Water gaps can be created in two ways. This water gap was made the first way: the stream was there before the hills began to rise, and it maintained its course, cutting down as fast as the land rose. Geologists call such a stream an antecedent stream. See three more examples: Del Puerto and Berryessa gaps in California and Wallula Gap in Washington.
The other way of forming a water gap is by stream erosion that uncovers an older structure, such as an anticline; in effect the stream is draped over the emerging structure and cuts a gorge across it. Geologists call such a stream a resequent stream. Many water gaps in the eastern U.S. mountains are of this type.
A water gap is not guaranteed. If the stream cannot cut fast enough, or if its course is abandoned because of stream capture or some other reason, a small gap might result without a stream in it, perched high on a ridge. That is called a wind gap.