An attractive flower sometimes found growing by shallow rivers, canals or ponds is the Flowering-Rush (Butomus umbellatus)   It can grow up to 5 feet tall in mid summer with each flower having three pale pink petals interspersed with three dark pink sepals giving the appearance of six petals. After being introduced as a garden plant to the USA it has become invasive and a real problem but is fairly rare in Britain in the wild and no problem.