Myriophyllum alterniflorum DC.
Family: Haloragaceae
alternate-flowered water-milfoil
[Myriophyllum alterniflorum var. americanum DC.]
Myriophyllum alterniflorum image
from USDA Plants website  
Myriophyllum alterniflorum image
University of Wisconsin-Stevens Point, Robert W. Freckmann Herbarium (UWSP)  
Etymology: Myriophyllum: from Greek myrios, "numberless," and phyllon, "leaf," alluding to the many divisions of the submerged leaves of these aquatic plants
Plants: perennial, submersed aquatic
Leaves: finely, pinnately-divided
Flowers: 4-parted; flowers from the axils of sharp, with toothed alternate bracts
Conservation Status: Native