THE SALTSTRAUM

Norwegian version of this page.

Saltstraumen (literally the salt sea current) is a powerful tidal current about 25 km east of the city Bodø in Nordland, northern Norway. Here the cosmic forces of the the Sun and the Moon display their might. In the narrow channels into the large Skjerstadfjord the current is reinforced and puts on a fascinating performance with spectacular current patterns; strong whirlpools, updraft, waves, flow separation, and turbulence. All of this is made visible by tiny shining air bubles drawn into the blue sea through the funnels of the whirlpools.

Saltstraumen with in-going (eastward) current in the main channels between the two larger islands; Knaplundsøya (right) og Straumøya, to the left. Photo: Eiliv Leren, Borealis Designstudio, © .

The Moon and the Sun govern the scene

The gravitational forces from the Moon and the Sun generate a tidal force which pushes the water masses in the world oceans. This force is not large, less than a millionth part of the gravitational force at the Earth surface, but it is enough to generate tides with amplitude of several meters in some locations on the coasts of the oceans.
The tide enters the Norwegian Sea from the North Atlantic in form of an extensive wave. The wave propagates northward along the Norwegian coast with a speed of 450-500 kilometer per hour and its height increases along the coast northwards. At Bergen the height of mean high water tide (MHW) is 0.5 meter over mean sea level (MSL) and at Bodø 1.2 meter over mean sea level. High water (HW) at at Bodø is delayed on an average 1 hour and 30 minutes compared to high water in Bergen. The time from one high water to the next is about 12 hours and 25 minutes on an average. Low water (LW) occurs about mid between to adjacent high waters, i.e about 6 hours after HW. At full and new Moon the forces of the Sun and the Moon reinforce each other and produce a higher spring tide. An extra reinforcement occurs when the Moon is nearest to the Earth (in perigeum) at the time of full and new Moon. In 2020 this happened at full Moon on 7th of April and in 2021 with new Moon on 4th of December.
Along the Norwegian west coast the Moon contributes ca. 60-70 percent to the tidal height while 30-40 percent is due to the Sun.
The regularity of the tidal motion enables reliable predictions of time and height of high and low water, long time in beforehand, see tidal tables . A simple, but practical rule says that it is high water at Bodø when the full Moon is seen in south direction, high up on the sky, and again when the Moon is in north low on the sky.

Flow through three channels

The sea level hight in the ocean outside Bodø is driving the current in and out of Skjerstadfjorden. When it is high water at Bodø the current flows in and at low water in Bodø the current flows out through the narrow channels. The surface area of Skjerstadfjorden with side branches is about 235 square kilometer. With a tidal range of about 1 m large amount of water masses flow in and out during each tidal cycle, approximated 470 million cubic meter. The main entrance channel between the island Knaplundsøya and the Storholme is narrow, about 100 m across and about 30-40 m deep. The cross section area of the main channel, including the branch between the Storholme and the island Straumøya, is about 5000 square meter. This implies a mean current in the cross section of about 6 meter per second. Saltstraumen can run with speed of 5-8 m/s (10-16 knots), but in the whirlpools the current speed can be stronger.

The other two channels or inlets to the Skjerstadfjord; one south of Straumøya and the mainland to the east (Sundstraumen), and another at Godøya (Godøystraumen), are narrow and shallow and the amount of water passing through these inlets is much less than through the main channel. But the current can be strong especially in Sundstraumen where at spring tide one may see a breaking bore at the inlet.

Whirlpools, funnels and updraft

From the low hill tops on both side of Saltstraumen as well as from the lofty bridge across the strait, one may have good views of the current, and hence enjoy watching the shifting character of the current pattern. At first sight the current may appear chaotic, but given time one will observe a well organized and recurrent pattern. In the shallow zones near land one can see boundary layers with small scale turbulent eddies. The flow separates from land at promontories and headlands, per example near the beacon below the eastern pillar for the bridge on Knaplundsøya, and likewise on the opposite side of the strait at the Straumholme, as see from the current charts and the pictures (below). In the middel of the channels, outside the boundary layers, the current is more rectilinear. Large eddies and whirlpools are seen to develop on the shear zones separating the interior flow and the boundary layers. At the center of the spinning whirlpool funnels ("kjeler") develop where the fluid is drawn downwards. Surrounding the eddies large bursts of upwelling fluid ("oppkast") can suddenly appear. Small light air bubbles which are drawn into the current by breaking waves and funnels makes the spinning eddies and the zones with upwelling fluid clearly visible.

Generally the current in the main entrance channel flows at its strongest inwards (east) ca. 1 hour before high water (HW) at Bodø and strongest outward (west) ca. 1 hour before low water (LW). The turn of the current direction occurs respectively about 1 hour after HW and LW in Bodø. Episodes with storm surges in the ocean outside Bodø, or heavy rain or snowmelting in the mountains around Skjerstadfjorden, will influence the flow through the channel. In such circumctances the times for maximum current and current reversals may deviate from the general rule above.

In cases when the direction of the wind and the current are opposite, e.g. outgoing current and strong wind (and waves) from northwest the sea can be very rough in the channel with choppy and dangerously breaking waves.

Predictions of current speed and direction in Saltstraumen with the time for maximum current and current reversals can be found on the website for Barentswatch

Current charts for in- and out-going current (ebb and flood). Places where the flow separates from land is marked by the letter S. Thin blue lines indicate the current shear zones with eddies (open circles, arrows indicating direction of rotation). Full circles mark zones with updraft ("oppkast"). The charts are copied from the book "Flo og fjære langs kysten av Norge og Svalbard" (2009), Farleia Forlag. © .

Photos of the out-going (ebb) current. Flow separation (S) occures at the promontory below the beacon under the bridge at Knaplundsøya. Turbulent boundary layer are seen near land with eddies and whirlpools along the shear zone further out. Up-stream of Kobbskjæret (right) a bow wave zone with strong turbulent flow and multiple breaking standing waves are seen. Photo: B. Gjevik, © .

A spectacular nearly 12 hour long film show, produced by NRK (The Norwegian Broadcasting Company), and first sent live as "slow TV" coverage of Saltstraumen in May 2016, is available from the NRK archive entitled; Saltstraumen minute for minute.

Tidal choking

In large fjords with a narrow entrance channel as in Saltraumen the tidal flow can be choked. The reason is that the large Skjerstadfjord with side branches is not completely filled to the same level as in the ocean outside the fjord in the time span of rising sea level. Similarly in the 6 hour of ebb the sea level in the fjord does not quite fall to the sea level in the ocean outside. The result is that the tidal range; i.e. the difference in height between HW and LW in the fjord, is reduced compared to tidal range in the ocean outside. The time for HW and LW in the fjord is also delayed. At Rognan, innermost in Saltfjorden, the tidal range is reduced to about 60 percent of the tidal range at Bodø. Hight water (HW) at Rognan occurs almost 2 hours after HW in Bodø. The renowned priest of Nordland, Petter Dass (1647-1707), already described this effect in his celebrated epic book; "The Trumpet of Nordland" (Nordlands Trompet). He was probably one of the first to give a written account of this special tidal phenomena. See the article in the Yearbook for Helgeland, 2011 (in Norwegian). .

Good fishing in the current

The strong turbulent flow provides a thorough mixing of the water masses of the current. Oxygen from the air and nutrients brought up from the deep water inside and outside the shallow channel are mixed. This provides nutrition for both bottom flora and floating planktons. Herring and plakton is also transported in with the current from the ocean outside and supplies nutrition for larger fish; pollack, cod, haddock and halibut. From early on the current was known for its rich fisheries and already Stone Age people were living near by and exploited the resources. In the Iron Age and Viking Time people living around the current hunted larger animals; as seals and whales.

To day sport fishermen and tourists are fishing with their rods from the rocks surrounding the current. Others are fishing from modern boats with powerful engines which can quickly be manoeuvered safely in and out between the eddies and whirlpools. Groups of tourists are taken out on tours with fast RIBs for even closer contact with the whirlpools.

Sport fishermen in a fast modern boat passing an area with stong updraft ("oppkast") Photo: B. Gjevik, © .

The design of the traditional fishing boat used in Nordland was modified to make it perform better and safer among the eddies and whirlpools. The bottom was made more flat and the keel got a markant swing upward in both ends. This old "current boat" is displayed at Saltstraumen Museum. close by the current. Photo: B. Gjevik, © .

Accidents and shipwrecks

In older days when small open boats were used (see picture above) accidents often happened and many perished when their boats capsized in the whirlpools and the updrafts. It also happened that large steam freighters was taken out of course by the eddies and run on rocks and sank. Today most boats have so powerful engines that serious accidents are more seldom. But the new generation of sportsmen - swimmers, divers and surfers do in many cases stretch their safety limits more than good is.

Other strong tidal currents

Many strong and renowned tidal current can be seen on our planet; Corryvreckan between the islands Jura og Scarba on the west coast of Scotland, the current in the Naruto Strait northeast on the island Shioku in Japan, Old Sow , on the border between the states Maine in USA and New Brunswick in Canada, the Nakwakto and Skookumchuck narrows in British Columbia on the west coast of Canada. Here , northeast of Vancouver Island, there is a maze of lagoons and channels with extremely strong tidal currents as displayed by beautiful scenic scenes in a video from the Roaring Hole Rapids . Very strong tidal currents are also found in Talbot Bay in West-Australia and in Canal de Chacao between the island Chiloe and mainland Chile, only to name a few. The world-wide known Maelstrom outside the Lofoten Headland should also be mentioned. The Lofoten Maelstrom was described in fantasy stories from the eightteens hundred by widely read authors; Edgar Allen Poe og Jules Verne .

Some claim, especially people associated with the local tourist industry, that Saltstraumen is the world strongest tidal current. This is, however, by no means documented and such comparisons are also difficult to make. Nevertheless it is true that Saltstraumen is an unique world class tidal phenomena, which relatively easily can be accessed and watched without advanced and expensive equipment. People have lived around the current and of its resources for several millenniums and fascinating stories of their life and struggle are carried down to us by the sagas.

Raud den ramme (the unyielding) at Godøy

The saga of the infamous viking king Olav Trygvesson tells us of a wealthy and powerful chieftain, skilled in magic, who had his farm at the fjord Salten about thousand years ago. Olav, then the king of Norway, tried to persuade him to convert to christianity and be baptized, but Raud refused stubbornly. When he and his men was beaten badly in a skirmish with the king's men south on Helgeland, Raud fled north with his ship for fair wind. He had always wind from the right direction whereever he wanted to go - this came from his witchcraft. Olav and his men followed after, but when they came to Salten [i.e.Saltstraumen] a furious storm stood out through the entrance to the fjord and it was impossible for them to sail in. They waited for several days, but the wind and the sea spray stood steadfast. Olav then asked bishop Øystein, who followed the king, if he could break the witchcraft. The bishop answered that he could try if God would lend them some of his strength.

Øystein put on his full pontificalia and stood in the bow of the king's ship with candles, crusifix and holy incense. The bishop read the gospel, said prayers, and sprinkeled holy water on the ship. Then they could row in and the others followed in the wake of the king's ship. They captured Raud home on his farm, in his sleeping quarter at night. He was again offered the king's friendship, but answered that he would never follow or believe in Christ. Raud was furiously mad and mocked and cursed Christ badly. He was then remorselessly killed on the spot in a gruesome way. His splendid ship, a drake (dragon) was taken by the king who gave it the name The Worm. When the sails were up they looked like wings on the dragon, and this was the finest ship in Norway - so says the saga.

Literature:

B. Gjevik (2009): "Flo og Fjære langs kysten av Norge og Svalbard". ISBN 978-82-998031-0-6. Farleia Forlag

Snorre Sturlasson: The saga of the Norwegian kings (section 78-80), written about 1200 in old Norse. Translated to modern Norwegian by Anne Holtsmark and Didrik Arup Seip. Gyldendal Norsk Forlag, 1964

Translated to English in December 2020 by the author.

A shorter version, with other picture illustrations, is published (in Norwegian) on the website of NordNorsk Reiseliv AS