Why is the Garonne river brown? Here is the explanation!

I came across an interesting article that was published by the local tourist guide Marie Hallier (Teleprotour Private tours) on her Facebook page. Marie kindly agreed to let Invisible Bordeaux reuse the article, which explains why the waters of the river Garonne are brown in response to one of the recurring questions Marie gets asked: why does the Garonne look so dirty?

Marie says: “To begin with, it is wrong to say the water is dirty, the exact term is “turbid” (cloudy or opaque). OK, it’s not easy to slip that word into conversations but it could prove useful the next time you play Scrabble! Other ways of describing the colour in French include “limoneuse” (loamy), “blonde” or even “café au lait” (milky coffee)…

And don’t listen to what your fellow passenger on the tram is saying. No, the Garonne has not suddenly turned brown because it rained in the Pyrenees last Saturday! The river is actually brown more or less 365 days per year!

The colour is the end-result of a natural phenomenon. To keep things simple, the fresh water (that flows in the Garonne from its source) is laden with sediment (mainly clay from the river bed). With the effect of the oceanic tides, the river comes up against an incoming current made of salty seawater.

In chemical terms, the salty water is heavier than the fresh water, resulting in a kind of undercurrent amplified by the riverbanks and which brings the sediment to the surface. This is referred to poetically in French as “les floculats” (microscopic flakes that form when particles coagulate) and this reaction is what gives the Garonne its lovely brown colour.

Further upstream, at the point where the tidal effect subsides (more or less around Castets-en-Dorthe), the water becomes distinctly clearer. The presence of salty water stops much further downstream, around Bec d’Ambès.

Sometimes, deposits of sediment have been known to latch on to bits of vegetation; this phenomenon is what gave birth to the islands in the Gironde Estuary!

And, it is interesting to note that the Garonne is ranked as one of the cleanest rivers in Europe! So now you know!”

> If you want to know more about the inner workings of the Garonne and the Gironde Estuary, Marie Hallier provides guided tours all year round. Further information available on her website: www.bordeauxcognactourguide.com or via her Facebook page. 

> All photos and illustrations © Marie Hallier/www.bordeauxcognactourguide.com
> Original version of this article available here. 
> Ce dossier est également disponible en français !