A terribly nice and passionate man called Richard Jenkins from the Environment Agency is, as we sit hunkered comfily like a couple of slack-jawed yokels sucking long grass fronds on the banks of the River Wear, telling me some astonishingly interesting things. Unfortunately the terminology isn't floating my boats, nor would it lift your skirts. There's a lengthy bit about reed-bed filtration and riparian buffer-strips. Then he says: "Oh, we also get to design meanders."

Hang on, I urge, leaving a nubbin of ripped hay between my two front teeth in the excitement, what was that? You actually get to … design … meanders?

Well, I've just found my, and I suspect your, dream job. I'm sure there was something in the works of the late Douglas Adams about someone who had a job designing meanders or, as we know them, better the "wriggly bits on rivers", and what bliss, and suddenly it turns out this job exists. Half a mile up from the Wear, people like Richard are introducing new wriggly bits on rivers for the benefit of the fish, who like their journeys to be interesting. What a job. It's like finding, at the jobcentre, that there are unicorns still needing shoeing, in liquid silver. This, I hope, serves as my – our – midlife plea for a job redesigning rivers, but also a way of leading into the fact that something rather fabulous is happening to them, if you hadn't noticed.

You might have done last week. The Environment Agency put together a nicely upbeat press release on the 10 most improved rivers in England and Wales, and it was picked up by many organisations rightly desperate this crammed news year for "non-killy" news, but, honestly, until you've come to one of these rivers, or unless you're an angler, you can't appreciate the tenth of it.

Statistics can be such traitors, triple agents, but one of Richard's sticks in my mind and more importantly in my notebook. There is, he says, carefully unfurling a couple of sheets of A4 in the warm breeze, a record catch of salmon going on in the Wear this year: the rod catch stands, end of August, at 1,531 fish so far. Fine. But what does that mean, in terms of comparison? How does that sit with the days when the Wear was a "dirty river"? Let's go back, mid-Seventies or something, mid-Sixties. The comparable catch in 1965, he says was … two. Count them: two.

The same story, though I don't have room here for all the statistics, is slowly repeating in all the other named rivers – London's once-toxic Wandle, the Taff in South Wales which once ran, like the Wear here at Durham, so black with coal dust that no life could survive; the Mersey basin, which powered the industrial revolution but had left it, as relatively recently as 1982, with the inglorious cachet of most polluted patch of Europe.

It is, like all interesting stories, the beneficiary of a double-edged sword; and, certainly like all environmental stories, the result of unintended consequences. Aspects of human society, in the Durham area and Merseyside of course and doubtless around the Taff, and both the Stours, have suffered, since about 20 years ago, and are in slowish recovery. Here in Durham and, the more I look into it, everywhere else where the fish are biting, it's becoming clear that the person we have to thank is Margaret Thatcher.

She it was who helped close down British industry. And, near the end of tenure, she privatised water authorities. It may be faintly possible she didn't do both with the singular aim of helping fish. But no one could have done more for those with gills. Here on the Wear, the ending of steelworks and shipbuilding at the estuary's mouth in Sunderland has led, unintended consequences, to the declogging of the muddy toxins of the river's mouth: salmon and sea-trout were able to get back in, to get instinctively upriver and to spawn.

For a few years, according to Richard Jenkins, this didn't work. The habitat wasn't right. The rivers were still at least faintly filthy, and there had been an orthodoxy, since the 60s, for irrigatory and other channels to be set in concrete canals. "And fish don't like that, particularly salmon. Fish want interesting lives, just like us."

But the privatising of water authorities necessitated the setting up of the National Rivers Authority – with privatised financial help the former rate-capped public authorities could only have dreamed of – and a marked crackdown on localised industrial pollution. Slowly the NRA, and the lack of any industry, and a rejuvenated environmental mojo, has had its effect, and the effect is clean, clean rivers, tens of thousands more salmon racing back from the west Atlantic to spawn; and kingfishers, and otters, and elvers, and a concomitant rise in the coarse fish, roach and dab and bream, normally thrown back these days, as are most fat salmon, by an equally addicted but kinder generation of non-bumptious anglers.

That this is good for nature, non-human nature (although I hope we still fit somewhere on the nature graph), is indisputable. What, though, of the human impression, on human society? I get good answers from young angler Gary Smith, up from the Hull area "because friends said this stretch was really working these days" and from 63-year-old Jim Wynne, who oversees for his angling club a discrete and discreet stretch of the Wear about a mile downriver, ie north-east, of the Durham river's coil. "It's not just the fish, and I'm not going to tell you how good it is, otherwise everyone's wanting here! It's the other stuff … kingfishers and the like. It's a clean river again. It's glorious."

Later that evening, wandering the shin-splint hell that is Durham, it's hard, because a good 20 years have passed, to find anyone who can't see the benefits. The death of industry is not quite … forgotten … but adjustments have been made; decades have passed. In the multitude of bars adjoining the Elvet Bridge, and in a glorious absence of yaying students, even the over-50s have adjusted to the north-east's lack of industry and found other if less Biblically manly work, and Barry tells me he's an occasional fisherman but doesn't want to say much more because he lives (rented) near a great spot. Charming, knowledgeable, diplomatic, knowing when to close their gills, anglers would have made far better spies than Cambridge types.

It's not that the once black-fogged Durham isn't wrapped in its mining history, any less than the other rejuvenatories are wrapped in their own heritages. Even in its cathedral, pride of place, in a place not even contemplating a lacking of prides, is given to a banner, right beside Prior Castell's 1650 Clock. The banner celebrates Haswell Lodge, part of the Durham Miners' Association. That's the body still behind the local miners' gala, an event venerated to this day as a magical touchstone over whether a new Labour leader a) attends or b) makes mimsy excuses because he wants to suck up to the Daily Mail. Anyway, the banner is at least – I'm rubbish with eye-measurements – 1,000 million miles high: it's big. "They Being Dead Yet Speaketh" runs the huge bottom bit, which to an upbeat mind is right there with The Simpsons episode which revealed their Amish ancestors' motto – "First Toil, Then The Grave" – but, still, essentially, Durham reveres its mining history.

But that mining history is over, and there were naturally downsides anyway. Bad bits. The eight collieries alone in this tiny city which had the Wear simply renamed the "black sewer" – they didn't actually wash the coal in it, just left the chipped tons beside it in stacks and let the rain clean it, the better to sell "rough small" at 15/6 a ton and "best" at 21 shillings, and then let the run-off darken and poison the river; and let the upriver tanneries proffer their interesting additions. Every fish died.

The fact they are coming back, here and in the other nine specified long rivers and many many unspecified happy rivers is, yes, testament to the accidental consequences of the 1980s – but also testament to increased understanding between those responsible. Concrete irrigation culverts have been destroyed, subtle salmon-leaps introduced, and meanders drawn and designed and, OK, fabricated, but so what, they'll soon be mossy and old and worn and as real as 1460. The drive to haul out every river-drooping willow as a "flood threat" is now seen as madness: they're now left, to give shelter for spawning fish and dripping fly larvae from above. The agencies now get on, roughly, with the farmers, and there's no room here truly to get into the five-metre riparian buffer-strips but trust me it makes sense and farmers like it and don't have to worry about erosion.

"Ive, are the anglers still there?" shouts a dauntingly paunched chum as the separated men lead their separately heel-teetering wives back over a sparkling bridge from Durham's centre to the one posh hotel, at way the wrong side of 11pm. Twelve hours before, same bridge, a complete stranger had come up to me near the weir to comment on the number of cormorants, feathers heavy with that languid sea-black, who'd flown in from the coast to crook fish. The locals used to talk about the mines. They're talking, today, about the river.