Most likely soft stem bullrush. It can get pretty tall (upto 6-10ft depending) and create a wide band fairly deep (upto 5-6ft) around the pond where it limits access to the shorline, thus I do not like it. In larger waters where a boat is usually used it is okay. I prefer shorter shoreline plants such as the spike rushes, shorter rushes, iris, pickerel plants, and arrowheads. The triangular rush Scirpus pungens is common and does not get real tall -2-3ft and stays in slightly shallower water - see below. IMO it would be better in a pond than softstem bullrush.
