Date: Tue Nov 4 00:04:59 EST 1997
Subject: A.Word.A.Day--estuary
es.tu.ar.y n. 1. The part of the wide lower course of a river where its
current is met by the tides. 2. An arm of the sea that extends inland
to meet the mouth of a river.[Lat. aestuarium [aestus, tide]

I looked this one up to find etymology.