The first big problem is HOW to get the water pass the Mississippi river. Truck, barge or Train are to expensive to move water on the scale needed. That leaves pipelines or canals. Pipelines have the advantage of being sealed, so that you can duck under something like the Mississippi and then the water in the pipe will raise to its own level (i.e. the pipeline can go down to the Mississippi, go under that River, then the pipeline can return to the same height it was before it hit the Mississippi. Canals, being open to the air, will have to flow into the Mississippi, but canals can be larger and haul the water needed.
After you cross the Mississippi, how do you get it to where it is needed, the Southwest? You have two choices, up the Missouri and Plate River Systems or up the Red River System. Either way you have to reverse flow either river i.e. instead of feeding the Mississippi, the rivers will be feed by the Mississippi. Reversing either can be done, but we are talking Trillions of Dollars, The cost of building the Panama Canal would look like a cheap tip compared to the Cost of reversing these rivers (And that is assuming we use constant dollars NOT the actual dollar spent, since the Panama Canal was built in the early 1900s when Gold was $20 an Ounce not the almost $1000 an ounce of today).
The Red River has an additional problem, how do you get over the Rio Grande? The Rio Grande flows forms the Border of Texas and Mexico, but before El Paso it is the central river in the State of New Mexico. IT is also the HIGHEST River in the nation. Whether you use a pipeline or a Canal, the water has to be pumped up and over the Rio Grande (A Tunnel could be built, but it will be about 500 miles through solid rock AND YOU WILL STILL HAVE TO PUMP THE WATER UPHILL for the ultimate goal, the Colorado River starts high in the Rocky Mountains, and the Colorado is how the water is going to be shipped into Nevada, Arizona and even California using existing irrigation systems (and expansions on those systems).
The Missouri Plate system starts further up steam of the Mississippi River then the Red River System, but you still have to tunnel under the Rocky mountains or pump the water OVER the Rocky Mountains. While you can avoid the Rio Grande of the Plate is used, but this is more a result of having to go even further up hill on the higher mountains of the state of Colorado.
My point is simple, the people who talk about shifting water from the Great Lakes to the Southwest have NEVER looked into HOW that is to be done. Once you do so, even if all you do is 1-2 hour research on the net, the cost of what is needed becomes clear (Not the Actual cost, but that it will be extremely high). Thus people who have looked into it don't mention it, or if they do what they say depends on the audience (i.e. if it is to a bunch of business men from Los Vegas, you bring it up as a solution to they water problem so to get they support for election, but in front of engineers and other people who have looked at the actual numbers you admit it is impossible).
The other great world wide irrigation plan is cheap and simple in terms of the above. All the Russian are planning on doing is diverting two rivers across a flat Plain (And pumping the water up a short climb). The Russians plan to divert the Volga and the Ob Rivers NOT moe water over or under them, and the water will flow over the Russians Steeps NOT pump up and over the Rocky Mountains.
Aral Sea diversion:
http://www.fragilecologies.com/oct09_95.html $40 Billion dollar cost and this is through the flat steppes of Russia NOT the Rocky Mountains:
http://www.newscientist.com/article/dn4637-russia-reviving-massive-river-diversion-plan.htmlQattara Depression water diversion (For Electrical production only):
http://en.wikipedia.org/wiki/Qattara_Depressionhttp://www.unu.edu/unupress/unupbooks/80858e/80858e0a.htmDigging a 30 mile ditch between the Mediterranean and the Qattara Depression is dirt cheap, cost would equal about the cost of building the Panama Canal (Distance is about the same). Once the water hits the Depression, it can be used to generate electricity.
Dead Sea water Diversion, an another salt water diversion for Electric production (Cost only 2-4 Billion Dollars and only 120 miles):
http://www.american.edu/TED/deadsea.htmhttp://news.nationalgeographic.com/news/2006/12/061214-dead-sea.htmlhttp://www.mfa.gov.il/MFA/MFAArchive/2000_2009/2002/8/The%20Red%20Sea%20and%20the%20Mediterranean%20Dead%20Sea%20canals