just thought I would provide an update based on my experience on this issue. The impeller was undamaged after approx 1 min of runtime with the water turned off. It is soft and zero evidence anything got hot.
Hopefully there is not a next time, but if there is I won't be as worried about it. We live on a river system that is 100s of miles long and has a class 5 Rapids at the mouth. I did not want to take the chance.
