IT all seems so alien. On the steep bank of the River Tay near Aberfeldy in Perthshire trees have huge bites out of their lower trunks. On closer inspection it seems the wood has been hacked away with hundreds of small chisel cuts in a dense pattern.

Around the trees lie what look like machined chippings from a lumber mill. Nearby a tangled raft of branches, some stripped of bark like white bones, protrudes into the surging river.

Odder still, 25 miles away on the Bamff Estate near Alyth, stick-and-mud dams have turned a burn – little more than a ditch – into a series of vast shallow ponds, some 100m long and 30m wide. Big dead trees, now rooted in water, claw upwards; bright turf runs under the flat, clear water.

Tree stumps in the little wooded valley look like pencil tips; fallen logs have the same whittled look. Older stumps sport bright fungi, and small trees sprout new shoots as if carefully coppiced.

The impact of beavers in the River Tay catchment is startling, and what they do clashes with what most of us think of as normal in our woodland. Despite this, last November Scottish Environment Secretary Roseanna Cunningham announced the animals are to be given native status.

The decision followed the beaver trial in Knapdale in Argyll, where 16 of the herbivores were released from 2009 and studied for five years. This summer the law changing the status of the animals will come into force. At the same time the Government’s nature agency, Scottish Natural Heritage (SNH), will set out rules on how beavers can be controlled.

Their presence on the Tay, and now on the Forth, in far greater numbers than in Knapdale, is the result of illegal deliberate releases or accidental escapes.

But all beavers in Scotland, including the 250 or so in these eastern catchments, will benefit from the protected status.

For Bob Smith they’re a golden opportunity. A wiry figure with the complexion of an outdoorsman, he runs the Nature Nuts tour business in Blairgowrie, showing clients beavers, otters, capercaillie, white-tailed eagles and more.

Smith’s beaver tours start on the Bamff Estate burn where captive beavers were first kept in 2002.

He explains that water engineering by beavers is for security – they like the entrance to their burrows in the river bank to be under water – and to transport wood, so they can eat the bark. At the first big pond Smith says standing water benefits wildlife: herons feed on frogs, and two hobby hawks nest to feed on proliferating dragonflies.

“People seeing this for the first time go: ‘Oh my God, it’s Armageddon,’ but when you see what’s here, the fungi, mosses, the wetland, the natural architecture the beavers put together, it’s amazing,” says Smith.

Some species lose habitat, but others gain, and overall it improves diversity, he says. Smith’s enthusiasm bubbles over: “Kingfishers, otters, grey wagtails, dippers, all of them thrive. There is just a wealth of wildlife.”

Along the burn he points to moats dug around soggy islands for safety; a beaver lawn of cropped grass; and a dam of stones, not sticks, which the beavers roll into place. Smith points to a tree stump. “I have watched a buzzard in one tree and a tawny owl in another, hunting mice. The birds sit waiting for the prey and the prey has been brought here by this amazing ecology.”

He is not the only one to find beavers help tourism. In Alyth, two miles south of Bamff, I find Piotr Gudan, who offers kayak trips to see wildlife through his company Outdoor Explore.

On local lochs and rivers in the past five years he has seen 20 beaver settlements. In his native Poland, he says, beaver dams are welcomed as a buffer against flooding.

“One of the best places to see beavers is in Perth, quite near the city centre,” he says. “Beavers only settled there last April, but because it’s a tidal section of the river, for a few hours a day you can see the entrance to the lodge.”

On the Tay at Aberfeldy I get my first glimpse of the impact of the rodents which can weigh 25kg and measure more than a metre, with Victor Clements, a woodland adviser and beaver expert.

The wooded area alongside the Tay is 20m wide, bounded by the A827. Trees up to 70cm thick have trunks carved out to a third of that thickness, the cut wood bright orange. Clements has seen trees over a metre in diameter felled by beavers; one 60cm trunk fell in four days. They mainly take willow but have a taste for the sweet bark of sycamore and beech, neither native trees. “It can be a benefit,” says Clements. “If the non-native trees are felled then hazels can come in and make the wood more diverse.”

The floating stick raft is a winter larder. “They take the trees down then store the branches and twigs to eat the bark.”

On the flat fertile flood plain above Aberfeldy, Clements shows me a ditch repeatedly cleared of beaver dams: they raised the water table and left the land too wet to plough. Moved on, Clements says they flooded part of a golf course, and blocked a culvert under the raised road, which acted as a giant dam to leave even more fields sodden.

Adrian Ivory knows such problems well. The tall, bluff farmer runs Strathisla Farms near Meigle, on the flood plain three miles south-east of Alyth, and has had beaver problems for the last two years.

In a low-lying field he points to a deep ditch with a tide-mark a metre up it. A land drain flows into it, but beavers blocked the ditch, the water backed up, and fields flooded. The drain’s gradient means if water is a metre over its outfall, fields will flood a kilometre away.

Ivory leads the way into a stand of willows, the swirling water half-way up his wellies thanks to beavers flooding the area. Several trees have been felled, and Ivory says the coppice regeneration beaver enthusiasts speak of hasn’t happened; the trees are dead.

With a hint of anger Ivory points out – and police confirm – that no-one who released beavers in the Tay catchment has been prosecuted.

“It’s me as a farmer who has to pay and it’s costing me a couple of thousand pounds a year. There are areas of the farm where there are beavers and they don’t cause problems, and there are other areas where I am pulling my hair out.

“The biggest impact is the cost of clearing out the dams.” Delays in planting due to sodden ground and flooded crops rotting are another cost. vory believes there are four times more Tayside beavers than the usual estimate of 250, and says the problem will eventually equal that of deer. Like deer, nothing preys on beavers “unless you want to reintroduce wild bears”.

His big fear is that new rules from SNH will constrain farmers’ actions: he wants long-term licences for dam removal covering whole estates.

Back on the Tay north of Dunkeld I meet ghillie Colin McFadyen, who leases angling rights on the river. he report on the Knapdale trial – the basis for Cunningham’s decision – drawn up by SNH suggests a potential impact on salmon and trout fisheries, without much detail. McFadyen’s biggest worry is safety. e shows me the now-familiar trees with well-chomped bases, and a sandy bank that collapsed. The animals arrived four years ago and he’s cleared six beaver-felled trees this year. "The worry is that they work on a tree for weeks and they don’t always drop it, and one of these 30 to 40 ft trees could fall on an angler.”

Angling organisations including the Tay District Salmon Fisheries Board fear the dams could block the way to spawning grounds. Hughie Campbell-Adamson chairs Salmon and Trout Conservation (Scotland), a charity whose members are mainly game anglers and fishery owners, and is a member of the Esk District Salmon Fisheries Board, north of the Tay.

Though there’s no sign yet of a permanent beaver presence in the South Esk catchment – there have been some sightings – Campbell-Adamson says it is only a matter of time until dams there block migrating salmon and sea trout. hat, he believes will put fisheries boards’ legal duties in conflict with the beavers’ status. “We have a statutory obligation to keep rivers clear for migrating fish. If beavers obstruct a river or burn, then we must have the right to remove that obstruction,” he says. “It would be lunacy if the actions of an introduced species were allowed to threaten the already fragile salmon and sea-trout runs."

Environmentalist Susan Davies says Scotland has a duty to reintroduce beavers because they were killed off by human hunting. They also provide a wealth of what she calls “ecosystem services”.

Davies is the director of conservation for the Scottish Wildlife Trust (SWT), which with the Royal Zoological Society of Scotland was the main sponsor of the Knapdale trial.

“SWT wanted to get involved partly because of the moral obligation, when you have had a species lost through human activity, to try to bring that back,” she says. “But it was also because beavers are seen to be good modifiers of the landscape and they can increase biodiversity.”

In the 1990s an SNH proposal to bring back beavers – prompted by an EU directive suggesting former native species should be considered for reintroduction – was turned down. By 2007 the Scottish Government was keener, and the trial was born. The site was chosen because it is surrounded by sea lochs and conifers, which beavers dislike. Their spread could be more easily controlled than in Tayside, where Davies says their presence, and objections by farmers, came close to scuppering the whole idea of reintroduction.

The SNH report on the future of beavers for Cunningham said a quarter of a million acres of woodland could support beaver populations. They could even range to islands such as Skye, Mull and Islay but that report admits their impact is not properly known. They could damage lichens in conservation woodlands, and beavers and deer grazing together could mean loss of tree cover.

Beavers will add to the issues SNH, on a tightening budget, has to deal with. Deer overgrazing is already a problem, as is invasive rhododendron; grey squirrels threaten native reds, American mink are still on the loose ... so should we introduce another set of management problems?

“In the overall conservation approach you have to balance protecting, restoring and enhancing, and there’s a sense it should not just be an either-or scenario,” Davies argues, admitting part of the rationale is inspiring the public by bringing back a charismatic species.

“Sometimes you need these large projects ... to raise awareness about the general decline we have had of our landscapes and habitats; and from a conservation perspective it would be a very sad thing if we held up our hands and said we’ve lost it and we’re not going to do anything.”

Pressure to spread beavers is already growing, with conservation charity Trees for Life raising cash so it can introduce them under licence to the north-west Highlands.

But the Tayside animals, and the handful left at Knapdale, are likely to be enough for SNH to cope with for now.

“The Tayside situation has left us with a problem, and we have to live with that,” says the aptly named John Burrow, one of the agency’s beaver experts. “Low-lying, high-productivity farmlands are probably the last place to have beavers, but that’s what we’ve got.

“What the Cabinet Secretary said in November amounts to her being content with them being where they are at the moment, and expansion through natural means, but she wants to let us get to grips with the management procedure and see how we cope.”

SNH wants to apply the “lightest touch” in terms of regulation, using a tiered approach with farms getting a standard licence to deal with beaver problems short of removing or killing them. Grants to pay for beaver management seem unlikely, and the aim will be for landowners to co-exist with them. Burrow says: “A pair or family group are highly territorial and will occupy a location and exclude other animals. If you remove a dam the response you trigger is they will try to rebuild it; they don’t use the old material, they fell new wood, and you create a whole new cycle. We say if you can manage their impact on this site without removing, and learn to live with them, you won’t get new ones coming in.”

Co-existence could drive other benefits. Rivers and burns in Scotland’s fertile farmlands get clogged with soil washed off ploughed fields. If farmers left a 20m margin uncultivated alongside watercourses it would suit beavers, and act as a barrier to soil run-off.

The SNH officer says people have fallen into beaver burrows, but, he adds: “It’s a public education issue. The landscape is going to change and how we approach risk in the landscape is going to change, and that will include the risk of collapsed burrow.”

Though he accepts the impact on fish is unclear, he says it’s worth remembering that beavers, salmon and trout co-existed for thousands of years.

Back at Bamff in the early afternoon no beavers can be seen. They are active between 7pm and 7am, so later Smith and I head to the River Ericht south of Blairgowrie.

We watch a beaver lodge for an hour in the fading light but it is a little too early in the year for the animals to be out in daylight. All we see is the bow-wave of a beaver a hundred yards upstream, a glittering line in the low light that raises our hopes then disappears.

The beavers divide opinion but there’s common ground. Ivory tells me: “There’s always going to be two sides, those who want them exterminated and those who want them completely protected. We need to meet in the middle.”

Smith, Gudan, Davies and other enthusiasts all agree culling will be needed sooner or later as their number expand. But will people accept hacked trees, flooded burns and the strange, soggy meadows?

“It’s just nature going back to how it was five hundred years ago,” says Smith. “It’s alien to us, but we’ll get used to it.”

HOW DID THE BEAVERS GET THERE?

How wild beavers first got into the Tay catchment is not clear. While SNH can tell whether a population originated in, say, Norway or Bavaria, once beavers are in the wild working out where they were released or escaped from is impossible.

Paul and Louise Ramsay, who own Bamff Estate near Alyth, set up the Scottish Beaver Network. The animals lived on the estate from 2002, in enclosures.

The Ramsays say the enclosure was breached during flooding and beavers could have got out, but not before there was already a population of wild beavers in the area.

Louise Ramsay said: “Movement could have been either in or out. The fence became a bit of an absurdity once we were surrounded by the wild beavers. So it’s impossible to know whether some of ours might have got out ... it’s possible they did but it was well after the establishment of the population from other sources.”

It has also been suggested the animals escaped from Auchengarrich Wildlife Park near Comrie in the early 2000s. Andrew Scott, who has owned it for nine years, says there were two beavers there before he arrived. One died and the other was rehomed to England: there were no escapes.

“There is a myth that they came from here but it’s not true,” he says emphatically.