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Grey-green colouring in rivers


The Flying Tench

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We all know the colour, and it tends to means the river is in good nick. But what causes it? And why does it drop out? Someone said to me today that a few frosts will cause it to drop out and leave a very clear river - bad for fishing as we know. But why should frosts have this effect?

john clarke

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Only a guess: but at 39.2°F water is at it's heaviest and will sink into water that is warmer - if the surface is being cooled rapidly by frosts, it will sink and maybe take whatever is causing the colour to the bottom.

 

If it's already colder than 39.2 then I have no idea. :D

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Chalk salts (calcium carbonate) dissolve in water and as the temperature drops they crystallise by coating fine silt particles causing the now heavier particles to drop out of suspension and the water too clarify. As the water warms up again the salts dissolve again and the silt is carried by the current again.

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