Across the Northern hemisphere our pagan ancestors understood that the humble beaver is the key to the healthy functioning of land and water. The modern term “keystone species” may as well have been invented to describe the pivotal ecological role played by beavers. Native Americans revered the North American beaver (castor canadensis), referring to them as “little people” and holding in the highest esteem the great parallel beaver society that lived and worked alongside and in symbiosis with people.

For humans aside, no animal so earnestly and capably engineers its environment to suit its own ends, such that we can scarcely conceive of the huge influence beavers once had on our landscapes. It is reckoned that a quarter of a billion beaver dams across North America once held back water sufficient to submerge fully the state of California three times over.

Then the Europeans came, and trappers fanned out across that great continent, nearly always the first of the colonisers to arrive in each place. They worked their way along rivers, streams, across wetlands, swamps and estuaries, searching for every last beaver, such was the value of their fur. And so by the time the documenters, cartographers, photographers and hordes of settlers arrived in their footsteps the beavers were long gone, wiped out across virtually the whole continent, and with them their dwellings and dams and canals, and all trace of that parallel society. With the beavers went the water, life drained out of the land, and the Western states, worst affected by the loss, became arid as we know them today.

Europe’s own indigenous beavers (castor fiber) had suffered a similar fate, albeit centuries earlier. Trappers sought not only their fur, but also the yellowish oil, castoreum, that beavers exude from sacs beneath their tail. This oil was in such demand for use in early cosmetics that the value of a single beaver in Dark Ages Britain was equal to an entire year’s earnings for the average peasant. By the time of the First World War only tiny remnant populations of European beavers remained in the remotest corners of Eastern Europe and Russia.

Beavers are highly territorial. Each territory is occupied by a single pair and their young (known as kits, born in late spring in litters of two or three), along with an assortment of adolescent offspring who tend to hang around until their second or third year. Beavers use water as a means of escape from predators, and therefore never stray far from it whilst looking for food – the soft inner bark of deciduous trees that they fell with ease (aspen, birch and willow are particular favourites), as well as shrubs, bracken, nettles, and all manner of aquatic plants. On the best territories along broad stretches of river, or on the edge of lakes or estuaries, no damming of water is needed, and beavers have an easy life. Here they busy themselves simply with tree-gardening or “coppicing”, opening up the water’s edge to precious sunlight and allowing wildflowers and tasty riparian plants to bloom.

When these high-quality areas are fully occupied, however, young beavers looking to establish a territory of their own are forced to make their way upstream, into the tributaries, streams and creeks that flow down into the larger rivers. It is here, further up the catchment, where beavers really make an impact. Without adequate deep water, beavers set about creating it, using rocks, branches, sticks and mud with almost unimaginable skill to construct first one and then a series of small dams along their territory. Behind each of these they dig out a large pool, complete with channels that reach out into the surrounding woodland.

Soon after the arrival of a pair of beavers, a small seasonal stream begins to resemble the immaculately flooded steps of a terraced rice paddy. These new permanent beaver wetlands quickly develop complex mosaics of wildflowers, grasses and scrub, and become havens for wildlife of all kinds. Fish, dragonflies and all manner of insects, amphibians, kingfishers, ducks, storks and myriad birds soon teem in numbers that are just unrecognisable to anyone unaccustomed to a landscape fashioned by beavers.

Streams braided in this way from top to bottom by successions of beaver pools are not only of huge benefit to wildlife, but they also protect us from flooding, seasonal drought and even wildfire. In the absence of beavers, winter rainfall brings torrents of water that flash down the creeks and streams at once, causing flooding further down the catchment, bursting the banks of our straightened and dredged waterways. Flash flooding in turn gives way to dry, lifeless gullies through the summer once the water has gone. Beaver dams dramatically slow and regulate the flow of water, holding it back in great volume, giving nature time to cleanse it of sediment and impurities such as the harmful nitrates and phosphates used in farming, and releasing it, clean, down the catchment through the year.

The water, passing slowly through the pools created by beavers, makes its way down into the groundwater too, which raises the whole the water table, swelling the aquifers that quench our thirst in times of drought. Recent satellite imagery shows that the steady return of beavers to America’s Western states after an absence of centuries is quite literally greening the desert, as great ribbons of beaver-made fire-break wetlands appear across the landscape.

In each beaver territory can be found a “lodge” in which the family finds refuge during the daytime: a large shield-shaped dome made of sticks, plastered with mud, and comprising a series of warm, dry inner chambers, built such that the entrance can be found on the underside, safely beneath the water. These great lodges provide a home for countless other species. Nesting birds, hibernating reptiles and amphibians, hedgehogs and small rodents all use beaver lodges for cover in this way. The beavers leave a section of the roof free of mud-plaster, for a ventilation shaft, from which on cold days a plume of steam gently rises from the sleeping beavers within – giving the impression of a miniature human home.

Since the beginning of the last century beavers have been granted legal protection in a growing list of places on both sides of the Atlantic, while the value of their fur has diminished. Consequently beavers are staging a remarkable comeback. Carefully planned reintroductions have taken place across Europe, and in North America, and while numbers remain at a tiny fraction of their former level, there now exist perhaps two million beavers in Europe and ten million in North America.

In Britain, where the last beavers were extirpated in the Middle Ages, there are small but viable and growing populations in Scotland and the South West of England. A growing understanding of the importance of beavers to the healthy functioning of the hydrological system, for mitigating flooding, drought and wildfire, and for breathing life back into our broken and depleted ecosystems, has led to calls for their restoration right across their former range.

Of course there are places in which beavers do present a problem, such as fish hatcheries, sewage works or man-made canals. Beavers can raise the water level on highly-engineered, low-lying arable farmland. In those places the activities of beavers need to be managed, as is now routine where beavers have already returned. Beaver dams can be altered to increase flow, and culverts, drains and specimen trees can be easily protected. In some cases it will be necessary to remove beavers altogether.

But for the most part opposition to the return of beavers arises from a lack of understanding. Some fishermen for example, presumably having read the Chronicles of Narnia in which Mr and Mrs Beaver eat all the fish, don’t realise that beavers are entirely herbivorous. Others worry that migratory fish such as salmon and trout will be unable to make it across beaver dams, forgetting that these fish co-evolved over millions of years with beavers. The beaver taught the salmon how to jump, and young fish depend on the cool, stable pools and gravel spawning beds created by beavers.

Indeed, lawsuits in Oregon and California have successfully argued that the destruction of beavers harms protected salmon, and is therefore illegal under the Endangered Species Act. Then there are the tidy-obsessives, the same people who demand that our road verges are regularly strimmed to perfection (at great expense), cleared of wild flowers and wild grasses. These people object to the perceived untidiness created by beavers along the water’s edge, not grasping that nature thrives on untidiness. Considering that the vast majority of our land is cultivated, tidied and managed by us, surely we can allow nature a modicum of free rein along our watercourses which, managed by beavers, present us with a ready-made nature recovery network. Farming right to the edge of the water is pure folly in any case, now prohibited in many countries.

The conservation movement has a tendency to flip that famous Martin Luther King line on its head, to tell us that it has a nightmare, and a lot of us simply can’t take it. So we switch off to the seemingly endless bad news. But here is some really good news: the beaver is back, along with all of its magic, and it is already transforming our landscapes. That is absolutely something to celebrate. So be happy, and if you don’t yet have beavers in your area, ask why not.