Scientists think they've solved the mystery of Devil's Kettle Falls
startribune.com
startribune.com
GPS could be helpful on a large scale if it did go underground for a distance, but I'm wondering about either setting up some kind of a local high-resolution tracking system that can track objects in the water, or some kind of a gyroscopically-aware tracking device that could be placed into the water and transmit telemetry from inside the kettle itself!
Could you model the current of the water while it's out of sight?