The Juruá River is a southern affluent

Tributary

A tributary or affluent is a stream or river that flows into a main stem river or a lake. A tributary does not flow directly into a sea or ocean...

 river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

 of the Amazon River

Amazon River

The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...

 west of the Purus River, sharing with this the bottom of the immense inland Amazon depression, and having all the characteristics of the Purus as regards curvature, sluggishness and general features of the low, half-flooded forest

Forest

A forest, also referred to as a wood or the woods, is an area with a high density of trees. As with cities, depending where you are in the world, what is considered a forest may vary significantly in size and have various classification according to how and what of the forest is composed...

 country it traverses. It rises among the Ucayali

Ucayali

The Ucayali River arises about north of Lake Titicaca, in the Arequipa region of South America. The Amazon River takes its name close to Nauta city , in the confluence among Ucayali and Marañón rivers....

highlands, and is navigable and unobstructed for a distance of 1133 miles (1823 km) above its junction with the Amazon. It has a total length of approximately 1500 miles (2414 km), and is one of the longest tributaries of the Amazon.