The named tributaries of the river are the Cunas, which enters the Mantaro at regional capital Huancayo, and the Kachimayu, which joins in near the city of Ayacucho.

The Mantaro Hydroelectric Complex is located in the Tayacaja Province of the Huancavelica Region, and produces 31% of all electrical energy generated in Peru.

[5] The definition for the distinction used at the time of survey was based on absolute length which the tributary added to the river, given a continuous and year-round flow of water.

With the Tablachaca Dam built in 1974, a portion of the Mantaro River suffers a dry spell for five months of the year, previously excluding it from the list of source eligibility.

However, research published in 2014 challenged this existing definition, and used advanced imaging and topographic data to establish the Mantaro as the true longest upstream source feeding into the Amazon Basin.