No matter where you go in Hampshire, you're never very far from water. Our chalk streams, in particular, are so entwined with the landscape that even miles from their banks you can still be close to them. But to understand how, we need to begin miles underground and millions of years ago.

Long before people roamed the earth, the space that is now Hampshire lay beneath a shallow sea. The warm water teemed with coccolithophores - tiny algae which thrived in the plentiful sunlight. Though each one was microscopically small, together they would build something incredible.

As eons passed, the exoskeletons of these algae settled in their trillions on the sea floor. The weight of millennia pressed them into a layer of chalk that in some places towered to hundreds of metres. Rich in calcium carbonate, this rock's bright white colour echoed its bony ancestry.

In time the sea receded, exposing the chalk to ice, wind, and rain. The elements wore away its soft surface, shaping it into rolling downs and winding valleys. In some places the rock was blanketed with material from retreating glaciers, while elsewhere it lay just below the soil. Through a happy collision of natural forces, wildlife had gained a mighty ally.

Buried treasure

True to its aquatic origins, chalk has an almost magical relationship with water. In our county, so much rain disappears into the ground that one might picture a vast lake deep below. The truth is rather more bizarre: the chalk is riddled with tiny holes, enabling it to soak up water like a giant sponge. This rock reservoir, known as an aquifer, has a truly staggering liquid capacity.

When the rock is fully saturated, its cargo makes a bid for freedom. It bursts onto the surface as springs - the sources of our chalk streams. In higher parts of the landscape, where the aquifer rarely fills completely, intermittent streams called 'bournes' can dry out into grassy hollows for months or even years at a time. Those that appear only in the very wettest seasons earn the title 'winterbournes'.

While this process is straightforward, it is anything but speedy.  A stream's resurgence can occur months after the wet weather that prompted it, and water can take decades to fully traverse the aquifer. The timescale is predictably unpredictable: the only certainty is that what goes in will, eventually, come out again.

Wonder water

When water does re-emerge from the aquifer, it has been fundamentally changed by its subterranean journey. It leaves the rock filtered of impurities and slightly alkaline; since rain is naturally acidic, it dissolves a little of the chalk during its passage. This suits species like the white-clawed crayfish, which uses calcium to harden its shell, and water crowfoot, which is hungry for bicarbonate ions.

Passing through miles of rock lowers the water’s temperature, causing it to arrive at around 11 °C regardless of the season. With this coldness comes abundant oxygen - a valuable quality for fish like brown trout and freshwater eels, which can suffocate in overly warm water. Aquatic plants are just as appreciative, and their increased growth lays the foundation for a vibrant ecosystem.

Sediment, which in excess can menace a whole host of species, is also found in lower-than-average quantities. In other landscapes much of the sediment in streams is washed in by heavy rain, but chalk is so absorbent that relatively little rain stays on the surface. This helps to keep the gravelly stream beds clean and the water itself famously 'gin-clear'.

While these qualities are all unusual, the consistency of the water's presence is also significant. The aquifer can maintain a flow for months after being topped up by rain, creating a fairly stable aquatic environment. Species in the intermittent streams, meanwhile, live in sync with their transient nature; many insects have drought-resistant eggs, which lie dormant until the water's return.

Time will tell

Just like the rock which held it, the water in our chalk streams is a window through time. When it returns to the surface, it carries not only the remains of ancient algae but also mankind's destructive legacy. Pollutants that entered the aquifer decades ago are still emerging today, and our own choices - whether good or bad - will be the inheritance of future generations.

It takes distressingly little human activity to undo the gifts the aquifer gives us. Fertilisers from agriculture can acidify the water, while eroding banks can introduce sediment. Poor management can leave streams shallow and sluggish, drastically raising the water temperature. Tapping the groundwater to supply homes and businesses can cause whole stretches to artificially run dry.

While the prevalence of chalk streams in Hampshire creates the impression of abundance, in reality they are vanishingly rare; the circumstances which formed them are so specific that only 210 exist worldwide. This scarcity makes them vulnerable, and their wildlife - so perfectly adapted - has nowhere else to go. If we're not careful, these waters born from prehistoric rock could soon be history.

So what can we do to protect these precious habitats? There are many ways in which our lives affect our chalk streams, giving us the chance to significantly brighten their future. One way to get involved is through Watercress and Winterbournes, a partnership scheme which is caring for seven such streams - explore our work to find out more.