Understanding Brook and Stream Wildlife

A Small Water Full of Life

A brook barely wider than a stride can hold more life, metre for metre, than any lake. Shallow, cool and constantly moving, it stays rich in oxygen — and that oxygen supports a cast of creatures that larger, slower waters cannot. Brooks are the small, steep, often spring-fed threads that tumble through fields and woods; streams are their slightly broader, gentler cousins. Both are headwaters, and headwaters are where the health of a whole river system begins.

They are also the easiest wildlife in Britain to watch. The water is shallow enough to see into, the banks low enough to see over, and most of the animals live within a few paces of where you stand. You need no permit, no hide and no equipment beyond patience and a quiet pair of boots.

Reading the Water: Riffles, Runs and Pools

A stream is not one habitat but several, arranged one after another along the same channel. Learn to recognise them and you will know where to look.

• Riffle. Shallow, broken, chattering water over gravel. Highest in oxygen, and the engine room of the stream: mayfly, stonefly and caddis larvae crowd the undersides of the stones here.
• Run. Smoother, gliding water, often with a glassy look. Small fish hold in these currents, facing upstream, waiting for food to arrive.
• Pool. Deeper and slower, usually below a riffle or on a bend. A refuge during summer heat and winter spates, and where fish gather when the water is low.
• Margins. Undercut banks, trailing roots, submerged branches and clumps of water crowfoot. This is the busiest real estate in the stream, and the easiest to overlook.

On the bank itself, look for tussocky grass, brooklime, water mint, and alder or willow overhead. Shade keeps the water cool, and fallen leaves feed the invertebrates that feed everything else.

The Cast of Characters

Mayflies are the classic indicator of clean, well-oxygenated water. Their nymphs live among gravel and stones for a year or more before the adults emerge on warm evenings, sometimes in clouds, sometimes a few at a time. Stoneflies prefer the coolest, fastest stretches; caddisflies build cases of grit and plant fragments, and turn up almost anywhere with reasonable water quality.

Among the small fish, look for brown trout in the runs, bullhead — the broad-headed "miller's thumb" — hiding under stones, and shoals of minnows flickering in the shallows. Stone loach and three-spined sticklebacks prefer quieter, weedier corners.

Water voles leave burrows in the bank, neat piles of nibbled stems, and a distinctive plop as they enter the water. A dipper bobbing on a midstream stone, a grey wagtail on a wet ledge or a kingfisher streaking low over the surface all tell the same story: this is a stream with fish in it. At dusk, watch for Daubenton's bats hunting just above the surface, and in the quiet hours an otter may pass through without you ever knowing.

Watching Well From the Bank

Most stream wildlife is shy, but it is not difficult to watch if you give it a moment to forget you.

• Stand still for ten minutes before moving. Ripples settle, fish come back out, and the whole stream resumes.
• Keep low and wear dull colours. Work upstream where you can, using bankside cover, and keep your shadow off the water.
• Polarising sunglasses cut the glare and let you see through the surface. Close-focusing binoculars help with bank-side voles and birds.
• Go early or late. Dawn and the last hour of light are the busiest times, especially in summer.
• Keep dogs out of the water, particularly over gravel. One enthusiastic swim can clear a stretch for an hour.
• Never handle fish or amphibians. Wet hands strip the protective mucous layer from a fish's skin, and a quick look can be fatal.
• If you lift a stone, put it back exactly as you found it, same way up, in the same spot.
• Note down what you see, with date and place. A few seasons of records become genuinely useful.

Gravel Beds Are Nurseries, Not Footpaths

Clean gravel is the most valuable and most fragile part of a brook. Brown trout and salmon cut hollows — redds — in the gravel between roughly October and January, then bury their eggs in them. Those eggs stay in the gravel all winter, and the young fish linger there for weeks after hatching before venturing out.

A boot print is not trivial. Walking over a redd compacts the gravel and releases silt, which smothers eggs and blocks the flow of oxygenated water through the spaces between stones. The damage is invisible and, for that year's brood, often permanent.

So cross at bridges, fords or established paths wherever possible. If you really must cross, choose a spot where the bed is bedrock, boulders or clean sand, and step carefully. Keep out of the channel altogether when flows are low and gravel is exposed — that is exactly when a stray foot does the most harm. And leave fallen branches and woody debris where they lie; they are cover, food and future gravel traps, not litter.

Small Habits That Help Your Local Brook

You do not need to be a scientist to make a difference. Recording what you see — the first mayfly of the year, a water vole, a stretch that has silted up — builds a picture others can act on. If something looks wrong, such as a sudden grey plume, a smell, an oily sheen or dead fish, report it promptly to the relevant authority and take a photograph with the location noted.

Clean your boots and kit before visiting another water, so you are not carrying silt, weed fragments or disease between catchments. Leave bankside vegetation tall where you can, resist tidying wood out of the channel, and let the alders and willows keep their shade. Above all, keep visiting. A brook that somebody walks past every week is a brook that gets noticed, and noticed water is protected water.