Legend has it that when the Danes
Blocked its flow and left them stranded.
Sailed up the river and landed,
King Alfred and a few of his Thanes

The River Lea rises at Leagrave Marsh in Luton, Bedfordshire, 52 miles (83kms) from where it joins the Thames at Bow.  Alfred the Great who died on 26th October 899 is best remembered for the enduring legend of the burning the cakes when he was on the run from Viking forces in Somerset. However one of his abiding legacies must be in establishing the River Lea as the established western border of Essex, which remained so until 1965.
            The Anglo-Saxon Chronicles describe that in 895, a substantial force of Danish (Viking) raiders abandoned their encampment on Mersea Island off the Essex coast in the Blackwater Estuary and took to their long ships and sailed south towards to the Thames Estuary. London was a very wealthy city holding out the prospect of much booty for the invaders. The Danish force consisted of around 3000 warriors and a fleet of at least 100 vessels of all shapes and sizes.
            Progress up the Thames was difficult and slow. Whilst the long ships were flexible and manoeuvrable, they were weighed down with fighting men, weapons and armour.  The vanguard of the Danish fleet reached Bow Creek, opposite the present day O2 Arena, and then for some inexplicable reason turned north into the River Lea where they proceeded to head up stream. The rest of the Danish fleet obediently followed. The Danes pressed on and followed the course of the River Lea through the middle of the area that is today’s Olympic Park at Stratford, on past Stoke Newington, Tottenham, Walthamstow, Enfield and Waltham Abbey.
At Ware, in Hertfordshire, 20 miles distant from the River Thames, it was impossible for the Danish ships to navigate any further. Ware itself was a border town between Anglo-Saxon Wessex and the Viking controlled, Danelaw. Danelaw was simply a historical name given to the part of England in which the laws of the Danes (or Vikings) held sway. The Danes remained at their base in Ware for six months unmolested. Alfred, with an army of Londoners, followed the Danes up the River Lea but rashly throwing his customary caution to the wind he attacked the Vikings in their newly fortified compound. It was a disaster. His forces were defeated and many of his senior commanders were killed.

Tottenham Lock today. Did King Alfred trap the Vikings in the River Lea here 1000 years ago?

            Alfred retreated, then, using tactics once employed by the Danes, he set to work diverting the course of the River Lea. He also built a series of fortifications on either side of the bank at strategic points. The Danes were trapped. They had sailed into a bottleneck and Alfred had sealed it with a cork. An 18th century historian speculated that the ‘works’ carried out by Alfred’s forces to block the Dane’s escape were built across the River Lea in Tottenham. With the onset of winter the Danish fighting men secretly decamped. They abandoned their ships and marched west across the country to the River Severn. The threat to King Alfred and London had been removed. There followed relative peace for nearly a hundred years and in the meantime the River Lea had become recognised western border of the county of Essex.

Extracted from London’s Metropolitan Essex – Wrong Turning
https://www.essex100.com/product/londons-metropolitan-essex/
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