As it turns out, the river running down the canyon literally "sinks" out of site into the limestone cliff bordering the river, hence the name of the canyon.
I've never seen such a queer river in all my wanderings. To add to it, the river re-emerges 1/4 mile downstream - on the other side of the road, it turns out. You would think that a river traveling at the velocity observable in the video wouldn't take long to travel a quarter mile, but die tracers indicate a 2 hour lag time from entry to exit. Subterranean explorations to understand this have been unsuccessful for obvious reasons. Weird.