Catabrosa aquatica, called
brookgrass,
water whorl-grass,
water whirlgrass and
water hairgrass, is widespread species of semi-aquatic grass in the genus
Catabrosa, native to the most of the non-tropical northern hemisphere, and to southern Chile and Argentina. As its scientific and common names suggest, it prefers to grow in wet areas, such as meadows, stream banks and lake shores.
(Wikipedia)