The Xingu Basin is the fourth largest tributary basin in the Amazon and with 507,000 km2 and it is nearly the size of France.
Two-thirds of the Basin is in the state of Pará; the remaining one-third is in the state of Mato Grosso. The Xingu River is almost 2,500 km long and contributes approximately 4 percent of the Amazon River’s total annual discharge. The confluence of the Xingu and Amazon rivers is approximately 420 km from the Atlantic via the main river. Oceanic tides are evident at least 100 km up the Xingu. Most of the Xingu Basin lies within the Amazonian rainforest zone but scrub-savanna dominates the headwaters area. The rainy season in most of the Xingu Basin is from December to May and this is also the period of highest river levels. Where tides are felt in the lower Xingu, average annual river-level fluctuation is only 2 meters. Elsewhere along the main stem, the average ranges from 4–4.5 m. The widest range between known water levels is approximately 8 m.
The outstanding aquatic features of the Xingu Basin are the large mouth-bay of the Xingu River, numerous cataracts, beautiful waterfalls in the hilly Serra do Cachimbo region drained by the Iriri River, and a low-lying headwater basin subject to seasonal inundation. The mouth-bay has a large archipelago and is bordered by sandy beaches for six to eight months each year. Some of the beaches are important nesting sites for the giant Amazon River turtle (Podocnemis expansa). Fishermen target mapará catfish (Hypophthalmus spp.) in the mouth-bay for the Belém and Macapá markets. The Xingu River and most of its tributaries are studded with cataracts. The most impressive cataract stretch on the main river is between the city of Altamira on the Trans-Amazonian Highway and the beginning of mouth-bay. A large part of the 5–20 km wide riverbed in this region is rocky and numerous islands split the main channel into a complex maze. During the low-water period, river depths in this stretch fall to less than 50 cm. The Volta Grande Rapids downriver of Altamira are a major geographic divide between aquatic life in the central Amazon Basin and that in most of the Xingu Basin, though a few migratory species are able to negotiate the cataracts. Large river turtles, dolphins and the Amazonian manatee, however, are not found above the rapids.
The Serra do Cachimbo is an ancient upland that forms the major divide between the middle stretches of the Xingu and the Tapajós. Weathered tablelands covered mostly by savannas are common at higher elevations. As tributaries descend from the tablelands, they flow over granite and resistant sandstones that form some of the most spectacular waterfalls in the Amazon Basin. Relatively little is known about the aquatic life of the Serra do Cachimbo. In comparison, botanists have studied the terrestrial plant life in the area and found many endemic species. Presumably, the tributaries above the waterfalls each have distinct populations of fish species and other aquatic life.
The headwaters of the Xingu River drain into a large, internal low-lying sedimentary basin that is reminiscent of Bananal Island in the Tocantins Basin. At least 30 percent of the Xingu headwater basin is subject to seasonal flooding for four to seven months each year. Major aquatic habitats include very large lake-like lagoons, flooded forests and grass savannas that are inundated during the rainy season. Open rainforest and savanna vegetation dominate the uplands. Water in the headwater basin is more nutrient-rich than water downriver in the Xingu and supports an abundance of aquatic herbaceous plants and mollusks, two groups that are relatively scarce elsewhere in the basin.
More than one-half of the Xingu Basin is classified as indigenous area. Gold miners, loggers and others have invaded most of the protected areas. There are no large national parks that protect wetlands of any size in the Xingu Basin. The headwater wetlands are largely under the control of indigenous groups but surrounded by agricultural lands.
Uses & Impacts
The three most extensively deforested regions in the Xingu Basin are (1) the region peripheral to the headwaters, (2) the region around São Felix do Xingu in the middle stretch and (3) the region along the Trans-Amazonian highway near Altamira in the lower reaches. Most of the Xingu Basin is still inaccessible by road and the Santarém-Cuiabá highway, the main north-south link, skirts only the western edge of the valley. Large soybean farms are found from the headwaters in the Cerrado to near Altamira above the large mouth-bay of the lower course. Other than roads the Belo Monte Dam at the Volta Grande Rapids near Altamira has been the main focus of infrastructure development in the Xingu Basin. Environmentalist groups have clamored against the dam for nearly two decades, though unsuccessfully as construction is now underway.
Most mining concessions in the Xingu Basin are for gold, though there is also interest in iron, zinc, cassiterite, copper and phosphates, among others. Illegal gold mining operations have damaged many small streams though there are few studies of their impacts.