More beach details. A small sandy berm, and not too far away, a small eroding scarp.
The river rises and falls about once a day in the canyon -- a remarkably regular diurnal tide. But it's tied to the sun, not to the moon. When the sun shines on Phoenix and other southwestern cities, the air conditioners turn on and the power grid demands electricity. More water is allowed though the penstocks at Glen Canyon Dam and the flow in the Colorado downstream goes up. When it cools in the late evening, power demand falls, and so does the river. The resulting tide is generated at Glen Canyon, but it takes a day and half for the wave to travel down the river to Lake Mead.
Blacktail Canyon is on river right at Mile 120. Unlike so many other side canyons, it does not end in a spectacular pool or a waterfall. What it does have is a sand and gravel beach, although not one most folks would recognize right away.
It is the base of the Tapeats Sandstone, lying on top of the schist at head level as we head up the canyon. This is the Great Unconformity - a boundary in the rock layers that represents a gap between incredibly old rocks and rocks that are merely ancient. The top of the underlying schist, perhaps 1.8 billion years old, is the eroded surface of a rocky coastal landscape caught in time half a billion years ago. As the ocean gradually rose, the waves planed off this surface, breaking off chunks of the metamorphic rock and spreading it in layers on the advancing beach. What we see now on the wall of Blacktail Canyon are multiple layers of sand separated by bands of coarse, angular gravel derived from the schist (there are quartz veins in the underlying rock and quartz clasts in the overlying beach deposits).
I suppose for each of the layers we see preserved, many more were formed and then erased. I wonder if each of these layers records a storm or a series of spring tides? Maybe on closer examination we would find thinner layers that represented the sediment moved by individual waves?
Imagine a rocky shoreline on the New England coast. The rocks are Paleozoic schists and phyllites, intruded by slightly less ancient granites. The Atlantic is at your feet, but the coast is gradually subsiding and the beaches advancing farther onto the land. With time, these rocks are buried by more layers of beach sand and then later, as the water deepens, with silt and mud and eventually limestone. Half a billion years later, the land has been uplifted, a river has carved a deep canyon, and some strange creature in a rubber raft discovers your fossilized cellphone (the size and basic skeletal structure of a 3" trilobite) on the unconformity that separates the schist from the beach.
The Tapeats was my favorite rock formation in the Canyon - partly because it told this wonderful story of ancient shorelines and partly because it was a distinctive and beautiful feature of the canyon. Downstream from Blacktail we float through Conquistador Aisle, with beautiful ledges of Tapeast Sandstone lining the river.
Some of my favorite beaches on the river were small ones adjacent to rapids. I wish I had had more time to explore and observe, but one of the downsides of an organized trip is that there isn't much unscheduled time, and when there is, it's either not in quite the right place or it rapidly fills with other things.
Here are two contrasting beaches just above Walthenberg Rapid, where a slot canyon on river right spews boulders and gravel into the Colorado. One is a pretty typical sand bar; the other a neat little arcuate gravel berm. Whereas the former is purely a fluvial feature, the latter required waves.
Waves are an interesting question on the river. On open stretches of river, wind waves can form, but something else is going on right around the rapids. Maybe this is obvious to river geologists, but it was new to me. Rapids are not just a bunch of big static standing waves. Sometimes these big rollers grown in size and then collapse, just like surf, except they do it over and over in one place. This pulsing generates waves that travel out from the rapids and seem more than capable of building small beaches in the vicinity. One evening at a camp farther down the river, I watched 5-6 inch waves break on the beach with a fairly regular 4 second period.
A trip down the Grand Canyon will force me to rethink how I use the term "beach." Here, they refer to sand bars along the river bank, deposited when the river is running higher and thicker with sediment. And then gradually eroded by currents and small waves until a sufficient flow comes along to rebuild them. Or at least that's how I understand them to work. Beaches are an important element in river travel, as they are almost always where folks pull over to lunch or to camp. Where the winds are right and there is enough sand, dunes can form, so some of these features get pretty high.
The beaches in the canyon are also a big problem. Their maintenance requires sand and big flows. Since Glen Canyon dam was built, sediment from the upper Colorado has been preoccupied filling up the upper end of Lake Powell. And with the exception of 1983, when spring floods overwhelmed the storage capacity of the reservoir and huge volumes were released, flows in the now-tamed river aren't enough to rebuild the beaches. So for the last decade, managers and scientists have experimented with designer floods, specifically intended to restore beaches. The flows aren't of the scale of larger historic floods and the lack of sediment is still a big problem, but apparently the effort is having some positive results. The last big release was in early March, 2008.
The last time I was on the Colorado River, or anywhere near it, was on the receding shorelines of Lake Mead back in January. Now D & I have come back to the river, at Lee's Ferry, in order to check out the free-flowing reach that links Lake Powell and Lake Mead. Glen Canyon Dam is 15 miles upstream. Lake Mead - or at least South Cove, where we will take out in two weeks - is almost 300 river miles downstream.
I've posted a full account of the trip at Grand Canyon 2009. I've also posted comments at hshipman. But I've been struggling with what to post here. Not that there aren't dozens of stories about geology and beaches and shorelines -- it's just that it's been hard to sort through it all. Too much material; too few summer evenings since we got back (four weeks tonight).
At Lee's Ferry, the Colorado emerges from the Mesozoic rocks of Glen Canyon. The river briefly runs across the Permian Kaibab Limestone, loading up on rubber rafts and maybe a little sediment from the Paria River, and then quickly begins to carve downward into the Paleozoic rocks that form Marble and Grand Canyons. By the time we float under Navajo Bridge a few miles downstream, we have already dropped down through the Kaibab and the Toroweap formations and are beginning to see the cross-bedded dunes at the top of the Coconino sandstone. Right now these rock cliffs (470' at the bridge) are impressive, but in a few days, they will be only thin bands many thousands of feet above us at the canyons edge. The Kaibab forms the rim of the Grand Canyon.
The river runs clear at Lee's Ferry, since the load of sediment it carried from the Rocky Mountains has settled out at the upper end of Lake Powell. It runs cold, since the flow through the dam taps the deeper waters of the reservoir. And it runs regularly, since the dam allows no more and no less water to flow than dictated by the Colorado River Compact and the air conditioning needs of Phoenix.
Grand Canyon 2009: June 17th
We took the class back to Tolmie this year. It's a good beach walk - especially on a nice June day. Bluffs, bulkheads, and a nice little barrier estuary. And some weird geology.
Two years ago (Butterball Cove), I was intrigued by the presence of a resistant geologic unit right below the surface of the beach. This year, the beach is lower, and this band of rock was much better exposed. I'm not sure what is most intriguing, the ledge, or the fact that the beach has fallen several inches. Between the geology of the site and the amount of bulkheading in the vicinity, its not like there's very much new sediment available to replace what gets lost.
