Words based on locations don't become part of the general language very often. You do get a few - I mean people talk about "Whitehall" meaning the government, or "the White House" in America, meaning the American government, but not very often, and certainly part of a river - 'estuary'! I think that's a first; I don't remember hearing that before ever.
Now the estuary in question is the River Thames, and during the 1980s the word estuary came into the language referring to the kind of speech that people are using around the estuary of the River Thames, in places like Essex, in the north of Kent, and it was a new kind of accent: a sort of cross between Cockney and Received Pronunciation.
And if somebody said he speaks estuary, it would mean he speaks this kind of mixed accent. In RP, in Received Pronunciation, you'd say that the word was 'wall' - the thing that holds a house up - a wall; in Cockney of course it's a 'wall', a 'wall' and in estuary English of course it's a sort of mixture of the two: a 'wall', a 'wall', with a 'l' sort of sound. It's one of the fastest moving accents of modern times.
Transcript (pdf - 42k)
Lesson plan - Teacher's notes, student worksheets with answers (pdf - 69k)
Audio - Professor David Crystal on "Estuary" (mp3 - 699k)
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