What makes water brown?

The surface of water reflects the colours of the sky, often making a lake appear blue or grey. To see the actual colour of the water, you can scoop some into a drinking glass.

Rather than ever being just pure water, water from natural sources always contains many other substances, some of which stain the water. Finnish river and lake water is often light brown or sometimes even the colour of tea, mainly because of humus.

Humus comes from the catchment of a lake or river. When rainwater runs along the ground, humus from the soil is dissolved or mixed in it. Humus, which is formed especially in peatlands, consists of partially decomposed organic matter. This is why particularly large quantities of humus are found in lakes and ponds surrounded by peatlands.

The tone of the brown colour may vary according to weather and by season. After rain and when snow is melting, plenty of humus is carried into lakes and rivers, darkening the water. Drainage and other soil excavation in the catchment may increase the humus load of a river or lake.

Iron from the soil layers and bedrock in the catchment may also accentuate the brown tones in water. Iron may be carried into a lake both with surface runoff and groundwater. Neither iron nor humus are harmful for human health.

Iron bacteria deposit iron and may sometimes stain an entire brook or ditch rusty brown, whereas large sheets of iron oxide deposits (limonite or ‘lake ore’) can occasionally be found at a lake bottom.