By permitting mass immigration, he said the country was 'heaping up its own funeral pyre'.
How far was his shadow cast?
Powell said people were experiencing 'hopelessness and helplessness in the face of the continued flow of immigration into our towns'.
In February 1968, eight-year-old best friends Mike Edwards and Raymond Comrie found themselves thrust into the spotlight when their class at West Park School in Wolverhampton was interrupted by a visitor.
"I can remember the door was open, someone has come in with a camera," recalls Mr Comrie. The camera "ended up pointed at me and Michael".
To the boys it wasn't a big deal - they didn't even know why the photographer was there, or who had invited him into the school.
The media attention had been prompted by something Wolverhampton South West MP Enoch Powell had said earlier that month.
Two months before his notorious speech of April 1968, he had spoken of a constituent he said had told him how her "little daughter was now the only white child in her class at school" - and the press wanted to find her.
The school was never identified by Powell.
Indeed, there were doubts about whether it existed at all, but it was suspected the MP might have been referring to West Park.
Other photographs taken that day show several white children in the class, suggesting that Powell's claim was exaggerated - or downright false.
West Park primary had recently been rebuilt on a new site, and head teacher Eileen May Llewellin-Davies kept a record in her logbook of the hostility her pupils faced from children at a nearby school.
"Some pupils of the Municipal Grammar School evidently objected to our presence on what used to be part of their games field," she wrote in one entry.
"The children were rather distressed when out to play to hear 'get off our field you dirty wogs'. Staff were very upset, but it was felt that if we ignored the taunts of the unintelligent minority and showed that we were well behaved and peaceable then the antagonism towards us would diminish."
As far as the two young schoolboys were concerned, the colour of a person's skin was irrelevant.
"All my friends were Jamaican heritage," says Mr Edwards.
The majority of pupils at their school were black, adds Mr Comrie. "We didn't even notice."
And for Mr Edwards, the irony that his extended family also attended the school with "one white child" was not lost.
"Powell might not have been specific," he says, "but it was a joke in our family - 'our Michael' being the only white boy in the school, when I had 12 aunties and uncles and a lot of cousins at the school."
The photograph of the two boys appeared in several newspapers, and soon after a meeting of 27 parents was held in the school library, organised by those against the numbers of immigrant children at West Park.
The boys remained oblivious to the furore.
"Some other families pulled their kids out of schools in Wolverhampton because of the high concentration of blacks or Asians, but at home my own experience was nothing - I've got no memory of it," says Mr Edwards.
He does, though, recall how the atmosphere changed following Powell's infamous speech, a few weeks after he and Mr Comrie were photographed at school.
Graffiti appeared almost overnight, he says, with the words "go home" painted on brickwork.
"I can remember saying to my parents, 'what's that sign?' and it was a swastika," says Mr Edwards.
"Things changed after that."
'The stars burst out'
A few days before the speech, Powell went to see his good friend Clem Jones, the editor of Wolverhampton's Express & Star newspaper.
The two men had grown close over the years - their families lived only a few streets away and would go on outings together.
During the visit, somewhat cryptically, Powell told Jones: "When a rocket goes into the sky, the stars burst out. This time they are not going to fall to the ground, they are going to stay up there."
Jones's son Nicholas says that while Powell wouldn't reveal what he was going to say in the speech he promised he would soon be giving, the MP was clearly hinting at its likely impact.
"He was making the point it was explosive," Mr Jones says.
The friendship between the two men was beneficial for both. It gave Jones a heavyweight political contact, while Powell learned how best to promote himself and exploit the media.
"My father says that he claims to have been Powell's spin doctor, and Powell was a very good student," Mr Jones says.
On the day of the speech, Jones and his wife Marjorie were looking after Powell's daughters.
It was only at this point the newspaper editor actually saw a copy of it.
"He brings it home and my mother devours this. She reads the speech through and she is appalled," says Mr Jones.
"This crossed the line for her and was the end of a friendship.
"My parents had been very close [to Powell]... but from my mother's point of view, there was no going back."
For the newspaper editor, it was not just his personal life that would be affected.
"All hell breaks loose and my father is in this terrible position," says Mr Jones.
"Ninety-nine percent of the letters are in support of Powell; he is trying to do a balanced job and it all gets really bumpy for him as an editor trying to produce balanced coverage."
Matters were to become even more difficult when the newspaper faced legal action from Powell.
"A few weeks later the Express & Star runs a Press Association story quoting a placard [which had accused the MP of adopting Nazi-like tactics]," says Mr Jones.
"Powell places a gagging writ on my father and this plays out for some years.
"This was eventually settled in 1970 when Powell stopped the case and accepted an apology."
Soon afterwards, Jones left his job.
"The fact that the Express & Star was under this gagging order and Powell was the hero of Wolverhampton made it very difficult for my father," says his son.
"My father accepted early retirement.
"It's a rather sad end."
'The whip hand'
Enoch Powell gave the speech that was to define him, on 20 April 1968, at a Conservative Association meeting at the Midland Hotel in Birmingham.
Flanked by two stony-faced colleagues, Powell told his audience about what he said was an exchange with an unnamed constituent.
"If I had the money to go, I wouldn't stay in this country," the MP claimed he had been told.
"In this country in 15 or 20 years' time, the black man will have the whip hand over the white man."
And Powell didn't stop there.
"We must be mad, literally mad, as a nation to be permitting the annual inflow of some 50,000 dependants, who are for the most part the material of the future growth of the immigrant-descended population," Powell said - his own words, this time.
It was "like watching a nation busily engaged in heaping up its own funeral pyre," the MP continued.
"As I look ahead, I am filled with foreboding: like the Roman, I seem to see the River Tiber foaming with much blood."
With the television cameras filming - ATV had been tipped off about the contents of the speech - Powell went on to tell his audience of the only white woman remaining in a Wolverhampton street.
A war widow, he said, who was abused by "negroes", had "excreta pushed through her letterbox" and would be taunted by children he described as "charming, wide-grinning piccaninnies".
The impact of the speech was explosive.
Powell was sacked from the shadow cabinet by Edward Heath, although he received strong support from the public, with dockers and meat packers marching in support of him.
For the immigrant community, Powell's rhetoric - and the reaction to it - was terrifying.
"It was the main subject - we might have to leave the country at any time," says Baldev Singh Bassey, who was 25 at the time of the speech and living in Wolverhampton.
"We didn't have suitcases [packed], but that was the only talk at the time.
"There was fear in our community, a lot of fear," says Mr Bassey.
"We didn't know what was going to happen tomorrow, if we were going to be kicked out [of Britain]."
Such fears were understandable.
In his speech, Powell had suggested the "encouragement of re-emigration" as a solution to what he saw as Britain's problems.
The MP also referred to a long-running campaign by Sikh busmen in Wolverhampton over the right to wear turbans on duty, which he said was a dangerous example of "communalism".
"When he made the speech the Indian community were really scared," Mr Bassey says.
"After that, we didn't know what was going to happen.
"People were talking about it in the pubs and suddenly the attitude of the [white] people living here was hostile."
One of the many industrious immigrants who came to post-war Britain, Mr Bassey arrived in the country from the Punjab in 1962, initially working at the Qualcast foundry and later on the buses for Wolverhampton Corporation.
"In the 1950s and early 60s, neither Powell nor any other prominent politician in the town showed opposition to the presence of Commonwealth immigrants," says University of Wolverhampton academic Dr Shirin Hirsch.
"They needed these workers for capitalism to function."
Despite this acceptance from the political class that immigration was necessary, Mr Bassey recalls that the prejudice he faced had been bad enough even before the speech.
"We were shouted [at] on the buses, we were spat at," the father of five says.
"I was shouted at, 'black bastard, go back to your country'."
Following the speech, Mr Bassey says, the hostility became even worse - a poll conducted by Gallup in the weeks afterwards found 74% of people agreed with what Powell had said.
Mr Bassey remembers being abused in pubs and in the street.
Walking home from work one night, he was accosted by two youths chanting "Enoch! Enoch!"
"We were scared," he says, "there was no doubt about it after that speech."
Lance Dunkley settled in Wolverhampton in 1955. Originally from Jamaica, he first arrived in London but was eager to make his way to the Black Country because of its name.
"At the time I didn't realise it was [because of the] mining of coal - I thought it was black people," he laughs.
"I was so happy to go to the Black Country. When I got here I didn't see any black people."
He was shocked by the overt racism he was to face.
"They would call us the niggers. Sometimes coloureds," he says.
"We thought we were in the 'mother's country', only to find out that the racism was as high as it was."
But he would go on to strike up a cordial relationship with Enoch Powell.
As a resident in his constituency, Mr Dunkley said he would act as a "go-between" for Powell and the Caribbean community.
By the time of the speech, both men were living on the same street.
"He was a nice guy before the speech," says Mr Dunkley.
"He would knock on my door and ask 'where is such and such?'"
After the speech, Mr Dunkley - already a community activist fighting against racism - refused to be intimidated by Powell's words.
"His speech was just another speech; we weren't going to flutter like the bird with one wing.
"We weren't afraid."
The only real surprise about Powell's speech was that it was so "openly said", says Newton Lyseight, whose father the Rev Dr Oliver Lyseight established Britain's first black-led church, in Wolverhampton.
"I think a lot of people were thinking like he was thinking, but it was an explosion for someone at that level to bring it into the open," he says.
"It was disturbing, but not surprising," according to Chester Morrison.
Mr Morrison, who was 17 in 1968, had arrived in Wolverhampton four years earlier.
"It was very significant because of the emphasis on the 'Tiber foaming with much blood'.
"We as youngsters had to think about what would that mean in reality and how are we prepared to deal with that," he says.
"From tolerating levels of racism, we were put in a position where we had to be more combative because we didn't quite know how the white folks would respond - in particular the skinheads at the time."
For Mr Morrison, who helped found the Afro-Caribbean Cultural Centre in Wolverhampton city centre, the speech "effectively legitimised racist behaviour".
"Many white folk felt they had a licence, because [it was said by] somebody who was well respected, because he was by no means a fool."
Powell's intellect and ambition had lifted him from relatively modest beginnings in suburban Birmingham.
Born in the city in 1912, he attended a grammar school and graduated with a double first from Cambridge.
By the age of 25, he was professor of Greek at Sydney University. At the outbreak of World War Two he returned to the UK.
"He had grown up with a strong loyalty to Imperial Britain," says Dr Hirsch.
"Rising up the ranks of the Army and removed from combat, his time in India [he secured a posting there in 1943] allowed Powell's love of Empire to blossom.
"When the war ended, Powell harboured dreams of returning to colonial India in the role of Viceroy.
"One way to protect the Empire, Powell calculated, was a career in politics."
Although by the time he was elected as an MP, in 1950, India had achieved independence, Powell channelled his energies elsewhere, and became Minister for Health in 1960. Five years later he was soundly beaten by Edward Heath in the contest for the Conservative Party leadership.
The timing of Powell's speech came at a sensitive time, days before the second reading of the 1968 Race Relations Bill.
The weeks that followed were dangerous for black people living in Wolverhampton - 12 attacks of racist violence in just two weeks were recounted to a national newspaper reporter, Dr Hirsch says.
Support for Powell came from various quarters, but there was also opposition to his speech, which an editorial in The Times newspaper described as "evil".
In the years that followed, Britain saw the strengthening of race relations legislation - and also unrest.
During the early 1980s, there were riots in several cities, including in Handsworth in Birmingham and Brixton in London, and the slogan "Enoch was right" was adopted by the National Front.
"Not everything he said in the speech is wrong," says author and journalist Sathnam Sanghera.
"He was right about the numbers. He was right that the character of Britain would change - he was wrong that it would lead to a fragmentation of society.
"He makes assumptions that immigrants and their descendants won't integrate - we have integrated.
"On my mum's road there's whites, blacks, Asians.
"I don't think there's rivers of blood flowing down the streets of Wolverhampton."
'A different Britain'
The MP in Enoch Powell's old seat of Wolverhampton South West is currently Labour's Eleanor Smith - who became the West Midlands' first female black MP at the last election - and between 2010 and 2015 it was held for the Conservatives by Paul Uppal, a Sikh.
It was this community that was held up as an example by Powell of why immigration wouldn't work.
But it's that same community that has become "so integrated and conservative", according to Mr Sanghera.
"The Asian community have been in Wolverhampton for 50, 60, 70 years. They are a conservative community - they believe in education, they believe in family.
"I [also] think the West Indian community are quite conservative."
The way the city voted in 2016's EU referendum - 19 of its 20 wards backed Leave - was not surprising to Mr Sanghera.
"It’s human nature to worry about immigration, to worry resources are going to be taken by outsiders," he says.
"Every phase of immigrants is worried by the next phase of immigrants – that’s human nature."
Five decades on from his speech, some of Powell's ideas have taken hold in mainstream politics, according to Dr Hirsch.
"The idea of 'immigration numbers' that simply need to be 'controlled' was once seen as an attack on people's rights," she says.
"It is now accepted within mainstream politics, and support for these immigration controls pushes ideas of nationalism and hierarchies of who is and isn't allowed into the country that are often connected with race."
For Dr David Wearing, an academic at Royal Holloway, University of London, Powell's speech was a very early example of a modern phenomenon, what he describes as "a racist member of the elite hiding behind the white working class".
"Read the full text of Rivers of Blood and what strikes you is the sheer familiarity of it.
"Among other things, it falsely associates hostility to immigrants with the working classes alone, contrasting their supposedly legitimate concerns with those of an out-of-touch liberal elite."
Dr Wearing adds: "Obviously Powell's speech was severely detrimental to the physical safety and wider well-being of people of colour in the UK, both immediately and for decades after he made it.
"It legitimised every racist beating, stabbing, arson attack, every denial of a job or housing, all the thousands of instances of these things that people endured up and down the country through the 70s, 80s and beyond."
And what of the two little boys who were inadvertently thrust into the limelight by Powell?
"At one stage I said, 'should I be here?' I questioned myself at the age of around nine or 10 - 'why did my parents come here?'," says Mr Comrie.
"Even though I was born in this country I still felt like I was born in the wrong place.
"Even though it's multi-racial, I still feel I'm in the wrong place today."
For Mr Edwards, Powell's speech helped to shape his life in a positive way.
Now a lecturer in union studies, he became a union rep at the age of 19.
"Going back, I think I was politicised by where I was from: my family background, white working class, boat people living a slum.
"Going to West Park School and the Rivers of Blood speech - this is a part of it."
At a recent reunion of West Park's class of 1968, Mr Edwards says he contemplated the legacy of Powell's speech.
"It was calculated, intended to create division, and to some extent it worked," he says.
"This is the importance of our story today and the kids that went to that school.
"If you think about Britain today, it looks very different to how it did in 1968, and certainly going back to the reunion was just amazing because we all told our stories.
"We are Powell's legacy because we have won."