Nyasa, Lake, or, a freshwater lake in the Great Rift Valley of East Africa. Lake Nyasa is bounded on the northeast by Tanzania, on the southeast by Mozambique, and on the south and west by Malawi. Nyasa is Africa's third largest lake, covering an area of 11,200 square miles (28,900 km2). It is 360 miles (580 km) long, 15 to 50 miles (24 to 80 km) wide, and up to 2,280 feet (695 m) deep. Portuguese explorers discovered Lake Nyasa in 1616. It was rediscovered by David Livingstone in 1859.