material.(1) When material is removed from one location, it is inevitably placed somewhere else; this is called deposition. Both erosion and deposition are natural processes. But when human activities substantially alter, increase or impede the natural movement of sediment, serious damage can occur.
How does erosion occur?
Over time, even the largest mountains are worn down by weathering and erosion. Weathering is “rock decay,” which occurs through physical, chemical and biological processes. Once the minerals have been weathered and broken down, they are prone to being transported (eroded) by wind, water and ice.
The impact of raindrops on bare ground dislodges soil particles and causes erosion on the smallest scale, termed rainsplash erosion. When water flows across sloped surfaces, it tends to form rivulets; these can create small channels in soft soil or sediment, and cause rill erosion. With enough rainfall on exposed earth, rivulets join together and gouge gullies in the land. Such processes can transport large amounts of topsoil and sediment from the land into water bodies.
In some places, such as prairies, deserts and coastal sand dune systems, wind causes erosion. The infamous “Dust Bowl” of the prairie states and provinces in the 1930s was caused by wind-driven erosion of soil that had been damaged by drought and intense farming practices.
As explained in coastal sediment processes, shoreline erosion is a natural phenomenon. Bluffs are continually worn away by the action of waves and rainwater, and their sediment nourishes sandy beaches. Sand and cobble beaches themselves are part of a constant cycle. Winter storms wash away the fine material, and smaller summer waves gently replace it.
Erosion caused by rivers and streams is also basically a natural process.
Especially during times of high rainfall, streams can move large amounts
of sediment.
Sediment removed by erosion is deposited when the flow of wind or water
slows down. For example, when a stream or river widens, the water slows
and sediment that was carried along drops to the bottom. In this way,
a delta is formed at a river mouth in a sea or lake.
How do human activities contribute to erosion?
Soils in this part of the world are usually covered in thick vegetation.
Herbs, grasses, shrubs and trees help to prevent excessive erosion caused
by heavy rains in a number of ways:
* Leaves and stems intercept the direct impact of raindrops, helping
to prevent rainsplash erosion;
* Roots create channels for rainwater to soak into the ground, help
to anchor soils, and protect stream banks from the force of rushing
water;
* Decaying plant matter accumulates in and on top of the soil, creating
a spongy “mat” that soaks up rain and prevents the soil
from being washed away.
When vegetation is removed, these natural functions are lost. Agriculture,
logging, construction and road building usually involve removing existing
vegetation. Unless these activities are carefully planned to minimize
damage, erosion often occurs.
When human-made structures stand in the way of coastal sediment processes,
problems arise. One worst-case scenario is a slope failure that damages
or destroys a house built too close to the shoreline. Compaction of
the soil, from the weight of structures, and excessive irrigation contribute
to this situation. Vegetation removal, infilling, shoreline armouring,
and construction of seawalls and groynes are practices that can cause
shoreline erosion (see altered shorelines).
Human activities substantially affect on the energy of a stream. Impervious
surfaces such as roads and parking lots cause unnaturally large volumes
of rainwater to flow into streams, increasing the stream’s destructive
power. Erosion can scour away the land beside a stream, and deposit
large amounts of sediment downstream, in areas of slower stream flow.
How does erosion affect ecosystems?
Topsoil is formed over hundreds to thousands of years, through complex
interactions among the bacteria, fungi, worms and insects, that live
among the roots of plants and help to break down organic matter. When
topsoil is washed away by erosion, the land becomes much less fertile,
and can support fewer plant and animal species.
Soil and sediment that is washed into natural water bodies degrades
water quality and fish habitat in a number of ways:
* Light penetration of the water is decreased in murky water, impeding
the growth of aquatic plants;
* Gravel spawning beds used by fish such as salmon and Cutthroat Trout can be buried and smothered;
* Fish gills become clogged, and sediment irritates the mucous membranes
of the skin and eyes, making fish more susceptible to disease and infection.
Excessively high water flows associated with impervious surfaces and vegetation removal can destroy and degrade habitat for a wide range of plants and animals. This occurs when land adjacent to the stream is destroyed, or characteristics of the stream bed are altered, either by excessive erosion or deposition. The effects of habitat loss for even a few species are felt throughout the food web, since biodiversity is lost and ecosystems cannot function properly.
Paradoxically, a lack of sediment can cause damage to ecosystems as easily as excessive sediment. Some types of shoreline rely on the deposition of sediment supplied by natural erosion to retain their habitat characteristics. For example estuaries, deltas, salt marshes, coastal sand dunes and sand and gravel shorelines all require regular, natural deposition of various types of sediment. Fine sediment such as sand and silt provides the primary material in which plants root and certain animals burrow. Human-made structures such as seawalls and groynes prevent sediment transportation, and starve nearby shores of fine sediment (see coastal sediment processes). This results in a gradual coarsening of beaches, and habitat loss for burrowing invertebrates living in the sand, and loss of a food source for the birds that feed upon them.
Dams also prevent the downstream movement of sediment, since it falls out of the water in the calm reservoir behind the dam. Areas such as floodplains and estuaries are then deprived of the gravel and sediment necessary for their proper function. For example, salmon require gravel beds for spawning, and fine sediment accumulates among tree root masses and log jams, creating new soil for riparian plants to grow in.
Erosion damage can create a window of opportunity for invasive species. Native species often cannot cope with the changes in soil characteristics and water quality. In contrast, certain invasive species may have evolved tolerances to these conditions, since they come from a different landscape. One example is Scotch Broom, which is able to grow in very nutrient-poor soils.
How does erosion affect people?
- When erosion occurs on a large scale such as a landslide, it can result in devastation of communities, roads, land and even lead to loss of life.
- Loss of topsoil can render the soil much less fertile for farming. Three quarters of the cultivated land in B.C. is considered to have a high to severe risk of erosion by water; 36 percent of farmland in the Prairies is at a high to severe risk of wind erosion.
- Habitat loss can negatively affect commercially important species such as salmon.
- Excess sediment in drinking water can increase the cost and reduce the effectiveness of water treatment.
- Coarsening and steepening of beaches, caused by shoreline erosion, makes them less attractive for recreation.
- The effects of erosion degrade the landscape, making it less attractive from a recreational and aesthetic point of view; this can also affect the tourism economy.
- Erosion damage to valuable ecosystems such as wetlands and estuaries reduces their ability to provide important services such as flood control, water storage and filtration, and protection from storm waves.
Additional Information & Links
Websites
- Stewardship of BC
- Factsheet: Marine Guide to Preventing Shoreline Erosion: Fisheries and Ocean
- Fish Habitat and the Effects of Silt and Sediment
- Ecology of Dams
- Shoreline Erosion and Ways to Prevent It: Living By Water
Articles & Books
- Christopherson, R.W. Geosystems, an Introduction to Physical Geography, MacMillan Publishing, New York, 1992.
- Trenhaile, A.S. Geomorphology – A Canadian Perspective. Oxford
University Press. Don Mills, ON. 2004.
