stream is the horizontal motion of water


Stream is the movement of water back and forth along the coast. This flow is powered by the wave that makes the tide – this can often mean that stream is misleadingly referred to as tide. If you sail you’ll often use tide for the vertical movement of water in harbours, and when you talk about tide out at sea you are really referring to stream (the horizontal movement of water). But to avoid any more confusion –we’re going to stick with stream.

Both vertical and horizontal motions of water (tide and stream) are created by the same wave flowing around the United Kingdom, and so they share various qualities. But to start with, we’ll talk about stream’s unique quality – the stream cycle. In most places with a pretty regular coastline and semidiurnal tide – the stream will flow for around 6 hours in either direction. In each direction, the stream will speed up for 3 hours and slow down for 3 hours. The direction of stream changes when the flow has slowed down and is at its weakest – this is slack water. The length (in time) of slack water is how long it takes for the tides to change.

Now we’ll look at the links between tide and stream. The biggest connection is time, slack water is always the same time before and after high tide. The time of slack water is unique to every inch of coastline, but the direction of the stream at high tide follows the tidal wave around the United Kingdom. This means the tidal stream flows north at high tide on the west coast, and on the east coast it flows south at high tide. In tidal rivers, the stream will flow upriver as the tide is rising, and downriver (out to sea) as the tide is falling.

As you can imagine, spring and neap tides can make a huge difference to stream. On springs, when the tidal range is greatest, the strongest streams occur and slack water is very quick. On neaps, when the tidal range is least, the maximum flow is much slower and slack water is longer.


Even on the calmest of days, when the sea is completely still, water is always moving. This is true even at slack water. Here are a few tips to spot the direction of stream.

Look out for...

Ships at anchor and buoys.

The anchor chain is usually tied to the bow (front) of boats, and when there is no wind the boat will spin around the anchor chain until it is facing the direction of flow. This is also true of fishing buoys. At slack water you can watch the boats spin around their anchor to face the new direction of flow.

pier legs and posts.

When water flows past a stationary object it creates a disturbance downstream – think of rocks in rivers. If you can see where the disturbance is, you can work out where the stream is coming from. The bigger the disturbance, the greater the speed of flow of the stream.


If you can grab a piece of driftwood from the high tide line, throw it as far offshore as you can and watch which direction it drifts. You can also estimate the speed of the stream by walking alongside the piece of driftwood. By counting how many seconds it takes to cover the distance, you can then work out speed.


If you can see swimmers casually swimming in one direction, the chances are they’re swimming with the stream. However if you can see swimmers furiously paddling in one direction, the chances are they’re swimming against the stream.


Another important stream is the Gulf, which has nothing to do with the tidal stream. The Gulf Stream is one of the strongest ocean currents on earth - it transports 150 million cubic metres of warm water per second from the Gulf of Mexico to the United Kingdom (and beyond). The Gulf Stream can be 10 kilometres wide and reaches speeds of 5 miles per hour.

The current is formed from two factors. The simplest factor is wind - southwesterly winds blow the water north towards the United Kingdom. The more complicated factor of the two is the NADW (North Atlantic Deep Water) channel. Water in the North Atlantic becomes denser because of cold Arctic winds and a rise in salinity (saltiness of the water) as icebergs form and expel the salt back into the ocean. This denser, saltier water sinks to the bottom of the seabed and flows south to the equator (the NADW channel). To keep the sea levels the same, water from the equator flows north to replace the water lost in the NADW channel. This flow of water from the equator is the Gulf Stream.

As more icebergs melt into the ocean, the North Atlantic becomes diluted and is less saline (salty); gradually decreasing the effect of the NADW channel and thus the Gulf Stream. This means less warm water arriving to the United Kingdom, which means we'll definitely need thicker wetsuits with the progression of global warming. Without the Gulf Stream, the winter sea temperature in the United Kingdom would drop by a chilly 5 degrees Celsius.  



At the very top of mainland Scotland, near Thurso, there is a channel where some of the fastest tidal streams in the world can be found. This is the Pentland Firth - a narrow stretch of dangerous water between the mainland and Orkney. As the tidal wave passes around the north of Scotland it is funnelled through the narrow Pentland Firth (which - to add confusion - is not technically a firth). This funnelling generates pressure and thus the tidal stream can begin to flow at rates of 5 metres per second.

These currents can have speeds of nearly 20 kilometres per hour and so cause eddies downstream of all the islands. As the fast flowing waters hit Stroma they are compressed and flow around the northern tip at even higher speeds. This leaves an area of low pressure on the other side of the island, and the gap is filled by water flowing upstream; creating an eddy. When the eddy meets the fast flowing stream, the contrasting currents create powerful eddlyines and the Swilkie whirlpool is created. Swilkie comes from the Norse "Svalgr", which means "the Swallower".

Many yachts will wait for neap tides to cross the Pentland Firth as the currents can often overpower both sail and motor if there are rough seas and spring tides. The Pentland Firth also poses a threat to surfers, as the huge speeds within the straits make rip currents off the coast of Thurso, and the rest of the North East corner, particularly dangerous.

For all the challenges of the Pentland Firth, there are also valuable opportunities with green energy. The tidal power of this stream can generate 16,000 gigawatts of electricity a year with today's technology - enough to account for half of Scotland's energy consumption.