Dean I worked down there fron 81 thru 89. Most of the info you are questioning was what i was told when training in by people who had worked down there in the 60's some even in the late 50's.
37 feet. the belknap guide first published in the 60's used 37 feet. I have never seen or heard of any scientific publication that disputes that. I was able to track down a national geographic from I beleive 62 that cronicaled the jet boat uprun. Most of the article was about lava as it took them 4 days to get up it. NG also used the 37 number. But again were thy using Powells number? Personaly it dosen't look like 37 feet but who am i to bust a myth and a billion egos
If one looks from the first swirly on river right to the last tail waves near the spring on river left it appears to me about 250 yards the ledge hole to cheezegrader about 50.
The fastest navigatable water was from the 60's. we thought it wrong in the early 80's it was never part of my schtick but I heard it often. The few times someone bothered to time aboat going thru all the times were in the 20 second range. Again a guide story i did'nt use and have no idea where it started but i heard it often.
We had a saying as guides down there"Never let to much truth ruin a good story" IMO most of these facts and figues and stories bear wittnes to that. But i will not be the one that changes anyof this
. peace sj