The Collaborative International Dictionary of English v.0.48:

Estuary \Es"tu*a*ry\, n.; pl. Estuaries. [L. aestuarium, from
   aestuare to surge. See Estuate.] [Written also
   [ae]stuary.]
   1. A place where water boils up; a spring that wells forth.
      [Obs.] --Boyle.
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   2. A passage, as the mouth of a river or lake, where the tide
      meets the current; an arm of the sea; a frith.
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            it to the sea was often by long and wide estuaries.
                                                  --Dana.
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