December 5, 2021

I, Science

The science magazine of Imperial College

Dead fish after an overflow event. Image: Thames Anglers Conservancy.

There once was a time when the Thames was teeming with life. Otters, eels, and even salmon were abundant. But time has not been kind to the river, and it has been subjected to the worst consequences of human development.

The use and abuse of the Thames started as far back as the 14th Century when only 12 rubbish carts served the whole of London and even then they dumped their rubbish in the river! Things got so bad that fifty years ago, the river was declared biologically dead. But, just recently, things have started to look up. The dirty, polluted river is coming back to life, and animals such as otters are returning to its banks. So what’s changed?

London’s Smelly Problem
It’s 1858 and a sitting of Parliament is scheduled to take place. But the House of Commons is deserted. The putrid stench emanating from the river is too great for the MPs to take, despite the curtains being soaked in chloride of lime.

Events such as this spurred previously listless MPs into action and they commissioned the engineer Joseph Bazalgette to plan and construct a new sewer system. The new gravity powered sewers carried sewage out of Central London and discharged it further down the estuary. Although this raw waste was originally emitted straight into the water, today it is incinerated on-site to create green electricity.

Bazalgette’s gravity powered sewers were an inspired solution to the city’s woes and formed the catalyst for the Thames’ recovery. The cleaner water allowed fish populations such as sprat to return and support life higher up the food chain.

But why, if Bazalgette’s sewers were such a success, was the river declared biologically dead fifty years ago? World War II is to blame. The bombing during the war caused massive damage to London’s infrastructure and this combined with a large population, meant that sewage once again was discharged straight into the river. It was not until the 1960s that funds were available to start repairing the sewers.

In recent years the sewers have once again started to fail. They are simply too small to handle all the sewage from London’s 7 million-strong population. During heavy rain, in order to stop the streets from flooding, the sewers overflow into the river. When the sewers were built, overflows only occurred once a year, but now raw sewage pours into the Thames nearly once a week. And when this effluent enters the river, bacteria break it down, using up dissolved oxygen in the process. This causes thousands of fish to suffocate.

In an attempt to prevent the death of fish, Thames Water utilise a fleet of “bubble boats” to oxygenate the river after overflow events. But oxygenation does not solve the problem, it only treats the symptoms. To stop sewage overflows in London once and for all Thames Water are constructing a massive 25 km long, 7 metre wide tunnel under London to capture all of the overflow sewage. A similar tunnel called “The Lee Tunnel” is already being constructed in East London. Once this tunnel is completed in 2020 it will significantly improve the Thames’s water quality with positive consequences for wildlife.

Alongside improvements in sewage disposal, recent strict legislation has prevented industry from dumping its waste into the river. But dirty water is only half the problem. In order to bring life back to the Thames whole new habitats have had to be created.

Creating New Habitats
Much of the Thames is hemmed in between concrete banks, making it difficult for plants and animals to colonise. Along sections of river, the environment agency have removed this concrete and replaced it with heaps of rubble. This rubble then traps sediment, to form mud banks, creating the perfect conditions for reed beds to develop. These reed beds support a vast diversity of animal life further up the food chain.

Where they have been unable to remove the concrete, the environment agency have been creative. Instead of creating horizontal banks, they have built them vertically, by placing rubble behind wooden planks clinging to the wall.

Due to improvements such as these, the Thames now supports 125 fish species and 400 types of invertebrate. These in turn provide food for bird species including herons and kingfishers, and a variety of mammals.

Seals are now frequently sighted in Central London. One has even been named “Sammy the Seal” by a worker from Canary Wharf, where it often visits. And seals aren’t the only surprising creatures found in the river, others include seahorses, goldfish, and even dolphins and porpoises which have been spotted as far upstream as Tower Bridge.

A Fishy Business
Not all conservation efforts on the Thames have been successful. One particularly high profile project to re-introduce salmon to the Thames (which cost over 3 million pounds), has failed to yield any results.

Salmon were present in large numbers in the Thames until the start of the industrial revolution when pollution caused their numbers to tumble. But come the 1970s and the river was once again deemed clean enough to support them. So in 1975, a number of juvenile salmon, mostly from Scottish hatcheries, were released into the Thames’s tributaries.

For a while their numbers began to rise, hitting a peak of over 300 in the 1990s. Success, though, was short lived and by 2005 no salmon were recorded. And despite further re-introduction efforts, the Thames still only sees the occasional “stray”.

The quality of the river’s water is not to blame for the failure of the programme, but rather the origin of the introduced juveniles. Salmon develop adaptations that are highly specific to their local river system, such as to its chemistry and temperature. So the Scottish hatchery fish stood little chance when placed in the Thames. It’s not all bad news though, as other fish are benefiting from the “ladders” constructed for the salmon. These allow fish to pass around the barriers by swimming and leaping up a series of relatively low steps.

Fish ladder on Thames weir (left). Image: Huw Waters.

A Bright Future?
The Thames still has its issues. Over the past century invasive species have taken hold and it doesn’t look like they’ll be going anytime soon. Chinese mitten crabs for example are voracious predators and so pose one of the greatest threats to the Thames’s native wildlife. Due to their high fecundity (ability to reproduce), removing the crabs completely from the Thames is described as “an impossibility” by the Thames estuary conservation group at ZSL. Additionally, until the Thames tunnel is completed in 2020, sewage overflows will continue to damage the rivers ecosystem.

But, despite these issues, things will only get better for the Thames, which over the past 150 years has been transformed from little more than an open sewer to an urban oasis for wildlife. When the Thames Sewer Tunnel is finally completed, the river will be exceptionally clean. This combined with continued habitat restoration and other conservation initiatives means that the future is looking bright for Thames’s wildlife. Maybe, with time, we will see salmon leaping up past parliament once again.

More > Kelly Oakes takes part in a local river clean-up in Residents Lead the Way.