{"id":32565,"date":"2026-04-21T15:00:27","date_gmt":"2026-04-21T14:00:27","guid":{"rendered":"https:\/\/research.reading.ac.uk\/research-blog\/?p=32565"},"modified":"2026-04-21T15:23:38","modified_gmt":"2026-04-21T14:23:38","slug":"five-warning-signs-that-rivers-are-polluted-even-when-they-look-clean-2","status":"publish","type":"post","link":"https:\/\/research.reading.ac.uk\/research-blog\/2026\/04\/21\/five-warning-signs-that-rivers-are-polluted-even-when-they-look-clean-2\/","title":{"rendered":"Five warning signs that rivers are polluted \u2013 even when they look clean"},"content":{"rendered":"

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After months of relentlessly miserable weather for most of the UK, spring brings renewed enthusiasm for spending time outdoors hiking, wild swimming, paddling or on walks.<\/p>\n

Millions of people<\/a> visit lakes and riversides every year. Yet with constant, and sadly necessary, reminders about sewage and water pollution<\/a>, it\u2019s not surprising that people are increasingly worried about whether the water they see is safe.<\/p>\n

Cocktails of contaminants created by sewage systems<\/a>, agricultural pollutants<\/a> and urban runoff<\/a> are currently at the forefront of public, scientific and regulatory<\/a> focus.<\/p>\n

Not one English river was free from chemical contamination, and only 14% were classified as having \u201cgood ecological status\u201d at the last assessment<\/a>.<\/p>\n

In 2024 alone, raw and partially treated sewage was discharged in to watercourses for more than 3.6 million hours<\/a>. With around 15,000 sites<\/a> regularly discharging effluent, in addition to ongoing inputs from agriculture, transport and other industries<\/a>, the 2025 results due to be published this year<\/a> are not expected to show significant improvement.<\/p>\n

Given this, many people who spend time<\/a> around rivers want to know how to identify pollution.<\/p>\n

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It\u2019s worth remembering that pollution isn\u2019t always visible.<\/p>\n

River contaminants take many forms<\/a>. Some, like oil sheens, excrement, sanitary products and fly-tipping are visible and often odorous. Others such as nutrients<\/a> including phosphates and nitrates, heavy metals<\/a>, microplastics<\/a>, and chemicals like ammonia and Pfas<\/a> \u201cforever chemicals\u201d are invisible to the naked eye.<\/p>\n

Pollutants may enter rivers from points such as discharge pipes, but also from farmland or roads, making them difficult to trace. Assuming you\u2019re not armed with specialist pollution testing kit<\/a>, identifying contamination often relies on observing indirect signs. Here are five indicators to look out for along with the conditions in which they are most likely to appear:<\/p>\n

1. Sewage fungus<\/strong><\/p>\n

Look out for a pungent, gelatinous, fuzzy carpet along the river bed, often found downstream of sewage outfall<\/a> pipes.<\/p>\n

Although not technically a fungus<\/a>, these slimy, brownish-grey growths of bacteria and microorganisms thrive in nutrient-rich waters. Sewage fungus reduces oxygen levels in water, suffocating and physically smothering aquatic life.<\/p>\n

2. Algal blooms<\/strong><\/p>\n

Look out for thick green mats or scum on the surface of the water, often looking like pea soup or spilled paint in green, blue-green, or brownish-black colours. Odours may be musty, earthy or grassy.<\/p>\n

Algae is a natural part of aquatic ecosystems<\/a>. Warm and dry weather combined with overloads of nitrogen and phosphorus nutrients, can trigger excessive algal growth. Algal blooms block light and use up available oxygen during their decomposition in a process known as eutrophication<\/a>. The effects of algal blooms, especially blue-green algae<\/a> can be catastrophic to aquatic life, and toxic to humans and pets.<\/p>\n

3. Murky water<\/strong><\/p>\n

Look out for colours<\/a> that are different to what\u2019s expected, and cannot be easily explained by what you know about the local landscape.<\/p>\n

Sewage tends to present as grey, cloudy or milky<\/a>, sometimes accompanied by bubbles, foam and a foul odour. Green or blue-green is indicative of algal blooms. Unusual colours like orange<\/a>, red, yellow, or black may suggest industrial leaching<\/a> or chemical dumping<\/a>. Soil erosion can cause large quantities of suspended solids<\/a> giving rivers a murky brown appearance that reduces light penetration and can suffocate organisms.<\/p>\n

However, not all colours are a cause for concern. Heavy rain will naturally suspend particles and transport sediments giving rivers a temporary murky appearance. Local geology and soils may result in tea-coloured<\/a> water caused by high iron content, or oily sheens<\/a> caused by aerobic iron-fixing bacteria in boggy areas. You may even spot harmless dyes<\/a> used by water companies and contractors to trace drainage leaks and misconnections.<\/p>\n

4. White foam<\/strong><\/p>\n

Look out for bright white or milky-grey foam that doesn\u2019t easily disperse. It is often accompanied by a perfume, soapy or detergent-like smell.<\/p>\n

Man-made foam<\/a> from sewage, detergents, fire-fighting activities, pesticide runoff and industrial processes causes oxygen depletion in water. They can contain noxious forever chemicals<\/a> known to cause serious health conditions<\/a> in humans and wildlife.<\/p>\n

Understanding river foam requires a discerning eye \u2013 not all foam is bad<\/a>. Natural foam, while aesthetically unpleasant, can be formed by decaying leaves and plants. White-to-brown in colour, natural foam smells earthy or slightly fishy. It can collect in large clumps and is common on windy days, following heavy rain and in turbulent and nutrient-rich waters. In the absence of other environmental impacts like dead fish, algal blooms, or obviously stagnant water, natural foam is rarely problematic.<\/p>\n

5. Aquatic life<\/strong><\/p>\n

Look out signs of distress including fish gasping at the surface, dead fish<\/a>, or unusual behaviour from animals that live in and around the river.<\/p>\n

Pollution causes a decrease in dissolved oxygen, which alongside drought and temperature extremes are leading causes of fish deaths and wildlife distress.<\/p>\n

The presence of species<\/a> such as kingfishers, water voles, frogs and riverflies<\/a> indicate a cleaner river environment. A prolonged and noticeable absence of wildlife using the river is a concerning sign.<\/p>\n

Interpreting the signs and taking action<\/h2>\n

Even with these indicators, identifying pollution is not always clear-cut. Many of these signs can overlap or have natural explanations, making it difficult to draw firm conclusions based on appearance alone.<\/p>\n

To build a clearer picture, it\u2019s important to consider additional information. For example:<\/p>\n