Formation of a Water Gap

To fully understand the drainage pattern displayed by a stream, it is often useful to consider a stream’s entire history. For example, river valleys occasionally cut through a ridge or mountainous topography that lies across their path. The steep-walled notch followed by the river through a tectonic structure is called a water gap (Figure).
Why do streams cut across such structures and not flow around them? One possibility is that a stream existed before the ridge or mountain was uplifted.
In this situation, the stream, called an antecedent stream, eroded its bed downward at a pace equal to the rate of uplift. That is, the stream would maintain its course as folding or faulting gradually raised the structure across its path.
A second possibility is that a superposed stream eroded its channel into an existing structure (see Figure). This can occur when folded beds or resistant rocks are buried beneath layers of relatively flat-lying sediments or sedimentary strata. Streams originating on the overlying strata establish their courses without regard to the structures below. Then, as the valley deepens, the river continues to cut its valley into the underlying structure. The folded Appalachians feature several superposed rivers, including the Potomac and the Susquehanna, which cut their channels through folded strata on their way to the Atlantic.

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