Re: What would happen if we flooded Death Valley?

Death Valley is a large closed basin.  If the tunnel you describe were 
constructed, water would flow into Death Valley until the level of the 
water in the valley reached the level of water in the ocean (sea level). 
Since the valley is closed (it has no drainage to the outside), any water 
that flowed in would stay in the valley and a long narrow salty lake would 
form. At the deepest point in Death Valley, known as Badwater, the water 
in this lake would be about 280 feet deep (Badwater is 282 feet below sea 
level). If you go to the National Park Service web site for Death Valley:
 http://www.nps.gov/deva/

and click on the link to maps and then download the PDF file that contains 
the park service map, you will be able to see what the outline of the lake 
would be.  The map has a darker brown color for areas below sea level and 
it is those areas that would be flooded. The lake would cover less than 
half of the floor of the valley, so there would not be wall-to-wall water. 
The valley would hold a bigger lake, but to make it, you would have to pump 
water in, it would not flow in on its own.

Initially, the new lake would probably only be a little saltier than sea 
water, and if the tunnel was left open, the saltiness would stay close to 
sea water.  Any excess salt on the Death Valley end would tend to migrate 
toward the less salty ocean, equalizing the salt levels.  If the tunnel 
were closed, the lake would get saltier and saltier as it gradually dried 
up.  Eventually the lake would become salty enough that salt minerals would 
start to crystallize out of it, as they do today in the little lake that 
already exists at Badwater.  Then the amount of salt in the water would 
stay about the same as in Great Salt Lake or the Dead Sea or other salty 
lakes.

The Salton Sea in the Imperial Valley of California actually formed in a 
way similar to what you describe, except it wasn't done with a tunnel.  
There, a dike on a very large irigation canal broke and allowed fresh water 
to flood the deepest part of the valley.  Evaporation has turned the lake 
salty over time. See  http://www.smithsonianmag.si.edu/smithsonian/issues96/jun96/salton.html 
and http://www.desertusa.com/salton/salton.html

I hope this answers your question.

Dave Smith
Associate Professor of Geology and Environmental Science
La Salle University, Philadelphia, PA 19141