Tidal Streams and Tidal currents

Some Thoughts on the difference between the terms

‘Tidal Streams’ and ‘Tidal Currents’

In the chapter, Currents and Tides, in my book, How to Read Water, I have tried to stick to my aim of covering sometimes technical subjects in plain English.

In one area of this chapter it proved difficult to do that without taking a potentially controversial position on the meaning of the words ‘current’ and ‘stream’. It is position that is likely to irk a few, not least those in the British nautical establishment and I would like to offer my sincere thanks to Peter Chapman-Andrews, Director of the Royal Institute of Navigation, for both taking the time to read a proof copy of the book and then kindly reminding me of this.

Below is my explanation of why, with little interest in irking anyone but a sincere desire to simplify things for all readers, I have found myself taking this potentially controversial stance. However, if you have little interest in nautical semantics and terminology and are just looking for a simple life then you may not wish to read on.

Let’s start with an area where there is harmony. Wherever people go to sea and English is used, the word ‘current’ will be understood to mean a horizontal flow of water. Unfortunately, this appears to be as far as the harmony reaches.

In large parts of the world, including much of the US nautical establishment, currents at sea can be caused by three things: wind, thermohaline (temperature/salt) imbalances and the tide. For simplicity, let’s call this the ‘US view’ (although there remain differing views there and indeed all over the world).

In this view, if water at sea is flowing horizontally it is definitely a current and the only question is what is causing it. If it is caused by wind or thermohaline factors, it is typically known simply as a current, or an ocean current, or if it is very large and influential, it may be given its own name, like the ‘Labrador Current’. If the horizontal flow is caused by tidal factors – ie. the rising and falling of the water level due to astronomical forces – then it becomes known as a ‘tidal current’.

This is the view I have endorsed in the book, because it seems simplest to me, for reasons that I hope will become clear soon.

In the UK nautical establishment, not least the Admiralty, the view has traditionally been taken that ‘current’ refers to the horizontal flow of water, but only when it is caused by two things: wind and thermohaline factors – NOT tides. In this, let’s call it the ‘UK view’, if the flow is caused by the rise and fall of the tide, then it is called something different – ‘a tidal stream’ - and it is not viewed as a current at all. The key difference is that in the US view ‘current’ covers all horizontal flows, but in the UK view it only refers to continuous flow and not the periodical flow resulting from cyclical tidal factors.

To my mind, neither of these views are right or wrong, it is a question of semantics and tradition. But as so often with such things, there are those that understandably take a strong view one way of the other, usually depending on their own nautical heritage. Since I first learned about the sea within mostly British waters and through British institutions, it might be expected that I would take the UK view of the words ‘currents’ and ‘streams’ at sea. But, after much deliberation, I have settled on the US view for two main reasons: firstly, the English is easier and secondly, since taking this view I have found the whole area more straightforward personally.

One of the reasons why I’m against the use of the term ‘tidal streams’ is that I think it is unnecessary. It doesn’t add any information, but introduces an additional term, which is usually unwelcome, especially for a beginner. There are no instances I am aware of where the expression ‘tidal stream’ will give any more detail or insight into a situation than the expression ‘tidal current’.

I imagine that the expression ‘tidal stream’ was created with the noble aim of eliminating ambiguity by associating the word ‘stream’ at sea with purely tidal flows. In that case, what does the person new to this area assume to be the cause of the Gulf Stream? The Gulf Stream, as a name of a major ocean current, is here to stay, but it has nothing to do with tides.

When a river approaches the coast, its flow is often influenced decisively by the sea’s tides. It is then often referred to as a ‘tidal river’. For the same reason, much smaller watercourses near the coast, ones that often disappear at low tide, are sometimes known colloquially as ‘tidal streams’.

But these two examples, whilst possible causes of ambiguity or confusion, are really only minor considerations. My main reason for taking the US view of the word ‘current’, is broader and can best be explained with use of a thought experiment.

Imagine you and some friends decide to while away a lazy afternoon by chartering a fishing vessel and enjoying a spot of sea fishing in an area you don’t know well.

When out to sea your skipper cuts the engine and you drop the long weighted lines in hope of a sumptuous fresh fish supper. Soon afterwards you are enjoying the hot sun on this windless day, when you notice that features on the distant coast appear to change very gradually and you deduce that your boat is moving very slowly. It is clear that the water below the boat is not stationary, it is flowing beneath you and carrying your boat very slowly with it.

At this point an animated discussion sparks up amongst your friends. It becomes very heated, and worried that you might be responsible for a loss of cordiality, you remain by the wheelhouse as one group move to the bow and the other to the stern from where each group shout at each other.

“We are being moved by a current!” The group at the bow confidently declare.

“Nonsense! It’s a stream!” The stern group retort.

You intervene in the heated debate by suggesting consultation with the skipper and his unlikely collection of weighty reference books and online resources. It soon becomes clear that the cause of the water’s horizontal flow changed over the three hours of your friends’ animated debate. At first the flow was mainly due to the tide, then it was joined by a flow caused by the wind and then temperature differences played a small part, at which point the tide’s effects were reduced to nothing. Finally the wind and temperature influences were reduced to nil, but the tidal effects kicked back in.

“It wasn’t so simple after all!” The group at the stern declare. “It was a stream, then both a stream and a current, then just a current and finally a stream again.”

“No.” The bow group reply, keeping their calm, but losing their fish. “It was a current all along.”

If you care about these things then I hope that the above goes some way to explaining why I have taken the view I have, even if you disagree with it. I will be humbly petitioning the Royal Institution of Navigation to schedule an official Institution meeting on a fishing vessel to discuss this matter formally and for the discussion to continue informally afterwards, for some time.