Message #33:
From: AzTeC SW Archaeology SIG
To:   "'Matthias Giessler'" 
Subject: Were NM PaleoIndians Ever Influenced by Rio Grande Lakes?
Date: Sat, 18 Jan 1997 06:43:55 -0700
Encoding: MIME-Version: 1.0

[ Were Paloeindians in New Mexico ever affected or influenced by Rio
Grande lakes forming three times over a 1,600-year period from 14,000 to
12,000 years ago? -- SASIG Ed.]

Landslide 43,000 years ago created huge lake on Rio Grande A giant wall
of volcanic rock once backed the river 15 miles to what is now Espa–ola
One day about 43,000  years ago, a portion of the western slope of White
Rock Canyon suddenly gave way and crashed down onto the Rio Grande. When
the echoes from the tremendous roar had faded, the river ran against an
insurmountable obstacle - a wall of volcanic rock and earth 200 feet
high. Within a year, the river had backed up 15 miles, approximately to
where Espa–ola is today. Eventually, the lake grew to a width of two
miles and rose about as high as the natural dam itself. The dam and lake
lasted for several hundred years, perhaps as long as a millennium. Then
the lake filled up with sediment and the great force that had created
the river in the first place - gravity - was able to reassert itself.
The dam might have been breached in a sudden massive flood or carved
down in pieces. Perhaps both events took place. Whatever happened, the
dam was erased so thoroughly that today the only evidence for its
existence is indirect and takes the form of thick, horizontal deposits -
the signature of a lake bottom. This remarkable bit of geologic history
has been fleshed out in greater detail than ever before by Los Alamos
National Laboratory geologist Steven Reneau. Reneau has published two
papers in scientific journals this year that present evidence not only
that the Rio Grande was dammed more than 40,000 years ago, but that it
also has been dammed on four more recent occasions: 17,000 years ago and
then three times over a 1,600-year period from 14,000 to 12,000 years
ago. That there are horizontal deposits in White Rock Canyon indicative
of ancient lakes was noted briefly by another geologist in a paper
published several years ago. But Reneau and geologist David Dethier of
Williams College in Massachusetts are the first to find evidence that at
least five lakes have formed in the canyon. Like many notable scientific
discoveries, this one happened by accident. In 1993, Reneau was
recruited by the lab's environmental restoration program to determine
the frequency of landslides in White Rock Canyon. The program's leaders
- charged with cleaning up contaminated sites at the laboratory - wanted
to have a better idea of the geologic stability of the region they were
working in. As Reneau and Dethier tromped through the rugged canyon
looking for evidence of past landslides - of which there were several -
they started seeing a surprising number of lake deposits. It wasn't long
before they made the connection. They were able to identify how large
the lakes were and where the dams were by studying the lake deposits and
the areas where the Earth had slumped, Reneau said. Thanks to the
presence of charcoal in the outcroppings - the remnants of ancient
vegetation - Reneau and Dethier also were able to determine the age of
the lakes. Charcoal, because it contains radioactive elements that decay
at a steady rate, is a natural clock. What caused the landslides is a
little less clear. Typically, slides are caused by earthquakes, floods
or heavy precipitation. Reneau and Dethier, in a paper published in the
November issue of the Geological Society of America Bulletin, argue that
the three most recent lakes occurred so closely in time that it's
unlikely that earthquakes triggered their formation. A more plausible
scenario is rain - relatively heavy rain compared to today. Reneau said
the presence of ancient lake bed deposits throughout the interior West
from this same time period - 18,000 to 12,000 years ago - is evidence
for a wetter climate. Rain in enough quantities can act as a lubricant
for rock and earth located on steep slopes. Additionally, the Rio Grande
- swollen far beyond the spring runoff levels we see today - could have
helped create the slides by cutting deeply into the sides of the canyon,
Reneau said. Wet weather probably also led to the formation of the
largest, longest-lived and oldest of the lakes, the one that formed
43,000 years ago. Oddly, while that lake might have lasted for 1,000
years, there are no fossils of fish or other lake creatures to be found
in White Rock Canyon. "It was there a long time, but not long enough to
develop a real ecosystem," Reneau said. One final bit of evidence for
the canyon's surprisingly dynamic past are large boulder deposits
stretching from White Rock to Cochiti Lake. These are the remains of the
catastrophic breach of the dam that created the lake 17,000 years ago.
In a single day, this 130-foot deep, 12 mile-long lake was drained. The
flood, "a wall of water," according to Reneau, would have destroyed
anything in its path. A similar catastrophic flood might have occurred
from the collapse of yet another dam in White Rock Canyon more than 50
to 60 million years ago, but the evidence for this is sketchy. The
canyon's more recent past - from 12,000 years to the present - has been
boring in comparison. The reason is a relatively arid cycle in the
region's climate. "When you've got a lot of water, a lot of things can
happen," Reneau said.
Published in The New Mexican Thursday, Dec. 26, 1996