SUPERIMPOSITION and PIRACY

"Transverse drainage incision associated with a regional drop in base level"

  A drop in base level can happen to all drainages, but required for a drainage to superimpose itself across a buried structure.  Once established across the buried bedrock high, a lowering of base level supplies the needed energy for the drainage to incise into the buried structure and excavate the covermass.  A drop in base level helps instigate stream piracy through headward erosion, but the long time delay for a drainage divide to retreat into the course of an upstream drainage complicates the timing.  Even though other piracy mechanisms  (aggradational spillover, sapping, and lateral erosion) do not directly depend on a drop in base-level, but likely plays some role.  The Goosenecks of the San Juan River  exemplify transverse drainage incision from base level lowering, in this case tied to the dramatic incision of Grand Canyon (upper image).  The lower image illustrates a physical model where drainage divide retreat into an upstream drainage, generating capture and transverse drainage incision.