Stretching from the Thames in London’s Docklands up to Hertfordshire, the Lea Navigation cuts through the East End of the capital. Not quite a canal and not quite a river, for centuries the River Lea has been a lifeline of industry taking goods to and from London’s port.

These days the “canalised” waterway and the surrounding Lea Valley have become more centred on leisure, with long walks, rowers and pubs a plenty. But it is when London has been threatened that the Lea has really stepped up, proving itself an all-important means of defence for nervous Londoners.

The Lea’s role as part of the city’s defences stretches back over a thousand years. Before England was even England, the wiley King Alfred the Great messed with the water levels of the river, causing some invading Danes to be trapped with their ships upstream. Stranded with no beard oil, the invaders had little choice but to leg it before Alfred had them truly surrounded.

In the closing decades of the 19th Century, as the British Empire stretched itself across the world, the powers at be started to get a bit more nervous about the neighbours. The London Defence Positions were envisaged to makeup for the fact London was practically indefensible, and consisted of a string of forts around the city, which would be linked up with trenches should the need arise.

When the First World War kicked off, the old scheme was resurrected and extended to incorporate the River Lea near Broxbourne. By that war’s end, though, trenches had become obsolete in the face of well armoured tanks, which could roll over most things. Deep ditches filled with soldiers were sadly not an exception.

Come 1940, with the fall of France, London faced a very real threat of being overrun by Nazi tanks. As panic ensued, the Lea was incorporated into the Outer London Anti-Tank Line under the direction of the aptly named General Ironside. The length of the Lea would form a physical barrier against any invading tanks.

Heavy WWII fortifications are more associated with Normandy than east London, but remnants of this defensive line can still be found along the navigation. Observant cyclists crossing the Lea on the Greenway to the south of the Stratford Olympic Park will realise they are pedalling right through WWII era defences. Further south in Bow there are still some pillboxes to be found.

The Lea saw further use in the wars, as a way of transporting ammunition safely from the Waltham Abbey Royal Gunpowder Mills: the smooth movement of barges was seen as a safer bet than the potholed roads for transporting volatile explosives. The defensive structure above was intended as an anti-aircraft position most likely to protect the Mills themselves.

So if you get the chance to take a stroll along the Lea, maybe you can now appreciate it for more than an afternoon activity. Between the hungry geese and the futile fishermen, try to spot the material remnants of Ironside’s defensive ring. And rest easy knowing that, should tanks ever threaten London again, the Lea has got your back.