Monday, June 29, 2009
We reached the high water mark of Lake Mead around Gneiss Rapid in Bridge Canyon. But we didn't reach the actual lake until the middle of the next day - more than 60 miles downstream. Its been a bad few years for the reservoir (see my posts from our January trip to Vegas) and it just keeps dropping. During its better years, and particularly in the decades between 1937 and 1963 before Glen Canyon Dam was built, the Colorado dumped its load in this section of the canyon. The tributaries built their alluvial fans to match the reservoir level. So the river now flows between banks eroded into these mid-20th century river deposits. And flows tens or even hundreds of feet above the rapids that used to be found on this stretch of river, which are now buried in silt.
Time to wrap up the Grand Canyon stuff. Between this blog and the other two, I've posted way too much material, much of it redundant, and its time to go back to the shorelines this blog was originally meant to address!
Saturday, June 27, 2009
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.
Wednesday, June 24, 2009
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.
Sunday, June 21, 2009
Grand Canyon is really a canyon within a canyon. The upper portion is cut through the thick stack of relatively flat-lying Paleozoic sedimentary rocks - from the Kaibab at the rim down through 300 million years. The Tapeats Sandstone is at the base of this sequence and also forms the rim of the lower, inner portion of the Canyon.
The inner gorge is cut through not just the Tapeats but also the underlying Proterozoic (Precambrian) metamorphic rocks, in particular the dark Vishnu Schist and the pink Zoroaster Granite that threads through it. This reach of the inner gorge in the eastern portion of the Grand Canyon is the Upper Granite Gorge -- the Middle and Lower Granite Gorges are much farther downstream. From the river, we usually could only see up to the Tapeats (a mere 1000' above us), but in a few places the upper cliffs can be seen rising (many more thousands of feet) up to the South Rim. Beaches are scarce in this steep, rocky portion of the canyon. And where they are found, they are often small and narrow. The dark rocks bake in the sun and hold the heat into the evening.
Grand Canyon 2009: June 21st
Saturday, June 20, 2009
We stopped for lunch on a sand bar on river left, just downstream of Chuar Rapid at the mouth of Lava Canyon (not to be confused with Lava Falls later on). We passed beneath the Paleozoic about half an hour ago and are now in the ancient, tilted sedimentary and volcanic rocks of the Grand Canyon Supergroup.
In a place this big, sometimes the most interesting stuff is the small stuff. Here's an oversteepened shoreline bluff and a small barrier beach within 20 meters of each other.
Grand Canyon 2009
Friday, June 19, 2009
The big alluvial fan (delta?) at Nankoweap Creek forces the main river into a big swift arc along the left bank. The canyon downstream from Nankoweap is straight, but the river meanders a weaving path through the piles of rock washed from the cliffs on both sides.
Grand Canyon 2009: June 19th
Wednesday, June 17, 2009
It is no coincidence that many of the rapids in the Grand Canyon are named after tributary streams. Soap Creek, House Rock, Hermit, Crystal. Virtually all of the Canyon's 160 or so rapids are products of flash floods and debris flows - even very small drainages can deliver more than enough boulders to partially block the Colorado. The result is a long series of riffles and intervening pools where the debris has dammed the river. Although many of the rapids are hairy, few are very long.
None of the rapids in the Canyon result from nick points (where a river flows over a particularly resistant geologic unit) and even deep in the metamorphic gorges the river runs smoothly except where blocked by debris. I guess the river has had more than enough time and power to smooth out those bumps and now just puts its effort into redistributing the stuff the tributaries pump out. This has become much more difficult with the Glen Canyon Dam upstream preventing large floods from periodically flushing out the debris, so over time the rapids are expected to get bigger. Someday a big flash flood will create a rapid so nasty it throws a real monkey wrench into the rafting scene. This almost happened at Crystal in 1966.
The House Rock Valley drains a large area northwest of Marble Canyon and forms a nice rapid at its mouth - pushing the Colorado sharply against the left bank. As always, click on the title of the post to see the bird's eye view.
Grand Canyon 2009: June 17th
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
Wednesday, June 10, 2009
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.
Friday, June 05, 2009
Parks are a far more appropriate use of big deep-seated landslides than subdivisions. Kopachuck, west of Gig Harbor, is a great example, and offers a nice counterpoint to the many big homes in this area built on gravity-prone hillslopes.
The beach at Kopachuck weaves in and out, the result of the uneven geology of the toe of the landslide (which is at or below beach level) and a series of gravel bars and proto-spits formed as the northward drift runs out of steam rounding the corner.
When we last visited with this class - four years ago - we used the failing, creosoted timber bulkhead as a discussion point about the effects of such structures and the opportunities for removing them. Since then it has been pulled out, allowing erosion to reclaim some of the old fill and re-exposing an even older concrete boat ramp. Some high school students (with some sledgehammers, wheelbarrows, and supervision) might be useful here. A couple small drain pipes need to be rethought - maybe combined with some plantings and some solidly built wooden steps.
A south-west facing beach on a summer afternoon with temperatures in the high 80s. The class clustered under the overhanging trees, evidence of both our skeptical attitude towards sunshine in this part of the world and the importance of riparian vegetation in influencing habitat choices.
I walked in from the gate, since the park was still a couple of hours from opening. An early morning walk on the beach was partial compensation for the third all-day meeting in Olympia in as many days.
The beaches at the park are pretty silty - reflecting an abundance of fine sediment from unstable bluffs to the north and the lack of sufficient wave action to efficiently remove the fines from the coarser beach material.
The bluffs are heavily forested right down to the water, and though most of the big trees were probably taken out 130 years ago, there were still several large firs along the shore. The head of Frye Cove is a small estuary - with a muddy spit sheltering it from the rest of Eld Inlet.
The park recently installed a "soft" structure of cobble and anchored logs to deal with erosion at the park's primary beach access. I'm still a bit puzzled by these efforts - the intentions are good, but they seem a bit like a solution in search of a problem. What if you just excavated the bank back a little farther, planted the heck out of it, and built a simple low-impact wooden stairway. And left out all the cobble and the big stainless chains - which are neither native to this setting nor particularly "soft."