Brian Parker, former oceanographer, of Crossparks, Dartmouth, writes:

Barry Vaughan, Letters, February 7, refers to Mr GR Hutcheson, Letters , January 24, as being confused about river flow increasing ambient tidal heights.

Unfortunately for Mr Vaughan, it is he who is confused. Mr Hutcheson is absolutely right about river flows adding to tidal heights within estuaries.

The process can be simply described. Imagine the tide flowing into an estuary, which has no river flow.

The height of the tide at any point will vary from prediction according to wind and atmospheric pressure.

Now add river flow. This will spread, by gravity, over the tide, as a fresh water layer on top of the salt water. This layering can be quite marked.

I recall diving off Sandquay at high water in conditions of high river flow. There was a substantial layer of opaque brown river water, about a metre, over clear greenish sea water.

At no time does the river water have to run uphill. Mr Vaughan's raising this point could be described as a clupea harengus rufus – red herring.