The Bombetoka Bay in Madagascar stretches many tentacles into the mainland via this tidal delta. While sometimes the water is a deep blue, at other times its Cthulhu-like rivulets turn deep red. What makes this strange body change color?
According to Anne Jefferson at the awesomely-named geo blog, Pathological Geomorphology, it all has to do with erosion:
What struck me about the delta was not just the nice tug-of-war between riverine and tidal processes in shaping the islands, but the dramatic red color of the water in the Google Earth image (and others as well). This red color is symptomatic of the massive erosion resulting from rampant deforestation of the island.
Also, the Deep Ones may have something to do with it.