Lake Chilwa (sometimes called Shilwa) is a shallow lake (maximum depth 2.7 m) on the border between Malawi and Mozambique in the southeastern part of the African continent. The water surface measures 1,750 km². Being in a tectonic depression south of L. Niassa, at the southern end of the Great Rift Valley, it lacks an outflowing stream; thus the water level fluctuates widely depending on the balance between rainfall and evaporation. Four steps of lacustrine terraces encircle the lake and indicate its development history. In the age when the highest terrace, now at an altitude of 650 m, was formed, the lake may have been nearly three times as large its present size, being then connected with the Indian Ocean by an outlet river.

The northern half of the lake is now fringed by a vast area of swampy vegetation dominated by a species of cattail, Typha domingensis, while alkaline mud deposits are found along the southernmost part. The drainage basin, with abundant production of rice, tobacco, groundnut and other crops, supports a population of about 400,000 people. Fishery is extensively carried on in the lake with an annual catch of some 20,000 tons.