Ruo River, largest tributary of the Shire River of southern Malaŵi and Mozambique. Rising on the slopes of the Mulanje Mountains, it flows south to Mulanje town, where it veers southwest, forming 80 miles (130 km) of the Malaŵi-Mozambique border before entering the Shire River at Chiromo. The Ruo’s catchment area of about 1,900 square miles (4,900 square km) drains the southern portion of the eastern Shire Highlands, with periodic heavy discharge causing severe flooding of the river.