Pegasus,
The picture you are showing is the low spot in the terrain outside Padre and Gunsite canyons. There is also a slot in the solid rock ridge near the back of Gunsite that, when the water level is 3685' or higher, you can run a fishing boat or Seadoo through there from Gunsite to Padre and vise versa.. It can be dangerous though as some try to take it at high speed and it's basically a blind shot through there. As I recall the passage is solid rock on both sides and about a couple hundred feet from one side to the other.
Chuck