The Greening of the River

I have no explanation for the phenomenon that affects the rivers and streams of Dartmoor in the early spring.  They turn green.  Bright zesty lime green.

All through the summer the waters are loaded with peat which turns them in extreme cases the same general tone as over stewed tea.  The tint is carried down the Dart, as far as its meeting with the sea at Totnes Weir.  All the growth and activity in the bogs presumably stirs the peat up and the reduced rainfall concentrates the effect, that seems fairly straight-forward.

In late autumn the rainfall picks up, the rivers run in torrents and the waters turn alternately grey-brown with stirred up sand, mud or mashed plants and leaves, or quite clear and colourless during the rare lulls in the rain.  That too is as expected, water does not have a natural colour or tint.

Then in spring the waters turn lime green.  Bright zesty lime green.

Why? is a good question well presented.  I’m open to suggestions.

It is still fairly cold and life in the peat is only just getting going after winter and maybe the winter rains have flushed out the peaty colour and that might explain why the water is tint free in February.  But what happens in mid-March to turn the water green through April and on to early May when the tea returns again?

If it was an algal growth then you might expect visibility to reduce, but it doesn’t ‘clear and green’ would be the best description, the clearest the water will be all year.

Dissolved minerals?  Some lakes appear blue because of clay flakes suspended in the water.  But what is special about spring that minerals would specially dissolve in the water?  Surely that’s going to happen all year.  Perhaps the acidity of the water changes, though why?

Perhaps most obviously it could be something to do with the plants but it happens up on the open moor above the ‘tree line’ so it’s mosses or grass or ferns, but why any of those emerging from winter would turn the water green is a mystery.

I’ve thought about it and thought about it and I still have no clue.  Answers please?


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