This handsome aquatic perennial, which is well-named for its clearly defined arrow-shaped upper leaves, flowers in July and August in still or slow-flowing fresh water. Its bright white three-petalled flowers (2 cm across) are in whorls of three, the upper flowers male with numerous stamens, the lower ones female. This plant is native and belongs to the family Alismataceae.
I first identified this plant while on the River Barrow in Athy, Co Kildare in 1978 and photographed it from a bridge over the Barrow in Athy town in 2018.
If you are satisfied you have correctly identified this plant, please submit your sighting to the National Biodiversity Data Centre