Cnut the Great was king of Denmark, Norway, England and part of Sweden from 1016 to 1035. He ruled over the North Sea Empire - a personal union of the aforementioned realm.
He was noted for strong governance, Christian piety, and raising the profile of the newly-Christianised Scandinavian kingdoms.
In England, he ensured full continuity of Anglo-Saxon traditions and customs, and was buried in Winchester Cathedral after his death. His remains, though, were spread amongst those of his royal contemporaries by Roundhead/Parliamentary forces in the English Civil War centuries later. His bones thus are mixed amongst many other sovereigns of the 10th and 11th centuries, in casks at the current cathedral.
One prominent story from his reign was the story of pushing back the tides.
The story first emerged via Anglo-Norman era historian Henry of Huntingdon in the 12th century.
Essentially, Cnut was trying to tell his flattering courtiers and thegns that his earthly power could never rival that of God’s. He demonstrated this via showing that he could not change the tide, as he was just a man ultimately. It was a mix of early-medieval PR, as much as teaching his thegns a lesson.
It seems reasonable on face value. Cnut was noted for his discernment, and it fits that mould.
However, there is no real evidence that it happened. Henry of Huntingdon was writing a century or so after the supposed event. And it was not recorded in the contemporaneous Anglo-Saxon Chronicle. Nor in any known Norse record of time, given he was king of three Norse kingdoms.
Moreover, nobody knows where this event occurred. There are several contenders here.
A prominent one is Gainsborough, in Lincolnshire. This was the initial capital, as it were, of Cnut’s father and predecessor King Sweyn Forkbeard. Sweyn had defeated and ousted King Athelred the Unready, who had fled to Normandy after the kingdom submitted to Sweyn in 1014.
Gainsborough is on the River Trent, which is a tributary of the Humber. As the Humber leads to the North Sea, a tidal bore named the Aegir moves up the Humber and the Trent, as far as Nottinghamshire. Cnut also spent much time there, and this tide may have been the current Aegir on the Trent.
Southampton is another site. There is a plaque in the city centre stating Cnut was there attempting to turn back the tide.
Thorney Island - which is current-day Westminster in London - was another locale. What are now Westminster Abbey and the Palace of Westminster were in Anglo-Saxons times a group of marshy islands. Cnut held a palace in this locale, and it could have been here that his tidal story occurred. Then, as now of course, the River Thames was tidal this far upstream, as is such up until Teddington Lock.
My own suspicion is that it’s a fable. A story concocted as positive PR for Cnut.
It may well have occurred but the lack of contemporary records is telling. Cnut was one of the most powerful European sovereigns of his day, so surely a story of this size should have been documented in the Chronicle, or via a poem. Or even a contemporary picture, painting, jewellery, etc. concerning it. It would have been to Cnut’s advantage to do this, from a religious PR standpoint, if anything else.
Of the era, other kings had relics, pictures, or churches, to denote their piety. This was especially important, as in this era kings were supposed to be upholders of the church and act as moral and religious guides accordingly. To have a longstanding relic denoting one’s piety boosted one’s hold on the church, and the people at large. The religious donations of Alfred the Great, Athelstan, Edgar the Peaceful, and even Athelred the Unready, all stood the test of time. St. Edward the Confessor, the last monarch of the House of Wessex, established Westminster Abbey. Cnut really missed a trick to cemented his legacy in his regard, and I doubt he would have been foolish or short-sighted enough to do this.
So overall, it makes an interesting story. Though perhaps one like Alfred the Great and his burnt cakes - something said long after the fact to emotively prove a telling point. And one that cannot really be proven sufficiently.