This may look like just another hogweed like flower but actually, from what I can gather, it is something of a rarity in Britain and the chalk rivers of Dorset are one of the few places you can find river Water-dropwort (Oenanthe fluviatilis). It may be uncommon but where it occurs it can be prolific and can start to block up a water course and on the Frome and Piddle near Wareham the Environment Agency have to cut it every year to control it!
River Water-dropwort is a member of the carrot family, or umbellifereae, and grows in the open water of fast moving rivers and it is especially adapted for this purpose. The stems are smooth and the leaves wedge shaped to reduce resistance to the flowing water and the flowers lack bracts that many of the carrot family have for the same reason. Much of the plant is submerged with the flowers being visible above the water level from July to September.
As a species that likes chalky water courses that have a clay bottom it is not very tolerant to pollution and that is one reason why it is not particularly common but as the health of our rivers improves it is hoped that it will begin to thrive and prosper. In Hertfordshire there is an action plan to actually help it progress.River Water-dropwort :the water carrot