As the riverboat chugs its way up the chocolate-colored channel, fringed with dense rainforest, the primeval depths of the Upper Amazon beckon. Our focus is the Pacaya Samiria National Reserve at the mighty river's headwaters, a 5-million-acre mosaic of flooded and dry forest, islands and tea-colored creeks laden with tannins. Containing some of the greatest biodiversity on Earth, it shelters the largest wildlife populations in the Amazon Basin. Explore by excursion boat and on foot, looking for pink river dolphins, monkeys, sloths, macaws and butterflies. Lagoons covered in giant lily pads teem with fish and caiman, while scores of marsh birds hunt from shore. Visit local villages, and learn about conservation challenges as we follow hundreds of miles of Amazon tributaries aboard a deluxe ship built for exploring the river system's farthest reaches.