Porto Velho is the capital of the Brazilian state of Rondônia, in the upper Amazon River basin, and a Catholic Metropolitan Archbishopric. The population is 519,531 people (as of the IBGE 2018 estimation). Located on the border of Rondônia and the state of Amazonas, the town is an important trading center for cassiterite, the mining of which represents the most important economic activity in the region, as well as a transportation and communication center. It is on the eastern shore of the Madeira River, one of the main tributaries of the Amazon River. It is also Rondônia's largest city, and the largest state capital of Brazil by area.