The broad Rio Madeira forms the western boundary of Porto Velho. Formed in the Bolivian Andes, it's 3200km long and has a peak average flow of 1.4 billion liters a minute – the sixth-largest river in the world. It enters the Amazon River 150km downstream from Manaus.
At 5:30pm on weekdays, and hourly from 8am to 6pm on weekends, riverboats make 50-minute cruises along the Rio Madeira from the dock in front of the Madeira–Mamoré train station (R$15 per person). While not exactly thrilling, this is a reasonable way to idle away an hour or so – with luck you’ll see a few pink dolphins. You can buy snacks and drinks on board.