And a day when I started to pay really close attention to the rapids on the river.
As a passenger on a raft (really, only a fool would allow me access to the oars in any rapid bigger than a riffle), we trust the boatmen. They are the ones who can quickly read and assess a rapid, either by standing up and observing just before entering, or by pulling ashore and scouting from above. They are the ones who make the snap decisions in the midst of chaos, deciding in an instant whether to pull left or right to get by the unexpected hole or pourover or eddy wall. They are the ones who keep their cool when the giant waves threaten to completely envelop the raft and sometimes tip it over (flipping is a highly undesirable outcome in a rapid; there's nothing fun about it at all). We trust them, and when they do their job really well, a passenger can actually become a bit complacent. If we've managed 40 or 50 rapids without problems, well, it can't really be that hard can it? And that's when things can get dicey.
Why was I suddenly watching rapids with a renewed interest? We had reached the point on the river where John Wesley Powell was inspired to write one of his most famous passages, the one which also inspired the name of this blog series:
We are now ready to start on our way down the Great Unknown...We are three quarters of a mile in the depths of the earth...We have an unknown distance yet to run, an unknown river yet to explore. What falls there are, we know not; what rocks beset the channel, we know not; what walls rise over the river, we know not.
The next morning he adds:
At daybreak we walk down the bank of the river, on a little sandy beach, to take a view of a new feature in the canyon. Heretofore, hard rocks have given us bad river; soft rocks, smooth water; and a series of rocks harder than any we have experienced sets in. The river enters the granite!
We can see but a little way into the granite gorge, but it looks threatening.
He and the mountain men who served as his crew had already been on the the river system for two months, and with their inadequate clumsy boats had run or portaged many dozens of rapids that were terrifying. They were running very low on food (the diet: unleavened flour, dried apples and rancid bacon). And now the nature of the rocks exposed along the river promised rapids far worse than any they had encountered upstream.
Indirectly, it was indeed the harder rocks. They were entering a part of the canyon composed of harder rocks than anywhere else along the river. It isn't the rocks themselves that make bad rapids, though. The river does not fall over ledges and waterfalls. Rapids on the Colorado River happen because of debris flows that enter the channel from the small tributary canyons. The debris in essence dams the river and forces the river channel to the side, making the cross-sectional area of the channel much smaller. Since the same amount of water in a river passes a given point in a given amount of time (cubic feet per second is one measure), the river must speed up to pass the barrier. You can see this effect in the picture above in Nevill's Rapid.
The severity of a rapid is determined by the volume and size of the boulders in the debris flows, and canyons cut into harder rocks produce larger boulders. Sprinkling a few giant boulders throughout a rapid turns a riffle into a terrifying roller coaster ride.
Geology of the Grand Canyon was actually one of the more difficult courses I had ever taken because not only did we need to master a lot of geology in a short time, but we also had to prepare for a challenging backpack down and then back up a series of officially unmaintained trails in the canyon (the New Hance and Grandview trails). The co-requisite for the class was a 2 unit physical education course in backcountry camping that including an entirely separate shakedown trip in the mountains of Southern California. When I came out of the Grand Canyon six days later I knew what I wanted to do with my life.
Of course, a few things have changed since 1976. Geotripper weighs, um, a lot more than that gawky teenager on the right in the picture below. Picture quality has improved, not so much because of better photographers, but it used to be expensive to take and develop pictures, so we never took very many. Plus we were using the old Kodak Instamatic cameras or something similar.
Still, seeing these pictures a few years ago on Facebook (thanks to J. Elson) brought a shock of memories, and now for the first time in forty years I was once again standing at the rapid that made a geologist out of me.
We had reached the base of the Grand Canyon Supergroup, and could now see the three formations that make up the oldest units: the Shinumo Quartzite, the bright red Hakatai Shale, and the basal Bass Limestone (intruded by basaltic dikes). The rocks are tilted about 15 degrees, giving the illusion that the river gradient is even steeper than it already seems. It can't have been a comforting sight to Powell and his men in 1869.
The rocks today are called the Granite Gorge Metamorphic Suite, and they formed in a series of collisions between a group of volcanic islands (called terranes), and the ancient North American continent around 1.7 billion years ago. The metamorphic schist and gness units were intruded by granitic magmas at intervals between 1.7 and 1.4 billion years ago. And now those rocks are exposed in the very deepest part of the Grand Canyon.
It was hard to find a spot to scout, so the boatmen checked out the rapid by standing up as they approached.
I turned in early once again...tomorrow we faced the biggest rapids so far on the trip.