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Why heavy rain changes everything about your brook

After a storm, a brook can look and smell like a different watercourse entirely. Within an hour of intense rainfall, a clear, gravel-bottomed stream can turn the colour of milky tea, carry a head of foam along its length and give off an odour that lingers in the back of your throat. Some of this is entirely natural. Some of it is a warning sign that something is entering the water that should not be there.

Knowing the difference is one of the most useful skills a river watcher, angler, dog walker or parish councillor can develop. The signs below will help you read what a brook is telling you after rain, and decide whether it is a case of watching and waiting or picking up the phone.

Discolouration: what the colour is really telling you

Rain falling on dry, baked ground picks up fine silt and clay and flushes it into the channel. This is why so many brooks run brown or ochre during the first hour of a downpour. Natural silt settles out quickly once the flow drops, usually within a day or so, and the water clears from the edges inwards as the current slows.

Colours that should raise your eyebrows include:

• Grey or milky white — often a sign of fine sediment from a construction site, a badly managed track or a collapsed bank, which can smother gravels where fish spawn.
• Blue-green or opaque green — may indicate agricultural runoff carrying nutrients, or in some cases dye from a misconnected drain.
• Yellow-brown or rust-orange staining — sometimes natural iron flushes from boggy ground, but also a possible sign of mine water or disturbed soil entering the system.
• Black or dark grey slicks that break into plates — a classic look for sewage or septic discharge rather than soil.

Note the colour, take a photograph if you can, and record the time. Discolouration that appears within minutes of rain and clears within hours is usually runoff. Discolouration that appears hours later, or persists long after the rain has stopped, deserves closer attention.

Foam, sheen and the difference between them

Foam on a brook after heavy rain is normal. Organic matter — leaf litter, algae, dead plant material — releases natural surfactants as it is churned up, and these create pale, soapy-looking patches that gather behind rocks and at the bottom of weirs. Natural foam is usually white to pale brown, collapses fairly quickly when you stir it, and smells earthy or faintly vegetal.

Suspicious foam tends to be bright white and stiff, persists for a long time, and can be blown off the surface in chunks. If it smells of detergent, chemicals or something sweet, treat it as a potential pollution incident. The same applies to sheens: a thin, rainbow-coloured film that breaks into fragments when you disturb it with a stick is often a fuel or oil derivative. Natural iron films tend to be duller and break into a fine powder rather than holding together as a film.

Smells: trust your nose, but interpret it carefully

Smell is the fastest diagnostic tool you have. A healthy brook after rain smells of wet earth, moss and cold stone. Anything else is worth noting.

• Sewage or rotten eggs — hydrogen sulphide and sewage odours point to foul water entering the channel, often through a misconnected pipe or a failing septic system.
• Chemical or solvent smells — petrol, paint thinner, or a sharp, clinical scent can indicate a spill from a yard, workshop or road drain.
• Ammonia — a piercing, cleaning-product smell in a rural brook is a strong signal of slurry or farm waste.
• Nothing at all — remember that some pollutants are odourless, so absence of smell is not proof of clean water.

Take note of where the smell is strongest. Odour is usually most intense close to the source and fades downstream as the water dilutes and aerates.

Flow, debris and the behaviour of the water itself

Heavy rain normally brings a rapid rise in level, a faster current and a brown tinge that clears from the top down. What you do not expect is a sudden surge with no rain locally, which can mean an upstream discharge or a burst pipe. Equally, a brook that stays abnormally low during a wet spell may be losing water to an abstraction or a blocked channel.

Watch the debris line. Sticks, leaves and grass are normal. Sanitary products, cotton buds, wet wipes and fat deposits are not, and they are often the most visible evidence of a sewage overflow. Also watch for dead or distressed wildlife — fish gasping at the surface, invertebrates drifting in unusual numbers, or a sudden absence of the usual minnows and sticklebacks under stones.

What to do with what you have seen

If you suspect pollution, act quickly and gather the details that matter. Record the date, time, location, and what you saw, smelled and heard. Photographs and short video are extremely useful, particularly of foam, sheen and discolouration at a fixed point. Note the weather over the previous 24 hours, as this helps distinguish runoff from a discharge.

Report suspected pollution to the environmental regulator for your part of the UK, and to the local water company if sewage is involved. Do not enter the water or let dogs swim if you suspect a spill. Keep an eye on the brook over the following days and note how long the signs persist — this recovery pattern is valuable evidence and helps confirm whether the cause was a one-off flush or an ongoing problem.

Most of all, keep watching. A brook that you know intimately in dry weather becomes a far better teacher after rain, and the person who notices a change early is often the one who stops a small problem becoming a serious one.