Saturday, April 2, 2011

Rivers, Old and New

I have come across the 1930 map that the Bureau of Reclamation produced in connection with its plans for Hoover (and Bridge Canyon) dam.
Since my copy is a very old Xeroxing of the original, it is not great on detail, but I wanted to look for changes that might show between this map and more current versions, so I PSEated it a bit in the section from the western end of the Canyon and Pearce Ferry (note the old version: Pierces):

The topography here is not great; the contour interval is 50'. Some relevant numbers for Lake Mead water levels are ~1100' currently; 1225' highest, in 1983; 1157' on USGS topos 1960's; 1170'-1215' supposed to be the normal fluctuation. (Thanks to Martin-Whitis river guide.)

Can that 1930 map be translated to more current depictions?

Does comparison with Google's satellite views show any differences in river course? To start, here is the P-Ferry piece of the above map, the river colored in and the high reservoir level (1200') emphasized:

My played-with version can be compared to more recent views, such as the 1971 USGS topos (ignore the reddish line):

Or the results of zooming in on satellite views presented by Google maps copyright 2011 (thank you 
very much), keeping P-Ferry centered. First, a magnification taken when the reservoir had lots of water.

This is a good one to see the topography I followed in drawing a proposed boundary correction, with the "Rampart" ridge coming up from the south (1), then turning east to cross the river and climb up the ridge to the Sanup level (2), where the P-Ferry drainage  (a dramatic X shape, 3) pushes the boundary right down to the Sanup's rim overlooking the Canyon's last side canyons (4). 

Now here is what appeared when the water went down, using the next step in magnification:

All that nice phreatophyte-covered silt is now exposed. The river seems shifted to a sharper southern bend at P-Ferry. What in the deposition of silt and the interaction with the side washes caused the channel to cut as it did? Compare with 1930 again:

Here is the next enlargment:

To close with some drama, we magnify two more steps to a close-up of the very active P-Ferry Rapid, this next showing the new ramp at the bottom:
 And the monster itself, though currently it is even more turbulent:

No comments:

Post a Comment