Severn, part of the Channel 4 series Rivers, will see the former Newsnight presenter meet the last mudhorse fisherman on the Severn Sea and investigate the country's largest ship graveyard.
It's part of a series of four films that started on Sunday with a look at The Tweed, which runs through Scotland and northern England.
Paxman travelled from the muddy source of the Severn to its mouth at the Bristol Channel estuary.
And he lingered in Shropshire to enjoy the river in all its glory, during a filming visit in August.
Sunday's show will include footage with volunteers at the Severn Valley Railway.
Lewis Maddox, event co-ordinator for the railway, said: "He was with us for nearly five hours. During the day he met with our volunteers at both Arley Station and Bridgnorth.
"We're hoping it gets a good showing, it takes a lot of work to put together something like this and any kind of coverage is fantastic for us. It shows the railway off in its best light to a brand new audience.
"The river is our namesake – we are the Severn Valley Railway. This will show people that we have amazing views of the Severn and that we're a fantastic day out in our own right.
"With a good year last year, 250,000 visitors, we're hoping all the coverage means we can match that again."
Lewis said the University Challenge presenter had been happy to take time to speak with fans during his time at the railway. He was really nice and wanted to know everything about the railway – he took a real interest," he said.
"He shook everybody's hand, signed autographs and took pictures.
"Luckily we weren't grilled on anything either!"
Paxman also visited Ironbridge at around the same time to take part in the annual Coracle Regatta.
Ironbridge Lions member Elaine Bradburn met with the presenter during the trip.
She said: "He was absolutely delightful. He was there filming. We spotted him and it was decided I should be the one to go up to him.
"He said that he had to go and film something, but asked where our stall was so that he could come and visit later. True to his word, he did."
The group had collected abandoned tents and chairs from V Festival with the aim to sell them for charity.
"Jeremy was very interested to hear about the V Festival tent collection we do every year, though he was surprised to hear just how many tents are abandoned by festival-goers," Elaine said.
"Having Jeremy visit our stand was a great end to a very successful bank holiday weekend."
Speaking about the documentary, Paxman said that many of the happiest days of his life had been spent around rivers. I long ago gave up trying to put people right about fishing," he said.
"If you told me it was a silly thing for a grown man to do, I'd have to agree with you. It's absurd. But consider the benefits. It gives you a reason to be in some of the most beautiful landscapes in Britain and to experience the watercourses that are our are nation's arteries.
"Britain may be cursed by rainfall. But as a result we are blessed by rivers. We have more than 6,000 of them, 30 of which are at least 70 miles long – longer than many of our motorways.
"Rivers are part of the scenery, sometimes bustling, sometimes meandering, all of them making their way to the sea in their own sweet time. Rivers made this country.
"The towns grew here because the rivers provided food and the current allowed you to take essentials like stone, salt, wool, grain, meat and timber great distances with much less effort than loading a wagon and hitching bullocks. When Britain became the first country in the world to industrialise, the revolution was made possible by water power."