The English chalk downland gives rise to 246 pellucid chalk streams and dozens of small, nameless rills and becks, comprising the vast majority of this river type to be found anywhere in the world.

Although chalk exists in other parts of the world, nowhere else is there such a mass of it – the remains of an entire sea-floor – exposed at the surface of the earth as rolling chalk hills, washed over by a temperate, maritime climate.

When rain falls on chalk hills it soaks down into the body of the rock and there undergoes a kind of alchemy, emerging again from springs as cool, alkaline, mineral-rich water, equable in flow: the perfect properties to create a richly diverse eco-system.

Botanically chalk streams are the most biodiverse of all English rivers. For invertebrates, fish, birds and mammals, they over a vast range of habitat niches. Upper ephemeral reaches, known as winterbournes, are global hotspots for a unique range of specialist plants and invertebrates.

But chalk streams are under immense pressure: they flow through one of the busiest, most urbanised, industrialised and farmed parts of our crowded islands. All these streams are impacted in one way or another by the activities of people.

We depend on them for public water supply, and have leant heavily on the resources of the underground body of water that feeds these streams. And yet every litre of water we take out of the aquifers – and we take billions and billions of litres to irrigate our crops, or run our taps – is water lost to the natural environment. Lost, that is, until we put it back – only by the time we return water it is no longer in the state in which we found it. It has passed through our sewage systems becoming rich in nutrients and other pollutants. We may treat it, we may even treat it to a very high standard in some places, but in many others we do not. Routinely, we put back into these wonderful ecosystems water which makes them eutrophic, so that oxygen is sucked away from the life which depends on it.

Even the water which we do not take out, which actually makes it to the underground aquifer or the stream, is unnaturally changed. Our heavily farmed landscape exerts a huge pressure on water quality, either because rain runs over bare, ploughed land and along roads, accumulating toxins along the way and rushes, unfiltered, into the river, or because it seeps down into the ground carrying with it the chemical fertilisers which have been applied to the land. There is now so much nitrogen in our chalk aquifers that we do not know how long it would take – even if we stopped applying nitrogen as fertiliser – for the aquifers to become clean again.

Finally, we have changed the rivers themselves, modifying them heavily over the centuries, because they are such gentle, malleable rivers.

We have used them for milling, for transport, to drive multiple agricultural and industrial revolutions. More recently, in the post-war decades, we made one of the most drastic and permanent changes of all: we dredged them. We took out the gravel river-bed – on which almost all chalk stream life ultimately depends – and dumped it on the banks, all in a misguided attempt to drain the landscape. Not quite understanding river morphology at the time, we mostly created a management nightmare, because the streams now fill with sediment – the only material at their gentle-natured disposal – to fill the void.

So, we have a job ahead of us if we are to leave our wonderful chalk streams in a better state than we found them.