estuary has definitions from the field of geography
1
[ noun ] (geography) the wide part of a river where it nears the sea; fresh and salt water mix

Used in print

(Clifford H. Pope, The Giant Snakes....)

R_._R_._Mole and F_._W_._Urich give approximately 20 inches as the average length of a brood of thirty from the region of the Orinoco estuaries .

*