Hydrilla is a submersed typically rooted perennial plant that can form monospecific stands with dense mats at the surface. Its heavily branched stems can grow up to 9 m (30 ft) long. Leaves are simple and arranged in whorls of 4-8 leaves around the stem. The leaves are small, 2-4 mm (0.1-0.2 in) wide and 6-20 mm (0.2-0.8 in) long, and pointed. The finely serrated edges and typically 1-4 small bumps or teeth along the underside midrib are diagnostic. The whitish or translucent 3-petaled-&-3-sepaled flowers are tiny, only 1.5-3.0 mm (0.0625-0.125 in) in diameter, and float on threadlike stalks that are up to 10 cm (4 in) long.
Note the serrated edges of the leaves.
A major infestation.