Yellow alder, russet beech and sessile oak; broadleaf trees stand guard over the stream in this steep-sided Welsh valley. Sunbeams pierce a rising mist, hinting at the winter chills to come. A high amber ridge-line is just visible through the canopy, raised against a gun-metal sky. The wet smell of scarred earth is all-pervasive.

As I stand here in the fold of a sliver of ancient temperate rain-forest, a tan sessile oak leaf falls gently to the river bank. I reach out to grasp it, but a squally wind whips it away and hurls it into the narrow headwaters we call Sgithwen.

A leaf is not a leaf for ever. It is in a state of flux between leaf and particulates, living and dead. A leaf that falls in the stream is the beginning of a chain. Spin, yaw, pitch and submerge, it’s hurried away downstream in a thrum of fluvial energy and I lose sight of it. The leaf has abruptly entered an ancient dynamic system of motion and unrest. Yesterday, Sgithwen was in full red-clay spate after downpours. Today, the water is clear and forceful. Tomorrow, who knows. The stream is different and yet the same. The water is also a blend of Sgithwen ‘tea’ percolated from surrounding organic and nonorganic matter, a mix of nutrients so unique that salmonids remember the smell, homing from the ocean to breed. The stream is different yet the same.

Heraclitus of Ephesus lived by the River Kayster in ancient Ionia. As I step into this stream, I hear his philosophy whispering down through two and half thousand years of algaeslippy boulders, ‘Upon those who step into the same rivers different and different waters flow.’ Heraclitus perceived, amongst nature’s constant flux and opposites, an entity or one-ness. This something, the source of everything, he called ‘Logos’. Life is an ever-changing continuum. Nothing remains the same, not least this stream in which I stand.

The river is a universal feature. Sgithwen stream falls to the River Wye, flowing to the River Severn and is channeled out to the ocean. But rivers are never exactly the same from one moment to another because their systems are constantly fluctuating. Evolving habitats downward along the gradient are described in science as a ‘stream continuum’.

The fallen leaf up here in the hills is now food energy to be rationed downstream. As consumers in the food chain, freshwater insects called macro-invertebrates are great indicators of a healthy river ecosystem. They tend to follow this continuum in particular ways. Some, like the white-clawed crayfish, are keystone species and, being polytrophic, fill a multiplicity of roles in the river. Most are grouped to reflect their labours.

Shredders, such as freshwater shrimp and the smaller stonefly larvae, begin by tearing away at coarse organic material like our sessile oak leaf. Not all the leaf is consumed and much floats away downstream together with faeces, later to be consumed by collectors such as midge larvae, nematodes and worms. More particles are scavenged in and around the sediments. Further downstream, the widening reaches open up to sky and sun, encouraging the growth of periphyton, a mix of algae, bacteria and other microbes. Grazers such as mayfly larvae, cased caddis-fly larvae and river snails scrape it from submerged rocks and roots for sustenance.

Filterers like pearl mussels sift out still finer grains left in suspense, along with floating algae (including single-celled alga called diatoms), and nutrients washed from the flood plains. Invertebrate predators, such as dragonfly and large stonefly larvae, ambush other insects that have consumed our leaf. And in lower reaches and depths, the flow is laden with yet more life, in microscopic phytoplankton and zooplankton composed of dissolved carbon, the carbon that once embodied a veteran sessile oak. And so the unseen continue to be nurtured.

Autumn gives rise to this annual pulse of life in the river; in time, leaves help to form a bounty of other living beings, way beyond the water’s edge. Predators move more easily up and down the continuum, feeding on in-stream and riparian life. There are pike, otters, the grey heron and, of course, humans, among others. Sand martins and Daubenton’s bats hunt adult aquatic insects just as they emerge from the water.

One person’s lifespan is a blink of an eye to the river, a leaf’s existence even less so. My own perceptions of time may be faster than the sessile oak or the 100-year-old pearl mussel, slower than the caddisfly or fleet sand martin. Each species experiences the river uniquely. Ebb or flow, whirlpool or riffle, all senses engaged, memories in the making. The river is different yet the same.

Long into the future, perhaps the winds may turn, continents will divide again, and the river’s entire length may disappear, devoured by geological action and climates. There may be traces of its ancient forms and life in the rock, and these may, in turn, erode to dust and silt the rivers a billion years from now. And so the story continues, beyond a simple lotic flow of water, of the autumnal leaf and the philosopher’s river; the beginning of a chain, the nurturing of the unseen and a myriad of woven fabrics of life, of place and of time.

Ginny Battson, 2016