This powerful tributary of the Amazon traverses a superb rainforest, affording incomparable opportunities to observe the wildlife. You have a good chance of spotting Amazon river dolphins or, more rarely, tucuxis. Keep your binoculars at hand if you want to see a whole array of typical fauna, including little squirrel monkeys, black-mantled tamarins, night monkeys, black agoutis, Amazon dwarf squirrels and pygmy marmosets.