According to Wikipedia, "Above Plucks Gutter, where the Little Stour joins it, the river is normally known as the Great Stour." The hamlet that is Plucks Gutter is about ten miles downstream from Canterbury.
Even so, I wasn't the only one to be surprised on one occasion, when "the Stour" was not accepted as an answer to this question. But it did happen!
Shortly after the confluence, the Stour takes a turn southwards before looping back northwards. Although the mouth is only about five miles from Plucks Gutter as the crow flies, the length of the river below the confluence is at least twice that.
© Haydn Thompson 2023