we have been diving an estavelle locally here and during full flow as well to aid in clearing sediments that have accumulated.i will post pictures to see if it seems similar to what some are coining boil holes.it will be a couple weeks though as i am post surgery at the moment.interesting subject though,thanks to contributers.the boil will elevate up to 2ft above the surface water level to give a sort of gauge.i should stick a pitot gauge on and get the flow
i apologize for single case, i am operating the net under sedation and medical bondage of a rather devious design