how does a water gap form? an antecedent stream; land is uplifted to form a ridge or mountain. the pre-existing stream downcuts into the rising structure at a rate equal to the rate of uplift. a stream lengthens its course by headward erosion, eventually capturing and diverting the flow of another stream by means of stream piracy. a superposed stream downcuts through flat sedimentary rocks into a buried, preexisting structure such as a ridge or anticline. all of the above choices can create water gaps.