The river nominally has its source at Lake Junin, but tributaries above Lake Junin extend as much as 70 km farther upstream, for a total length of 809 km. The named tributaries of the river are the Cunas which enters the Mantaro at regional capital Huancayo, and the Kachimayu which flows near the city of Ayacucho. The upper Mantaro is 432 km long extending from Lake Junin to the Kachimayu inflow while the lower river shown on the map is 307 km long. The river runs through the provinces Junín, Yauli, Jauja, Concepción and Huancayo in the Junín Region, then through the Huancavelica Region and the Ayacucho Region. The river then returns to the Junín Region in Satipo Province, where it unites with the Apurimac River to form the Ene River. Its hydrographic basin also includes some of the Pasco Region. The river belongs to the hydrographic basin of the Amazon River. Its principal tributaries are the Cunas River, the Vilca/Moya River, the Ichhu River, and the Kachimayu.