aquatic plant in Sliver Lake
I consulted with the Yakima County Noxious Weed Board and with the Department of Ecology on the identity of this plant. They both came up with the same diagnosis- Your flowering plant is Menyanthes trifoliata, Buckbean, Bogbean. It is a native species found in bogs and wet environments here in the PNW, as well as throughout temperate North America.
It is quite an attractive plant when in bloom!