Subject: How are the headwaters of a river determined or defined?

I am originally from MN and always thought it curious that Lake 
Itasca could be the source of the Mississippi River.  It is 
neither the highest point in the watershed nor the furthest 
distance from the delta.  I know that determining the headwaters 
had great significance for explorers (Nile, Amazon, Mississippi) 
and in the case of the Mississippi, it even settled a border 
dispute between MN, MB, and ON.  I even asked my geology 
professor when I was in college, but he couldn't provide a 
satisfactory answer.  I'm hoping I'll finally get a good 
explanation.  Thank you.

Re: How are the headwaters of a river determined or defined?