Wednesday, June 17, 2009
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