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A Whole World in a Handful of Gravel

Kneel at a riffle on any decent British stream, scoop up a handful of gravel from the shallow, fast-running water, and you are holding one of the richest habitats in the country. Hidden among those stones are the larvae and nymphs of insects that will one day take to the air, plus crustaceans, worms, flatworms and snails that live their whole lives underwater. For the curious naturalist, learning to recognise the most common of them is one of the most rewarding skills you can pick up. It costs almost nothing, it works on almost any stream, and it turns a walk along a brook into a proper investigation.

You do not need expensive kit or a permit-free stretch of some famous chalk stream. A small net, a white tray and half an hour is enough — though you should always have the landowner's permission and never sample where it is too deep or too fast to stand safely.

Kick-Sampling: The Gentle Art

The standard technique is called kick-sampling, and the name is slightly misleading — it is closer to a firm shuffle than a football kick. Here is the method that works.

• Use a net with 1 mm mesh and a reasonably stiff frame. Coarser mesh loses the small creatures that are often the most informative.
• Stand facing upstream in a riffle — the broken, bubbly, shallow water running over gravel or small stones. This is where oxygen and life are richest.
• Hold the net vertically with the mouth facing upstream, frame touching the riverbed, and the handle held downstream so the current presses the net open.
• Disturb the bed just in front of the net with your heel and hands, rubbing and lifting stones for roughly thirty seconds to a minute. The current carries everything dislodged straight into the net.
• Lift the net carefully and rinse the contents into a white tray with a centimetre or two of water. Let it settle for a minute, then keep still and simply watch.

Movement is your first identification tool. Sit quiet and the animals start crawling, swimming and shuffling, and their manner of getting about sorts them into groups quickly.

Mayfly Nymphs: Three Tails and Fast Legs

Mayfly nymphs are among the most familiar finds. Count the tails at the rear: most have three long, fine tails, and along the sides of the abdomen you will see a row of plate-like or leaf-like gills that flicker gently. Once you have seen that combination, you will not confuse it with much else.

Within the group, shape tells you about lifestyle. The flat, broad-headed nymphs that cling tightly to stones in fast water have flattened bodies and legs splayed wide to grip. The slimmer, rounder-bodied ones swim with a distinctive wriggling, side-to-side dart — these are the strong swimmers of quieter glides. Both are excellent food for trout, and both are sensitive to pollution, so a tray full of them is a good sign.

Caddis Larvae: Cases, Caddies and Net-Spinners

Caddis larvae look like small caterpillars with hard heads and three pairs of proper legs behind them. The giveaway is the case: many species build a portable tube from sand grains, tiny pebbles or fragments of vegetation glued together with silk. Watch one drag its case across the tray and you will see the head and legs poking out of the front like a tenant checking the weather.

• Stone-cased larvae tend to live in faster, cleaner water and build sturdier homes.
• Vegetation-cased larvae are common in slower reaches and among weed beds.
• Caseless larvae — the net-spinners — live in silken nets stretched across stones in flowing water, and their larvae are smooth, greenish or yellowish, without a portable home.

Do not prize a larva out of its case. Let it walk, and you will learn more from the way it carries itself.

Freshwater Shrimp and Other Familiar Faces

The freshwater shrimp Gammarus pulex is a favourite. Look for a curved, comma-shaped body flattened from side to side, and a creature that hops, scuttles and swims on its side. Abundant shrimp usually mean reasonably clean, well-oxygenated water, though they are not as demanding as mayflies.

Compare them with the water hog-louse, which is flattened top to bottom and scurries about more slowly, like a miniature woodlouse. Hog-lice tolerate lower oxygen and poorer conditions, so the ratio between the two says a lot about a site.

Other regulars to look for include:

• Stonefly nymphs — two tails, flattened body, strong legs, clinging to stones in the fastest, cleanest water.
• Blackfly larvae — small, anchored to rocks in flow, with a pair of fan-like feeding brushes.
• Bloodworms — bright red midge larvae that thrash and thrice in the tray, tolerant of silty, low-oxygen spots.
• Flatworms — soft, gliding, with two eye-spots, harmless and fascinating.

Reading the River from Your Tray

A sample is not just a list of species; it is a snapshot of river health. Plenty of mayfly nymphs, caddis larvae and stonefly nymphs, with shrimp to spare, suggests clean, cool, well-oxygenated water. If your tray is dominated by hog-lice, bloodworms and snails, with few sensitive insects, the water is probably warmer, siltier or carrying organic pollution.

Sample different habitats too — riffle, run, pool, weed bed, leaf pack — because each holds a different community, and richness lies in the variety. Spring and early summer give you the widest range of sizes and species.

When you are finished, return everything gently to the water, well away from where you took it, and rinse your net between sites to avoid moving organisms around. Take a few notes, sketch what puzzles you, and check it at home. The next time you pass a stream, you will see it entirely differently.