Lake Nyasa is the most southerly of the great African Rift Valley lakes. It is about 560 km long and has a greatest width of about 75 km. In contrast with Lake Tanganyika, it consists of a single basin with greatest depth of about 706 m near the western shore about 45 km north of Nkhata Bay. It lies between 9 deg. 30min. and 14 deg. 30min. S at an altitude of about 500 m in a tropical climate. However it lies far enough south of the equator to experience marked seasonal variations in wind, temperature and precipitation.

The lake occupies part of the southern end of the Rift Valley system and is to a large extent delimited by faults, particularly to the north and on the eastern coast. In these areas the shores are steep and depths in excess of 200 m are found close inshore. At the southern extremity and along the southern half of the west coast the shoreline is more gently shelving. From here the bottom rises gradually to north and south and, except for a ridge some 20 m high at 10 deg. 25 min. S, there is no trace of separate basins as in Lake Tanganyika. In further contrast to Lake Tanganyika, where a depth of 200 m is found within 20 km of the southern extremity of the lake, in Lake Nyasa such a depth is not encountered within 110 km of the southern end (8, 1).