Some 60 years ago, parts of the River Thames were declared biologically dead. But the famous waterway that cuts through London has been revived and it is now home to hundreds of wildlife species, such as seahorses and sharks. The latest State of the Thames report, released by the Zoological Society of London in November, found that cleanup efforts over recent decades have brought down levels of chemicals like phosphorus and conserved salt marshes for birds and fish, making the river "home to myriad wildlife as diverse as London itself."

The report also highlighted many challenges the Thames faces, including rising water temperatures and sea levels due to climate change. For instance, summer temperature in parts of the river have increased an average of 0.34 degrees Fahrenheit each year since 2007, researchers found. Even slight alterations in seasonal heat may upset the river's ecosystem and erode living habitats. The researchers also found elevated nitrate concentration that threatens water quality. Much of London's drinking water comes from the river.

Among modern cities, the Thames hasn't always been a model for successful environmental protection. It became heavily polluted during the Industrial Revolution as toxic runoffs from tanneries and human waste found their way to the river. The "Great Stink" of 1858, caused in part by human sewage flowing into the Thames, forced the British Parliament to build better waste-water disposal systems. But even in 1959, oxygen levels in the Thames had dropped so low that the British Natural History Museum declared it biologically incapable of sustaining marine life. At around this time, authorities began investing in better sewage treatment facilities and better monitoring of key environmental indicators, sparking a turnaround.

The Thames may be free of much toxic waste these days, but it has one of the higher concentrations of microplastics in the world, reportedly ahead of other urban waterways like the Chicago River and the Danube in Europe, according to a study from British scientists last year. These tiny fragments of plastic, mostly broken off from larger pieces of trash, could be ingested by animals, posing "potential physiological and toxicological threats," the Zoological Society report warned. Rivers carrying plastic waste are the most common way for the pollutant to enter the world's oceans, which puts even more wildlife in danger.

As environmental consciousness grows and people are increasingly drawn again to riverside urban living, many other cities in recent decades have tried to clean up their waterways. New York's picturesque Hudson River was contaminated for decades by a now-banned chemical coolant called PCB that neighboring General Electric plants dumped into its streams. It was only in the early 21st century that the U.S. government ordered the conglomerate to dredge the river. Meanwhile, the Anacostia River in Washington, D.C., recently received a passing grade for the third time in the past four years, with a local environmental organization saying the water is on its way to being "swimmable and fishable." The city has poured money into projects like building a $2.7 billion tunnel network to prevent waste from overflowing in its river system.