If you have spent years living near a stream or river, you know that the channel doesn’t stay put. Rivers and streams are dynamic – “healthy” streams are a constantly changing system. Streams are rarely naturally straight; rather, they wind their way through the landscape, meandering back and forth. Bank erosion, where the soil along a stream or river is washed away, is a natural process and is important for creating and maintaining habitat for salmon and other fish and wildlife.
Erosion: How It Happens
Erosion naturally happens at the outside of river bends where water is moving quickly. At the inside of stream bends, where water is moving slowly, sediment builds up. The result is a stream channel that is always slowly migrating in the direction of the outside of the bend. In some cases, especially during floods, the stream or river may make a sudden jump into a new channel.
While erosion and channel movement is natural, alterations to the landscape can cause erosion to happen more quickly. One of the most common areas where rapid erosion occurs is along unvegetated stream and river banks. Grass or sparse vegetation along a stream or river bank creates the perfect opportunity for erosion – sparse vegetation and grass do not provide a soil-stabilizing root system. Without a tree-shrub root system to hold soil in place, surface water and the energy of the water in the stream or river can work together to erode soil and gravel.
Removal of large pieces of wood from the stream channel can also lead to increased erosion. Wood in the channel slows water down, lessening the impact of high velocity water on some banks.
Structures installed within a stream channel can also accelerate stream bank erosion. Large rock or rip rap along a bank, a bridge or culvert, or other structures that confine the channel and water flow can cause erosion downstream, and may create problems for downstream properties.
How Can I Reduce Erosion On My Property?
If you are concerned about erosion on your property, leaving vegetation intact is your first line of defense. The root systems of trees and shrubs reduce the risk of erosion by holding the soil. Trees and shrubs slow surface water and increase water infiltration into soil, thereby reducing erosive energy. Native streamside plants also “drink” a lot of water – some species of native trees can absorb over 20 gallons of water per day during the growing season.
Planting additional trees and shrubs along your bank can provide additional protection, but where active erosion is occurring, new plantings are not likely to provide the short-term help that is needed. There are many soil stabilization techniques that may help reduce bank erosion. Many of these techniques require technical or engineered design work, and all work in a stream channel requires permits. These techniques vary from low cost – low complexity willow fascines (willow cuttings bundled and planted along a streambank) to high cost and highly technical bank stabilization projects that incorporate planting and in-channel structures including large wood.
If you have an eroding stream bank, contact Snohomish County Surface Water Management Cooperative Bank Stabilization Program, Washington Department of Fish and Wildlife, or your local Conservation District and schedule a visit to your property.
Which Native Plants Are Best For Reducing Erosion?
Some native plant species are better suited to help reduce the risk erosion on your streamside or riverside property. Fast-growing species and plant species with strong root systems provide the best for reducing erosion. Planting vegetation along a bank is only effective if the vegetation buffer is wide enough for the watercourse, and if the vegetation is planted at the appropriate density. For smaller streams, a streamside buffer should be at least 35 feet wide.
For larger streams and rivers, the planted buffer may need to be as wide as 100 feet or more to effectively protect property from erosion.
Do not plant non-native, invasive plants or noxious weeds – oftentimes, these plants hide erosion issues or accelerate erosion problems; at the very least, non-native invasive plants and noxious weeds will compete with native streamside species (working against your goal to establish a native vegetation).
Be sure to check out our fact sheet highlighting “erosion superstars” – native plant species that provide good soil stabilization. Consider plant requirements (soil, light, moisture) and plant characteristics (mature height and width, ability to spread, tolerance to flooding/disturbance, susceptibility to wildlife, wildlife habitat benefit) as you develop your planting plan. Check out the following resources for more information on plant characteristics and erosion techniques.
• Department of Ecology’s “Slope Stabilization and Erosion Control” online publication: http://www.ecy.wa.gov/programs/sea/ pubs/93-30/index.html
• King County’s Native Plant Guide: http:// green.kingcounty.gov/gonative/
• To find low-cost plant materials, look for wholesale nurseries and check out Conservation District plant sales or Washington Native Plant Society plant sales. A list of some local nurseries is available at: http://www.betterground.org/native-plantresource-list/
For more information
Contact the Habitat Team at 425-335-5634 or email firstname.lastname@example.org.
Better Ground is brought to you by the Snohomish Conservation District. 528 91st Ave NE Lake Stevens, WA | 425-335-5634, ext 4 | www.betterground.org
Sound Nature Idea #01