I was cruising around in GE looking for a contribution to the current meme of Structurally Controlled Drainage, when the first image caught my attention: an entrenched oxbow! Entrenched or incised meanders aren't all that common, but neither are they all that rare. The typical sequence of events is listed as 1) the development of a relatively flat surface near base level, allowing a stream (which includes rivers, in geospeak) to meander, 2) relatively quick uplift or lowering of base level, causing the stream to incise rapidly, leading to 3) locking the stream into a "fossilized" record of its meandering in a previous life. I have rarely considered the idea of an entrenched oxbow, since they would require a fourth step, eroding across an often substantial volume of rock to connect a slightly higher elevation section upstream to a slightly lower elevation downstream section, providing a slightly more efficient flow path.

Still, in geology as in physics, if it can happen, it will happen, and I've now found three examples. The first is along Glenn Canyon in southern Utah. The second, of which I've posted two images, is in northern Nevada. GE has improved the imagery of this area, and someone has kindly posted photos from the length of Thousand Creeks Gorge, confirming something I've wondered about: that is a cutoff in an entrenched meander. I included the first picture to point out a couple of other interesting bits of geology. The basin to the west, Virgin Valley, is one of the few places in North America that produces gem-quality opal. It is filled with layers of Pliocene tuffs, lahars, and a few rhyolite flows. It drains through Thousand Creek Gorge, which is carved into one or more massive rhyolite flows that form the prominent ridge from ~WNW-ESE through the middle of the picture. In the northeast of the image is an excellent example of inverted topography: Railroad Butte is the result of a basalt flow in a valley, which diverted the drainage and created a resistant caprock. Subsequent erosion in the area has literally inverted the original topography. The past low areas are now high, and past high areas are now low. The third picture is a closer zoom, showing that the meander is clearly cut off. If the pictures are accurately placed, I've been much closer to this than I had realized, but the lower portion of the gorge becomes more of a scramble. I just never took the time to walk all the way through.

Finally, the fourth image is a GE view of the fifth picture, which I found in Google Images, and is posted here by Roger Weller at Cochise College. The location is about 11 and a half miles (~18 km) WSW of Bluff, Utah, along the San Juan River, and about halfway between Bluff and the famous Goosenecks of the San Juan.

http://skywalker.cochise.edu/wellerr/pglnx/chapter10x.htm