Looks like
This annual aquatic plant has clusters of floating leaves that blanket the water's surface. Plants are anchored in the soil by a long, tough stem, and typically grow in dense mats. Spiky, black seeds can be viable for a decade and wash up onto the Hudson's shores.
Lives in
Quiet bays, coves and shallows along the Hudson River, as far south as Haverstraw. Also occurs in some rivers, ponds, and lakes in the Hudson Valley.
Arrived
Introduced as an ornamental in the 19th century, first in Cambridge Botanical Garden in Massachusetts, then again in Collins Lake near Schenectady, New York. By the 1930s, water chestnut was established in the Hudson River.
Threats
Crowds out native plants and prevents recreational access by boaters, anglers and swimmers. Dense beds block sunlight from penetrating the water, preventing photosynthesis and leading to oxygen-depleted waters that are inhospitable to fish and other animals.
Control
Labor-intensive, usually controlled by cutting or hand-pulling.
Interesting fact
Water chestnut is not eaten by native waterfowl or fish.