Yesterday morning, life-giving mug of coffee in hand, I stepped onto the deck for my usual look-about. The sky overhead was a radiant azure, with faint daubs of gossamer white clouds to the west. A waning quarter moon traced its arcing path toward the horizon. These next few nights will be the final appearances for this year's version of that fabled Harvest Moon; our next full moon, which will occur on October 11, will be the almost equally storied Hunter's Moon.
Time moves steadily on. Just like the river, which last Friday did its astonishing overnight color switch, turning from opaque brown to bright green. This annual phenomenon—so dramatic it's nothing short of natural magic—happens at that point when summer is transitioning onto autumn. The weather cools into the mid-50s for several nights in a row with a concurrent drop in water temperatures.
I've never seen any mention of this sudden "greening" of a stream in anything I've read, so I'm not sure exactly what's going on to cause such an abrupt color change. My best guess is that the water's heretofore impenetrable shade of gray-brown—which, if you didn't know better, might be mistaken for being muddy—is due to some species of algae that's been thriving in the stream's nutritious summer warmth. Then, as the water begins to cool, a critical point is reached which triggers a widespread die-off—like a group of houseplants you forgot to take indoors at night succumbing to frost. That would explain why one day the river is instantaneously a delightful luminous chartreuse—a green so impossible it looks unnatural, as if the water had been colored with dye.
Too, I've noted the peak of this brilliant crazy green lasts only a few hours. By the following day, the color has toned down considerably. This may indicate the glowing temporary green is the color of the dying algae rather than of completely dead plants, and might explain why the green continues to gradually fade over the next couple of months. By mid-November, and on through the heart of winter, the river's clarity is pristine—a colorless mirror which can look blue or gray, gold or black, pink, orange, turquoise or purple, depending on the sky and angle of light.
Today is cloudy with sprinkles. So…no waning Harvest Moon in the morning sky. But the river, now the color of old jade, is whispering the news of seasonal change.
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6 comments:
As good an explanation as any I have ever read for that phenomenon. Here what happens is deep, deep brown with peat or crystal clear, depending upon the amount of rain. But that is a lovely green - rather Spring-like I think.
Weaver…
Peat bogs are rich with tannin from decaying vegetation, a process that can be affected by sunlight. A tannin-rich stream is acidic, having a higher pH. It's the astringency of tannins that makes dry red wines "puckery." Wood tannins from oak are used in the tanning of leather.
I'm not sure whether my algae theory for the water green coloration is even close to being correct—and if it is, whether the transformation is precipitated by a change in pH level, photoperiod, or temperature.
I understand a bit about water chemistry and the biology of streams—what's called limnology—but only enough to occasionally impress myself and then sound laughingly stupid when engaged in a conversation with a real expert.
so that isn't tree reflection on the water but the colour of the water itself?It should be happening around St Patrick's day to be really effective.
The only 'green' river I ever see is the Chicago on St. Pat's Day. Yours is beautiful and I envy you....
AfromTO…
Oh, no…you ARE seeing green vegetation reflections in the water—and those little blue bits are reflections of sky, while the light-colored blobs are bankside sycamore trunks.
But a lot of the green is also actual water color. In the photo, almost the entire topmost third of the image is the water's green—which is exactly how it appeared on the first day.
By day two, the color had turned a bit more olive. Now, it's the same olive but slightly faded. Of course it looks different depending on the angle and intensity of the sun, or whether it's cloudy, in shade, etc. But for the first day especially, the river's color is always as surreal as that intense blue you often see on golf course and office complex ponds which are treated with some commercial product such as Aquashade to keep down plant growth—except here it's an electric green.
Robin…
I've seen photos of your Irish green river; mine's more chartreuse…and, of course, the wrong time of year for St. Paddy's Day—though there must be some old saint or battle or tale of ancient devilment to celebrate. Then again, what Irishman ever needed an excuse to party?
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