Hi Dean, great posts. Here's why the Martin Whitis Guide says what it says. Side Note: Duwain does the Maps, and I do the text. I asked Duwain where he got the 13 feet on the map page, and here is his reply:
"I originally got it from Belknap, but they got it from the maps produced from the 1923 Birdseye expedition
(see current Belknap guide, page 27, top left photo caption). It's funny you should ask since RiverMaps just mailed a check to Dan Casidy to purchase a set of those maps, which I have never seen. You know that USGS pick inscription under the overhand below Brown’s Riffle? Yeah, that expedition… That was part of a larger effort by the USGS to map the plan and profile of many western rivers to identify potential dam sites. It was the only survey that mapped the profile and tabulated the drop through each rapid in the Grand Canyon until the GCMRC LIDAR survey in 2000 (more on that later). The Birdseye survey also established the mileage system used until 2003, too. The maps were first published in 1927, but we’re purchasing a set reprinted during WWII. These maps were the basis for most of the river maps and guides published until our third edition including the maps used by Norm Nevills, the Les Jones scroll maps, Belknap, and probably Stevens.
Back to GCMRC… The LIDAR survey actually mapped the river bottom and ground along the river corridor. GCMRC used that data to map the river’s thalweg (deepest channel) and establish the new mileage system. LIDAR penetrates water, but there were reflections from the surface that were used by USGS researchers to look at changes to the river profile since the 1923 survey. They published a paper on their work in Water Resources Research
in 2005. The upshot is that aggradation due to debris flows from tributaries and decreased peak flows in the river channel have caused all rapids to increase in drop by an average of 0.26 meters (almost a foot) with House Rock and Badger increasing the most by about six feet. A couple of rapids actually decreased, but they are the exception. The reason I’ve been looking into this is that I plan to try to contact the researchers to see if I can get a comprehensive list of all the rapids so I can update the guidebook with current info, but I want to see the original USGS maps before I do that.
BTW, the 13’ drop in Lava is just the main rapid itself and doesn’t include Son of Lava. It looks bigger than other rapids with more drop since it happens so abruptly."
A fun Lava story that i recount in Big Water Little Boats
(also called Vulcan Rapid, below Vulcan Anvil and Vulcan Throne) is what happened on the 1957 motor run through the Canyon made by Otis "Dock" Marston when the river was running 104,000 cfs. While up on the high scout on river left, Marston's crew watched in horror as they noticed one of their boats, the Boo Too, had come loose from it's tie up and was floating into the gut of Vulcan, hatches open, cameras laying on the decks. They got some good photos of the wayward ghost-boats run (and flip).
All the best, Tom