Apurímac (apōrēˈmäk) [key], river, c.430 mi (690 km) long, rising in the Andes, S Peru. It flows generally northwest in a narrow valley. After joining the Mantaro River, the Apurímac becomes the Ene; after joining the Perené River, it becomes the Tambo. When it joins the Urubamba River, it becomes the Ucayali, which is one of the main headstreams of the Amazon. The headwaters of the Apurímac, which rise from glacial meltwater on Nevado Mismi, were determined by a 2000 expedition to the most distant from the mouth of the Amazon.