From: mdhangton@gmail.com   
      
   On Friday, July 4, 2014 11:33:27 AM UTC-4, Joy Beeson wrote:   
   >    
   > How large are the available patches of farmland?  Arbitrary expanses   
   >    
   > as on the Great Plains,   
      
   In the modern world, we don't realize that, without well water and modern   
   irrigation, Kansas is pretty dry.  The aquifer beneath the Plains is currently   
   being depleted, and takes on the order of 3 millennia to be fully refilled.    
      
   >  Do you   
   >    
   > have a confluence of two navigable rivers with a big flat farmable   
   >    
   > space around it?     
   >    
   The nature of the rivers is what is important.  When water comes from mountain   
   rainfall, the water may be fresh, but it will contain trace minerals.  As the   
   surrounding farmland is continuously irrigated, as in the case of the Tigris   
   and Euphrates in    
   Ancient Mesopotamia, the minerals become concentrated over the course of   
   centuries, and the groundwater becomes saline.  It then becomes increasingly   
   difficult to grow crops, and, each year, the land must be flushed with river   
   water to drive back the    
   salty water table.  This is the origin of the Babylonian myth of the god   
   Marduk defeating the underground serpent Tiamat each year.   
      
   The Nile, before the Aswan Dam was built and canceled the process, ran over   
   its banks each year to wash its floodplains and fertilize them with loam   
   carried down from Ethiopia.  It never had the salination problem, and   
   supported continuous cultivation    
   for 6000yrs.   
      
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