Lake Sandoval: Oxbow Ecology in Peru’s Amazon Rainforest

A short boat ride down the Madre de Dios River from Puerto Maldonado, then a walk through the forest, brings you to the edge of Lake Sandoval. The lake is long, calm and dark, ringed by palm swamp and rainforest, and on still mornings it reflects the whole forest edge. It was once part of the river itself. Lake Sandoval, in Peru’s Tambopata National Reserve, is an oxbow lake, a loop of river abandoned when the Madre de Dios changed its course, and understanding how it formed explains a great deal about the Amazon around it. On Day 6 of our South American Small Group History and Culture Tour, the group spends a half-day here, walking through the forest to the lake and crossing it by canoe with a naturalist guide.

What an oxbow lake is, and how Lake Sandoval formed

Lowland Amazonian rivers rarely run straight. Across the flat floodplain they swing from side to side in great loops, or meanders. The current erodes the outside of each bend and deposits sand and silt on the inside, so over time the loops grow and migrate across the valley. Eventually a loop can become so tight that only a narrow neck of land separates one bend from the next. In a flood, the river breaks through that neck, takes the shorter route, and the old loop is sealed off. What remains is a crescent-shaped lake, an oxbow, known in Peru as a cocha.

Lake Sandoval is one of these, a former meander of the Madre de Dios, covering about 125 hectares. It lies within the Tambopata National Reserve and is the reserve’s most visited destination, about 30 minutes by boat from Puerto Maldonado.

Oxbow lakes do not last forever. Cut off from the main current, they slowly fill with fine sediment and plant matter, becoming marsh and eventually just a curved scar in the forest. Across the floodplain, then, there is a constant cycle: rivers carve new loops, abandon old ones, and leave lakes at every stage of filling. A landmark 1986 study in the journal Nature by Jukka Salo and colleagues found that more than a quarter of the Peruvian Amazon lowland forest showed signs of recent river erosion or deposition. The authors argued that this endless reshaping is one reason the region has such a variety of habitats and species.

Canoe-scale observation of water and forest

Reaching the lake is part of the experience. On the tour, the group takes a boat on the Madre de Dios, then follows a trail of about three kilometres through the forest. Part of it is a raised boardwalk, built in 2018 by the reserve authority and local tour operators; the rest can be muddy, especially in the wet season. At the far end, canoes wait at the edge of the palm swamp.

Once on the water, the pace changes. A paddled canoe moves quietly and slowly, close to the lake’s edges, which is exactly where most of the life is. Your naturalist guide steers along the shoreline, watching the water’s surface, the overhanging branches and the canopy, and pointing out what would otherwise be missed: a heron frozen in the shallows, a kingfisher’s flash of colour, the ripple of something just below the surface. It is the most intimate way to see the rainforest on the journey.

How lake habitats differ from river habitats

The Madre de Dios and the Tambopata are fast, sediment-laden rivers, their levels rising and falling by metres between seasons. An oxbow lake is a different world: calm, deep, dark with organic matter and sheltered from the current. That makes it a nursery for fish and a feeding ground for animals that hunt them.

Around Lake Sandoval, much of the shore is fringed by an aguajal, a swamp forest dominated by the aguaje palm, Mauritia flexuosa. These tall palms grow in waterlogged ground, often in almost pure stands, and their fruits feed a wide range of animals. The Amazon Conservation Association notes that blue-and-yellow macaws nest almost exclusively in the hollows of dead aguaje palms. On the canoe approach to the lake, you pass through this flooded palm forest.

Not every resident of the lake is native. Arapaima, or paiche, one of the world’s largest freshwater fish, was introduced to the upper Madre de Dios in the second half of the twentieth century and spread through its lakes and floods. A 2025 study in the journal PLoS ONE found that its effect on native fish in the region’s oxbow lakes was smaller than often claimed, but it is a reminder that even protected lakes have a human history.

Wildlife interpretation without wildlife guarantees

Lake Sandoval is known for a remarkable range of animals, but nothing about wildlife can be promised. The most famous residents are giant river otters, the largest otters in the world, which live in family groups and depend on oxbow lakes like this one for fishing, denning and raising young. A family group of eight or nine lived on the lake when researchers from the Frankfurt Zoological Society studied it in 1999, and visits have been managed under a plan drawn up partly to protect them. Black caimans also live in the lake.

Birds are more reliably seen. Among the strangest is the hoatzin, a pheasant-sized bird with a ragged crest that lives in vegetation over the water. It feeds mainly on leaves, which it ferments in an enlarged crop, much as cattle do. Its chicks have small claws on their wings: when threatened, they drop into the water and later use the claws to climb back up to the nest. Herons, kingfishers and macaws are often seen around the lake, and monkeys, including capuchins and squirrel monkeys, live in the surrounding forest.

Will you see giant otters at Lake Sandoval?

Possibly, but it cannot be guaranteed. Giant otters range widely, spend long periods out of sight, and can be shy of boats. Seeing them depends on where the family group is that day, the time of morning, and luck. If you do see them, your guide will keep the canoe at a respectful distance; the species is among the most threatened mammals of the Amazon and is easily disturbed.

How muddy and demanding is the walk to Lake Sandoval?

Moderately. The trail is about three kilometres each way, on level ground, part of it on a boardwalk and part on earth paths that can be deep in mud after rain. Rubber boots are often provided, and the walk is unhurried. Getting into and out of the canoes takes a little care. The whole outing takes around half a day, returning to the lodge for lunch.

Why access conditions need current checking

Lake Sandoval is a protected area under active management, and visiting arrangements can change. Entry to the Tambopata National Reserve requires a fee, boats on the lake need authorisation from Peru’s protected-areas service, SERNANP, and some parts of the lake may be closed to canoes to protect wildlife. Water levels also vary with the seasons, changing trail conditions and canoe routes. Your tour’s local operator arranges the visit and follows the current rules on the day.

The lake also reflects wider pressures. Research on another oxbow lake near the Tambopata found that, although fish there carried less mercury than fish in the main river, many species still exceeded safe limits, a legacy of gold mining in the region. Our article on the Tambopata National Reserve and the Madre de Dios looks at those pressures in more detail.

From the Amazon to the Andes

After the morning at Lake Sandoval, the afternoon is free at the lodge, with an optional night walk in the evening. The next day the group flies to Cusco and drives up onto the high plateau, stopping at the weaving town of Chinchero, known for its textiles, on the way into the Sacred Valley. In two days, the journey moves from an Amazonian lake less than 300 metres above sea level to Andean villages at more than 3,700 metres.

Our overview of the South America cultural tour route shows how the Amazon fits into the wider journey. On the South American Small Group History and Culture Tour, naturalist guides and a Program Leader help a small group of up to 16 travellers make the most of these rainforest days.