MICHAEL Hance questions the use of the term "estuary" to describe the Forth Estuary (Letters, May 20).

The answer is straightforward. An estuary is defined as "a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with fresh water derived from land drainage" and originates from the Latin word "aestuarium". The word firth (or frith) originates from the Old Norse Word "fjoeror", which also gave rise to the modern Norwegian word of fjord. Both firth and fjord are now generally defined as an arm of the sea.

The difference is crucial - a firth (or fjord) is made up of undiluted sea water, but an estuary has a gradation of salt water dilution from fully salt water at its mouth to fresh water at its head, and experiences tidal f luctuations. For the sake of completion, a river is a freshwater body of water which flows in one direction (towards the head of the estuary) and does not experience tidal fluctuations.

Thus for the Forth the agreed geographical areas of definition are:

River Forth - upstream of Stirling;

Forth Estuary - Stirling to Queensferry; Firth of Forth - seawards from the Queensferry bridges.

And for the Clyde, the areas are:

River Clyde - upstream of the tidal weir in Glasgow; Clyde Estuary - Glasgow to Greenock; Firth of Clyde - seawards from Greenock.

I hope that this helps.

Donald McLusky (editor, Estuarine, Coastal and Shelf Science), Ardoch Cottage, Strathyre, Callander.