Floating treatment wetlands are increasingly being implemented, tested and improved upon to treat stormwater and runoff. Plants suspended on floating mats of soil-like material float above the water offering water quality benefits as a makeshift wetland. Plants take up excess nutrients in the water, and the associated biofilm works to remove metals and petroleum-based contamination. The floating rafts and vegetative cover also shade the water, helping to cool water temperatures. In dense areas where the landscape is playing many roles – providing space for parking lots, buildings, surrounding forest, and stormwater ponds – floating treatment wetlands help expand the benefits native plants have on water quality and habitat. With the help of many volunteers, the Snoqualmie Tribe’s ENR Department built our own floating treatment wetland. Now 3 years into the process, we’re checking in on how this manufactured wetland is changing our local landscape.
Birdsong from the Floating Treatment Wetland
Why a floating treatment wetland?
In 2023, the Snoqualmie Tribe installed its own floating treatment wetland with the goal of reducing temperatures in the stormwater pond downhill from the Snoqualmie Casino and Hotel. The stormwater pond receives water from surrounding modern development, attempting to cool and clean water (improving water quality) before it joins other waters downstream. In typical restoration projects, if we want to create shade, we plant trees. However, stormwater ponds are required to have access for dredging and most facilities prefer to have few to no trees to keep a clear line of site to the water for safety purposes. Some deep-rooting large trees can also damage the structure of the engineered stormwater pond, making it less functional. Working within these conditions requires creativity on how to maximize shade in the pond beyond the small amounts provided by cattails and willows on the pond’s edge. Despite the unusual nature of this miniature landscape, volunteers planted many native species of wildflowers and shrubs, now blooming and providing habitat on the floating treatment wetland. In the past, the Tribe also considered shade netting across the pond and floating blocks to reflect light away from the surface of the water. Ultimately, a floating treatment wetland was chosen to create both shade and wildlife habitat, rather than impeding wildlife from using the wetland.
Floating treatment wetlands borrow lessons from Indigenous Knowledge related to water filtration and aquatic farming techniques. The Mayan people of modern-day Guatemala used pond lilies as part of a multi-tiered filtering approach to keep water reservoirs clean. The Aztecs used floating islands called chinampas, or floating gardens, to grow crops, which are becoming more common modern-day Mexico. As in many places, these once-familiar land-management tools are returning to the landscapes they used to be widely practiced in.
Floating wetlands also adopt the benefits of local native plants to improve water quality. All the plants chosen for the Tribe’s floating treatment wetlands are native to the Pacific Northwest. Our plants were chosen for their love of wetland environments, their cultural use, their ability to root in and shade open water, but also for their varied flowering times and established plant-pollinator relationships.
Borrowing from both Traditional Ecological Knowledge and modern research on ecosystem services and water quality benefits, these floating treatment wetlands use Two-Eyed Seeing to bring positive ecological change to the Snoqualmie Tribe’s historic and cultural landscape.
This year, Canadian Geese welcomed five new goslings to their nest on the floating treatment wetlands. Redwing blackbirds can be seen perched and singing on the willows and fruiting twinberry planted on the floating wetland. Bees have even taken residence in the structure of the floating wetland, but lucky for us and our monitoring efforts, decided not to stay. The floating treatment wetlands have been visited by many types of waterfowl, songbirds, snakes, and pollinators, who have welcomed this new habitat in the pond.
More data is needed to determine the effectiveness of the floating treatment wetlands for reducing pond temperatures and other contaminants. We are using a combination of temperature loggers and on-site sampling to monitor results, tracking both the physical conditions of the ecosystem and what wildlife is showing up to use it. These monitoring efforts are currently in place, with results pending. Floating treatment wetlands tend to improve in their effectiveness over time as roots, biofilm, and shade-forming canopies develop. Look for future updates on our little oasis in the stormwater pond as we continue to track its growth, filtration ability, and temperature impacts over time.
If you want to learn more about Floating Treatment Wetlands, check out the Snoqualmie Tribe’s curriculum on Floating Treatment Wetlands, intended for lessons in High School. ENR’s Water Quality Department also has a website that talks more about this floating wetland in particular. And as always, a huge shoutout to our volunteers and restoration partners who made this project possible.
