Surface Water Management Division
Landowner Guides to Streamside Living
Frequently Asked Questions about
Erosion and Sediment Problems
Real questions from streamside landowners
How can I stop streambank erosion?
Streambank erosion is influenced by many factors including water velocity, flood frequency and flow, streamside vegetation, soils, sinuosity (how your stream bends and weaves through the landscape), gradient (how steep your stream is), and your specific location on the stream system.
Bank erosion is a natural process that cannot be stopped. That is not necessarily a bad thing. Streams continually erode in some areas and deposit eroded material in others. Natural amounts of erosion are a necessary part of any healthy, productive stream, providing a consistent supply of salmon and trout spawning gravel.
Bank erosion can often be predicted, and with good planning, we can minimize problems associated with bank erosion. By manipulating the factors that cause bank erosion we can reduce it in places where it is particularly troublesome.
Removal of streamside vegetation almost always increases erosion. The single best thing people can to reduce and prevent excessive erosion is to leave existing streamside forests intact and plant trees where no streamside forest currently stands. Roots stabilize soils and fallen trees reduce water velocity and erosion. Occasionally, a fallen tree along a stream will cause some localized erosion. The long term benefits of a fallen tree in a stream or on a streambank far outweigh the local erosion caused when it first falls.
How do I repair streambank erosion?
Streambank erosion and how to address it varies from site to site.
Since every site is unique and treatments in the wrong place may accelerate erosion instead of repairing it, your best option is to contact an expert for a site visit. Contact one of the resources in the How To Get Help page for technical assistance. They can offer expert advice and, in some circumstances, may even contribute labor and materials to fix the problem.
Can I dump rock along my streambank to stop erosion?
It was once common to dump rock along streambanks anywhere there was erosion. Research now shows that this practice has negative impacts on streams, salmon, flooding, and downstream erosion. Many rock-lined banks have caused serious erosion problems for downstream property owners. In many instances, the erosion caused by lining streambanks with rock is worse than the erosion the rock was intended to fix.
Today, streambank rock may be used in specific locations, where erosion problems are unusually severe or where unique conditions prohibit alternative solutions. In most cases, however, a technique called bioengineering is cheaper and more effective than rock.
Bioengineering uses carefully designed placements of trees, shrubs, and logs to stabilize soils. Bioengineering has the added benefit that it enhances fish and wildlife habitat. It is also self-sustaining, since trees and shrubs grow and reproduce over time, whereas rock needs to be replaced periodically.
Like any other stream project, careful engineering is essential, and permits are required. Contact one of the resources in the How To Get Help page for assistance.
How can I deal with upstream development that caused sediment and erosion problems on my property?
Snohomish County’s Department of Planning and Development Services implements building, land use and other codes that are applicable to residential and commercial building construction. Specific services include building and site inspections, drainage and grading, and permit and site review.
If you suspect problems on your property are the result of upstream activity, contact the Department of Planning and Development Services at 425-388-3311. You should try to contact the department when the problem is occurring. Keep a careful record of when you see sediment in the stream and take photographs so that department staff can properly diagnose the problem.
If you see dirty, polluted, or discolored water, or if you are unsure of the source of the problem, you may also call the Water Quality Investigation Line at Snohomish County Surface Water Management, 425-388-6481, so an investigator can determine the source of the problem.
My stream has changed a lot in the past few years. Is that normal?
Yes. You should expect change since streams are dynamic systems. The changes and movement we see in streams often create the best salmon habitat. Streamside vegetation grows, reproduces, and dies. Trees fall into the water, sediment moves around, pools are created, spawning beds shift, and new channels form. Our floodplains are criss-crossed by historic stream channels, long abandoned.
Experience is a good teacher, and we now know that attempts to alter or stop these natural processes frequently create problems for ourselves and our neighbors. Good property management strives to predict and accommodate movement and change in streams. It allows these natural processes to continue and works with them rather than against them.
If you see changes that you don’t understand or don’t seem right, contact your County Watershed Steward at 425-388-3464. Your Watershed Steward can answer questions and help you resolve problems.
How can I prevent my stream from jumping out of its channel?
What should I do if sediment is accumulating in my stream?
These two questions are closely related.
Accumulations of sediment, such as sand and gravel, that can cause a stream to leave its channel are sometimes caused by upstream activities. Certain portions of streams, however, naturally tend to accumulate sediment regardless of upstream activity by people. This is most commonly seen at the base of hills or slopes.
Fast-moving water carries higher volumes of sediment, and slow-moving water tends to deposit sediment. Where streams flow quickly down a slope, they move sediment. When those streams reach the bottom of a slope and the water moves more slowly, the sediment drops out of the water and accumulates in the stream bed.
Sometimes, accumulating sediment can raise a stream bed above the surrounding land. Since water flows downhill, the stream will leave its old channel and flow to lower ground. This is a natural occurrence in specific parts of streams and is one way that healthy habitat is developed. It often goes unnoticed and doesn't cause problems.
Unfortunately, if this occurs in your backyard or next to your house, it can be cause for concern. In the past, the problem was solved by dredging sediment from the creek bed or building a levee to contain the creek. Now, we know that these solutions can cause more problems than they solve (see the Sediment and Dredging question). In addition, dredging is usually a short term solution, since the stream bed will likely fill with sediment again.
Getting to the root of the problem is important. It may involve doing a site survey and looking at the entire stream system to determine the source of the sediment. Often, there are solutions to protect yards and buildings and protect water quality and stream health as well.
Since sediment problems are usually site-specific, your best bet is to contact one of the resources in the How To Get Help page for advice. They can look at your situation on the ground and help you develop a site-specific solution.
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