Amazon river dolphin widespread river dolphins. They are present in six countries in South America: Bolivia, Brazil, Colombia, Ecuador, Peru, and Venezuela, in an area covering about . The boundaries are set by waterfalls, such as the Xingu and Tapajós rivers in Brazil, as well as very shallow water. A series of rapids and waterfalls in the Madeira River have isolated one population, recognized as the subspecies "I. g. boliviensis", in the southern part of the Amazon basin in Bolivia. They are also distributed in the basin of the Orinoco River, except the Caroni River and the upper Caura River in Venezuela. The
Red River of the South receives little precipitation. As a result, much of the river above the Texas–Oklahoma border is intermittent, and until the river is past its great bend south in Arkansas, the flow varies widely. Most of the agriculture in the basin is sustained by groundwater, which is recharged with rainfall and riverflow. The lower course of the river flows through a series of marshes and swamps, where its flow is dramatically moderated. Native American cultures along the river were diverse, developing specialized adaptations to the many different environments. By the time of European contact, the eastern Piney Woods were dominated by the