Brazilian Elodea - Also known as Brazilian Waterweed, its a fully submerged ornamental that is used primarily in fish aquariums.
Leaves: Lower leaves grow in a whorled formation and typically consist of 4 leaves per whorl. Upper portions of stem often appear more dense than the base. Color can vary from bright to dark green depending on light availability, time of year and growing conditions.
Flowers: Small white flowers with 3 petals and yellow center, on thread-like stems will often breech the water's surface when in bloom. Blooms in summer.
Habitat: Can infest the beds of freshwater lakes or rivers or any other location with year-round fresh water. It can be found as deep as 7 meters or as shallow as a few inches.
Weed Classification: B Designate
Why is it a Noxious Weed?
It can infest a variety of fresh underwater habitats, reproduces readily from fragments and is often difficult to eradicate due to its prolific nature and ability to grow in areas difficult to access. Brazilian elodea forms dense mats on the beds of lakes and rivers which can affect flow, trap sediment, and easily outcompete native vegetation.
Plummer lake is likely the point source for the infestation of the Chehalis river downstream of it. It is believed to have been introduced via the dumping of aquarium contents into the lake.
WA State Noxious Weed Board
UC Davis Weed Report
King County Noxious Weed Board - Best Management Practices