What is the largest tributary of the Amazon River by water volume?

The largest tributary of the Amazon River by water volume is the Madeira River.

The Madeira forms in Bolivia from the confluence of the Mamoré and Beni rivers, with the Guaporé contributing to the wider system. It flows north through Bolivia and Brazil before joining the Amazon near Itacoatiara, east of Manaus. Its basin also reaches into Peru.

The river is enormous by discharge. Near its formation, its mean annual flow is about 18,000 cubic metres per second, and it contributes roughly 15% of the water in the Amazon basin. Its muddy, sediment-rich water reflects material carried from the Andes, making it an important influence on Amazon floodplains and aquatic habitats.

A common mix-up is the Rio Negro, which is the Amazon's largest left-bank tributary and its most famous blackwater tributary. The Madeira is the overall largest tributary by water contribution. The name Madeira means “wood” in Portuguese, a reference to the floating timber once seen in its waters.