Plants that grow in running water form a subset of aquatic plants as a whole, and are generally understood to be classified as freshwater plants, as the oceanic tides are not considered to be running water. Like plants anywhere, they form a vital part of the ecosystem and are placed into many diversified subgroups based on outward characteristics of roots and leaves.
Plants rooted to the mud and muck in running water are broken up into three more subgroups based on their leaves. Leaves that are entirely submerged are extremely thin and flexible so as not to be damaged by the water's flow. These leaves are said to be highly dissected, meaning they are crinkle-cut so as to provide as much surface area to sunlight as possible. Examples of this subgroup would be European milfoil, parrot's feather, and hornwort.
The next subgroup has leaves that float on the water's surface. These leaves are much broader, undissected, and contain small air chambers called lacunae to stay buoyant. Examples are water-lily, Victorian water lily, water-shield, and floating heart.
The final subgroup is comprised of rooted plants with leaves that extend up beyond the surface of the water. Examples of these are cattails, hundreds of different species of reed, lizard's tail, arrow lead and sacred lotus.
Water lettuce is a perennial which floats along the surface of running water completely unrooted to the mud below. It produces a series of broad, lettuce-green leaves in a spiral around a central head and can grow up to six inches in diameter. It's known as an aggressive invader of new territories.
Water hyacinth are native to tropical regions of South America. They produce a clump of round, inward-curving, ovate leaves with a spike of roughly a dozen six-petaled flowers, typically in pink or lavender.
Amphibious plants are those capable of growing both in running water and on land. Great yellowcress is a member of the mustard family native to Ireland and England and is found in parts of North America growing wild. They grow up to a height of two feet from a central stem with immediate junctions sprouting from it. Leaves are long, narrow, and jagged. Flowers appear as miniature marigolds containing seeds in clusters at the end of each stem only.
Mermaid weed is native to much of New England. It grows to a height of one foot, with lower leaves forming a seaweed-like spray of light brown. Leaves higher up on the plant are more blade shaped and a bronze-red with red-purple flowers half an inch in diameter emerging from the newest growths.