Ohio's Cuyahoga River used to be so polluted it regularly went up in flames. Images of one dramatic blaze in 1952 shaped the US's nascent environmental movement, long after the flames went out.

The last time Ohio's Cuyahoga River notably caught fire was 22 June 1969. At midday, debris floating in an oil slick on the river ignited after sparks fell from a nearby passing train. While locals reported the flames reached five stories high, firefighters extinguished them in less than 30 minutes. Two railroad trestles sustained a total of $50,000 in damage ($428,500 in today's money, or £321,000). But the event was over before reporters could arrive.

"All we have photographically of the Cuyahoga fire in '69 is pictures of firemen mopping up, spraying the trestle, and then Carl Stokes, the mayor, on the tracks the next morning talking to the press about it," says David Stradling, professor of history at the University of Cincinnati, US. Earlier images of fires on the same river, such as that pictured above in 1952, nevertheless began to be circulated when the 1969 blaze occurred.

And yet, this short-lived fire on the Cuyahoga River became a powerful moment in the growing environmental activism movement in the United States. The images of the river's previous fires ignited national conversations on pollution and social justice, just as the US's nascent environmental movement was gathering pace.

The history of the river's dire pollution stretches back long before its final blaze. By the late 1800s, Cleveland and the Cuyahoga had become a hub for the Industrial Revolution. Stradling says it started with steel mills towards the head of navigation (the farthest point upstream on a river where boats can travel, often defined by a dam or other physical barrier). The river made it easy for boats to transport ore. "Once the steel industry became established in Cleveland, all these other ancillary industries were good to have nearby," he says.

With all of this steel production came excess chemicals and grease, says Stradling, and since there were virtually no environmental regulations at the time, a lot of it ended up in the river, along with raw sewage.

"We didn't treat our wastewater, so anything that you flushed down the toilet was piped straight into the river," says Adam Schellhammer, Mid-Atlantic regional director of the non-profit, American Rivers. Then, in the early 1900s amid World War One, industrialisation ramped up and the river pollution got worse. "With that uptick in manufacturing, these rivers were bearing the brunt of all that, and there were no restrictions on what went in," says Schellhammer.

By the 1930s, the Cuyahoga had essentially become an open sewer, infamous for its foul smell and telltale sheen of oil slick. Unsurprisingly, the fauna that typically reside in and around the river steered clear, or died. "The Cuyahoga was a completely dead river for decades," says Stradling. "There were no fish, no water fowl." Those who lived nearby cautioned that if you fell in, you had to be rushed to the hospital.

And, occasionally, the massively polluted river burned.

The first fire on the river was in 1868, and there were at least 12 subsequent fires, according to the Environmental Protection Agency (EPA). The biggest ones occurred in 1936, 1941, 1952 and 1969.

Local protests and dogged advocacy by Mayor Carl B. Stokes had already led to a $100m bond for river cleanup passing in 1968 (equivalent to more than $900m today). When the river burned again in 1969, Stokes acted quickly.

"It was a right place, right time situation," says Anne Vogel, former director of the Ohio Environmental Protection Agency and current EPA region 5 administrator. "Cleveland was also a place, at the time, where environmental and social situations intersected."

Stokes was the first black mayor of a major city in the US, and he was acutely aware of how racial inequality in the

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