“When within the mouth of the river, I was interested by observing how slowly the water of the sea and river mixed. The latter, muddy and discoloured from it’s less specific gravity, floated on the surface of the salt water. This was curiously exhibited in the wake of the vessel, where a line of blue water was seen mingling in little eddies, with the adjoining fluid.”

Charles Darwin, Voyage of the Beagle, July 5th 1832