I run down the mud and gravel beach and into the water at speed, my knees raised and teeth gritted. When it’s not possible to jump, the only option is to sprint; to get in quick, before the scream of protesting nerve endings can turn the body around. The water is bone-chillingly cold as it reaches my waist and moves over my stomach. I take a deep breath, lowering my torso into the river and then, a spilt second later, floating my legs up in front of me. My breathing is short and fast; snatched shocked gasps, as I kick slowly away from the weir.

I’ve swum before in this pool, at the point at which the Little Ouse sweeps into Knettishall Heath. My kids taking daring jumps off the weir’s foaming step, shouting with joy and cold. Pink piglets to be rubbed back to squealing warmth with rough towels. But today it is just me. Even the dog walkers have gone.  A grey wagtail jinks along the weir, a skittering, feathery drop of lemon. I lie still in the water and watch him; enjoying his shell-shocked bobs and the flicking tail. In Japanese folktales the wagtail is a sensual bird of love, of passion. A bird that was sent to direct husbands and wives in their conjugal duties with its fluttering, dipping thrusts. Some men were even known to carry the delicate bones of this river cupid in boxes as love charms.

The air, sweet with pollen, feels loose. Relaxed. A breeze rumples the surface of the pool, pushing the water up into dark bars that fan out away from the weir and into the reeds. I flip onto my front and head for where the pool empties into the river, keen to explore further. I kick through reeds, enjoying the feeling of the banks closing in. I’m getting into my stride now, warmed by effort and the sun that scalds the surface, turning the soft greens and tannin browns to gilded, burning gold.

At the second weir I tread water, considering my options. It’s tempting to climb over and plop back in the other side but I can’t shake the slight nervousness of jumping into the unknown. I stand, feeling the river’s spongy bed; the silt swirling up into the water like plumes of black ink. I scramble up the bank and race around the other side, my feet slapping on the mud of the path before half stepping, half jumping back into the river. A crack willow has slow dived into the water. But the tree still lives.  Catkins, like insectivorous sundew, or tiny green bottlebrushes, thrust out from the water. The tree’s branches strain the river’s flow, collecting sticks and weeds; gluing them together in a sticky, beer-brown foam. I take a breath and sink below it, kicking out hard to head further downstream. My hands tinted green under the surface and flashing like fish.

Roger Deakin, in his great ode to wild swimming, Waterlog, called the breaststroke, the naturalist’s stroke. It was, he said, a position that provides a frog’s-eye view of the world. Borne by the river it is a new perspective on everything from water vole holes and otter slipways, worn smooth by the passage of furred chests and ruddering tails to the silver glow of skudding fish. But I also know there’s so much more to see. Last week, while Knettishall’s ranger Sam Norris and I looked for the brook lamprey that are thought to have spawned in this restored river, we netted all manner of life: caddis fly larvae in sleeping bags of twig and stone; shrimp the size and colour of fingernails and Pan-headed damselfly nymphs with antlers like thorns. I think about how it won’t be long before the nymphs crawl from the water to shed their skins and transform.

I swim on; eager now to reach my own changing point, the dipping platform where I’ve left my towel.