Treatment wetlandTreatment Wetland

A stormwater treatment wetland is a shallow man-made pond designed to treat stormwater through the biological processes associated with aquatic plants. These facilities use dense wetland vegetation and settling to filter sediment and oily materials out of stormwater.

Treatment wetlands take advantage of nature's approach to removing pollutants. They also provide habitat for amphibians, aquatic insects and birds. Occationally, vegetation and sediment removed, primarily to maintain the original storage capacity of the wetland. In general, stormwater wetlands do a good job of removing sediment, metals and pollutants.

Treatment wetlands need to be maintained to stormwater facility standards to avoid redesignation as a natural wetland, where permits are required for maintenance work. 

Virtual tour

Click on the image below to get inside of a treatment wetland and learn about common components and maintenance tasks.

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Maintenance needs commonly associated with treatment wetlands:

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Inlets and outlets

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Control structure


Maintenance sheet


Maintenance is needed if you see these signs  

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Bare, exposed soil - vegetation becomes sparse and does not provide adequate filtration
Slopes that are deteriorating
Sediment that restricts flow or clogs inlet and outlet pipes
Sediment buildup; the facility is not draining or conveying runoff
Unhealthy or dead vegetation
Blackberries or other problem weeds
Overgrown vegetation
Holes in berms or slopes, or berm/dike has settled 4 inches or more than designed height
Leaves, trash and other debris
Water surface is discolored or has an oil sheen
Water not retained to a depth of about 18 inches during the wet season
Bottom of slopes show signs of seepage and leaking
Trees, often alders, growing on the slopes



Tips for fixing problems and general maintenance

Completely remove invasive species such as blackberries and English ivy, then check for them on a regular basis and remove new vines.  Contact Clark County Vegetation Management for questions about controlling weeds at (360) 397-6140 or email 

Remove cattails before they start to dominate a facility.

Remove all unplanned trees or saplings that block parts of the facility or hinder maintenance.

Make sure banks, slopes and areas designed for vegetation are planted with native or easy-to maintain species. Avoid trees near the pond and on berms.

Plant at appropriate times during the year so vegetation can get established.

Replace vegetation damaged or removed during maintenance.

Check access roads and fencing, if the facility has them. They should be free of overgrown vegetation and other materials so that the facility is easily accessible for maintenance.

Control erosion by reseeding areas where soil is exposed, especially on slopes around a facility.

Fill in eroded areas and cover them with sod, mulch or other erosion control materials.

Removing sediment is best done in July and August before winter rains set in.

Pick up leaves before rains begin.

Inspect and remove debris regularly, particularly after storms.

Remove any material clogging drains, outfalls and channels.

Avoid using fertilizers, herbicides or pesticides in or near the facility. Chemicals pollute the water and can cause unwanted plant growth.

Identify sources of leaks or spills and contain them as quickly as possible.

Check inlets and any open or confined standing water for mosquito larvae (see photos below). If mosquitoes are a concern, contact Clark County Mosquito Control District for information. The 24-hour service request line is (360) 397-8430.

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Check roads and fencing in the facility. They should be maintained to allow easy access.