It's the element whose symbol is the postal abbreviation for the "Land of 10,000 Lakes"
[DOC] [TLE] Our Mississippi Educational Guide (entire pdf) - Mississippi Valley ... [PAR] A large wall map of the watershed places the Mississippi River in its ...... Land of 
10,000 Lakes ..... in the watershed on the map using the two-letter postal code 
abbreviation ...... Indicator species: A species whose ...... A safe supply of water is 
the most important element of ...... symbols to understand and explain their world.