It’s almost winter, which means we’re knee-deep in the middle of the rainy season. In an average year, Snohomish County gets 35 inches of rain, and most of it falls during these next few months.

All of that rain has to go somewhere and much of it becomes stormwater, traveling across the landscape and eventually into Puget Sound.

This stormwater isn’t Britta-filtered. Instead, the average cup of water collected from rooftops, driveways and other impervious surfaces can be contaminated with bacteria, toxins, and oils that are harmful to fish and wildlife.

This is where green stormwater infrastructure like detention ponds are important tools that help filter water before it flushes back into Puget Sound. But like most tools , detention ponds need maintenance to continue to work properly.

How to Recognize a Detention Pond

If there’s a large ditch in the middle of your neighborhood that looks intentional, you probably have a detention pond. Despite the name, detention ponds aren’t always filled with water like their counterpart (retention ponds).

Instead, a special restrictor device located downstream from the pond controls the water levels. The restrictor, a small hole for water to filter through, is similar to an overflow drain for a bathtub. When the pond fills with more water than can pass through the restrictor, the water backs up and is stored in the pond until it stops raining. It should then empty to its normal level.

When working properly, these systems allow for stormwater to drain slowly into the ground, where bugs and microorganisms living in the mud then help naturally dispose of the pollutants within the stormwater.

How Do I Know If the Detention Pond Is Working?

As mentioned above, the water should always be draining. If not, you may have a problem. Maintaining a stormwater pond has some risks and may result in injury if not done properly. Please work with your local municipal stormwater authority and a licensed contractor to carry out maintenance!

Plants: Plant control, including weeding, mowing, and other maintenance, can ensure that the pond works as intended. Poor management can result in clogged pipes, stagnant water, and overgrowth.

Mow regularly to maintain 3-inch grass height
Remove noxious weeds
Cut back branches and shrubs that hang over the pond
Remove small trees from pond banks
Reduce/eliminate use of herbicides near pond

Soils: Detention pond soils are designed to clean and soak in water. If the soils are not cared for, they can become clogged with pollutants.

A hired professional can…

Rototill compost soil mix into ruts
Check that embankments are stable
Mow steep banks and trim high/large branches that reach into the pond.

Control Structures: Control structures are manholes at the pond outlet. They control how fast water leaves the pond, so that nearby creeks do not get flooded during a storm. Control structures need to be maintained by an expert, to make sure the complicated pipe system inside does not become plugged.

A hired professional can…

Remove animals that have burrowed or nested within the pond
Inspect structures and pipes annually

How Is This My Problem?

If you are a member of a Homeowners Association (HOA), you are responsible for maintaining your neighborhood detention pond! The city will regularly inspect public and private drainage facilities like this to determine whether it meets performance standards.

Think of detention pond maintenance as an investment towards your neighborhood’s value. If the pond breaks, that can mean standing water, a new mosquito breeding ground, and thousands of hard-earned dollars to fix what could’ve been prevented.

You are not alone in this work!

If you live in Granite Falls, please reach out to Charles with the City of Granite Falls Public Works at charles.white@ci.granite-falls.wa.us. You can also contact Snohomish Conservation District at outreach@snohomishcd.org for assistance.