In 1803, renowned engineer John Rennie was asked by the Government to survey the lower parts of the River Lea; to see if it could be flooded as a defence against a feared invasion by the French. This was the time of the ‘invasion’ scare and the height of Napoleon's European control and influence.
Whilst Rennie raised a number of objections to such a scheme, the Government went ahead with the works; and at great expense.
The flooding of the valley reputedly comprised one link in a chain of defences around London's eastern flank, stretching from the Thames, up the Lea, and turning west at Springfield Park across the fields north of London.
A line of entrenchments was constructed to protect London itself and a letter despatched from the Horse Guards, dated 25 August 1803, provides details of their line; 'up the north bank of the river Lea as far as Stamford Hill, thence through Highgate and Hampstead; from there it passed near Willesden Green and turning, came to Holland House, before descending to the Thames at Little Chelsea. The eastern face of the capital’s defences was further enhanced by a variety of works; notably, arrangements were set in hand to flood the Lea Valley should need arise; to block the river Thames by the construction of a dam and floating gate at Bow; and to provide a ferry at Blackwall Stairs, manned by the River Fencibles, to be used as a communicating link between the north and south banks'.
The scheme for flooding the valley was prepared. But the works were only partly carried out, and never completed. The work was abandoned when the scare passed off.
The nature and extent of the works conceived by Rennie seem to be unknown and therefore a matter for speculation. How could a dam or dams across the valley impound the river or tide to create a great lake without being overwhelmed by the same forces of the river and the tide? Perhaps a series of dams were planned, topped with connecting causeways? Or was the scheme an elaborate hoax, designed to deter the French and reassure the citizens of the Metropolis that London was well defended?
Rennie's attention to the Lea, in the national interest, was later to prove fruitful. In 1804, his designs for the improvements to the the River Lea Navigation were executed at a cost of £102,714. Rennie also developed plans for the development of the Royal Small Arms Factory at Enfield in 1811.
John Rennie, 1761-1821: The Life and Work of a Great Engineer By Cyril Thomas Goodman Bouche