The mouth of the Orinoco in the Atlantic Ocean was documented by Christopher Columbus on 1 August 1498 during his third voyage. He was puzzeld by the fact that such a huge amount of sweet water could come from a small island (he was assuming he was going around an island in the ocean). By August 13, he had come up with the conclusion that the several islands he was seeing were part of a huge coastline of a big continent and that the amount of sweet water pouring into the gulf had to come from a mighty continent that was hitherto unknown (as such a newly discovered continent).
What's peculiar is that Columbus assumed first on August 13 that he discovered a new continent. A few days later on August 17, he changed his idea and concluded he discovered the oldest continent: Asia.