King Cnut the Great — he of the holding back the tide fame — is also claimed to have constructed what could have been on of the most impressive feats of civil engineering in Saxon England, a vast 4 mile long canal through South London.

If it ever existed that is.

Denmark’s Cnut the Great lead an invasion of England to reclaim the throne briefly seized from King Aethelred by his father, landing at around Southampton in September 1015, and moving northwards, crossing the upper Thames in early 1016.

Bypassing London, which was still strongly protected from land based attack by its Roman wall, he moved northwards to Northumbria, but later returned south to deal with the city after it chose Edmund as its King after the death of Aethelred in April 1016.

Cnut lead a fleet up the Thames, but a river based invasion of London was going to be hampered by the presence of the early medieval London Bridge (the precursor of the more famous Old London Bridge).

What is claimed to have happened is the Cnut’s army then dug a trench all the way from Rotherhithe through south London to Vauxhall – a canal that would have been about 4 miles long.

In the Chronicles of London Bridge, William Maitland cited the Saxon Chronicle that said that having arrived in Greenwich, the army sank a deep ditch on the south side and dragged their ships to the west side of the Bridge.

In the same Chronicles, a William Maitland is cited as having found some evidence of Cnut’s Trench at the Rotherhithe wet dock – today’s Greenland dock — and then it continued along Old Kent Road, and then continued in a crescent form to Vauxhall.

The difficulty with this is that presumably he dug the huge channel while also seeking to subdue London to the north of the river, and probably having to deal with forces still loyal to the English King attacking his fleet.

So, being doubtful that a canal was constructed – as we would understand such a thing – was it possible for Cnut to still get his fleet around London Bridge? I think the answer is almost certainly yes.

South London is a very low lying area, and despite some raised banks being constructed to protect the land from the river, it remained largely marshes and flood plains until being drained in the 18th and 19th centuries.

Most of Rotherhithe was certainly marsh, with the Bermondsey and Rotherhithe Eyots being the only notably dry land in the area.

A significant dip in the ground can also be found at Elephant and Castle where the Rockingham Anomaly exists.

If the river were breached at Rotherhithe and the low-lying land deliberately flooded, it would not be overly difficult to imagine shallow hulled boats being able to be rowed across Southwark and the Lambeth Marshes to Vauxhall.

Indeed, a number of now covered up rivers, such as the Earl’s Sluice and Efra come close enough to each other to assist in the passage across South London.

It’s possible that bypassing London Bridge in this manner is how Cnut was able to engage King Edmund at the Battle of Brentford sometime in the summer of 1016, which Edmund won, but the victory was short-lived as the two sides eventually settled a peace later in the year.

So, did Cnut dig a 4-mile long canal across London? I think not. But I do think that there is a good chance that he might have dredged parts of already very wet marsh lands and flooded them to enable ships to pass across.

Remarkably, despite not being mainstream knowledge, a cigarette company did recall the event, when they released a series of collectors cards – one of which imagined what the progress would have looked like.


Whatever the truth of the matter, it is now nearly 1,000 years since that original passage across South London.

Wouldn’t it be wonderful to recreate it in four years time — in the summer of 2016 — with floats and parades?


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  1. Joanna Cronin says:

    Very interesting. I love the thought of them cutting through using the marshes and rivers. I agree it would be great to have a celebration with parades. etc.

  2. Richard Ash says:

    At current rate of progress, the Thames might permanently invade quite a bit of London by 2016 …

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  1. […] temporary respite, as Cnut returned in 1016 and defeated the English — allegedly by digging a huge canal though South London — and his line ruled the country for the next 26 […]

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