I don't know enough about snakes but I assume that this is an Anaconda - or rather was an Anaconda. It was lying under the first bridge on the Transpantaneira Highway (on which there are about 120 bridges on its 130 km length) and looked as if it might have been digesting a meal. You can just make out that it has something in its stomach. However, it wasn't moving and the fact that its head was under the water should have given me a clue.

About 4 days later we passed by the bridge again and the anaconda had been dragged out from under the bridge and had been partly eaten by something.

Anacondas are normally about 5 metres (16 feet) long although there are stories of individuals 9 metres long. They kill their prey of capybara, pigs, young caiman, etc., by constriction then swallow them whole and go off to digest their meal. This can take several weeks depending on the size of the kill.

There is some on-line information at the University of Michigan site.