The most prominent feature of the Altiplano is the large lake at its northern end, Lake Titicaca. At 3,810 meters above sea level, it is the highest navigable body of water in the world. With a surface area of 9,064 square kilometers, it is larger than Puerto Rico and is South America's largest lake. Lake Titicaca is also deep, about 370 meters at its maximum, but with an average depth of 215 meters; its volume of water is large enough to maintain a constant temperature of 10° C. The lake actually moderates the climate for a considerable distance around it, making crops of corn and wheat possible in sheltered areas.

Lake Titicaca drains southward through the slow-moving, reedfilled Desaguadero River to Lake Poop�. In contrast to the freshwater Lake Titicaca, Lake Poop� is salty and shallow, with depths seldom more than four meters. Because it is totally dependent on seasonal rainfall and the overflow from Lake Titicaca, Lake Poop�'s size varies considerably. Several times in the twentieth century, it nearly dried up when rainfall was low or the Desaguadero River silted. In years of heavy rainfall, however, Lake Poop� has overflowed to the west, filling the Coipasa Saltpan with shallow water.

Most of Bolivia's important rivers are found in the water-rich northern parts of the lowlands, particularly in the Alto Beni (Upper Beni), where the land is suitable for crops such as coffee and cacao. The northern lowlands are drained by wide, slow-moving rivers, the three largest of which--the Mamor�, Beni, and Madre de Dios--all flow northward into the Madeira River in Brazil and eventually into the Amazon. Riverboats along the Beni and the Mamor� carry both passenger and freight traffic; rapids on the Madeira prevent river traffic farther into Brazil. Near the Paraguayan border, shallow sandy streams carry the seasonal runoff into the Pilcomayo or Paraguay rivers.