It was the only warship in sight, but far away to the right over the smooth surface of the sea—for that day there was a dead calm—lay a serpent of black smoke to mark the next ironclads of the Channel Fleet, which hovered in an extended line, steam up and ready for action, across the Thames estuary during the course of the Martian conquest, vigilant and yet powerless to prevent it.
There are no more uses of "estuary" in the book.
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Michael Ondaatje -- Running in the Family
A second shell screeches overhead, and Marie-Laure shrieks, and the chandelier above her head chimes as the shell detonates somewhere across the estuary.