That would be a green anaconda (Eunectes murinus). This was actually a relatively small anaconda (though huge compared to any other wild snake I've seen), a young female perhaps 8 feet in length. Females grow bigger than the males so some day she may reach the gargantuan proportions you hear about and be the apex predator of the Llanos. You can see below why green anaconda are also sometimes called horned anaconda, those beautiful head stripe patterns.
Even as we moved around her taking photos she was totally relaxed and never made any move to strike. We just got the odd tongue flick for our troubles as she had a taste of the air to see what we were. I was more worried about walking backwards into caiman range as there were a couple of those around in the mud too. At this size she was still very thick and as she moved you could see the muscle moving under loose skin. The strength of a big one must be phenomenal.
One thing that surprised me was how pretty she was. I've seen hatchling captive anaconda before and been impressed by their rosette patterning but this snake was just as pretty as a young adult. In addition to the beautiful rosettes along her body she displayed some rather intricate patterning and attractive scaling from her nose....
all the way to the tip of her tail.