The Nanay River is tributary

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...

 to 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 Napo

Napo River

The Napo is a tributary to the Amazon River that rises in Ecuador on the flanks of the volcanoes of Antisana, Sincholagua and Cotopaxi.The total length of 1075 km. Catchment area of ​​100,518 square kilometers...

 in Peru

Peru

Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

. The Nanay is one of the three rivers that surround the jungle city of Iquitos

Iquitos

Iquitos is the largest city in the Peruvian rainforest, with a population of 370,962. It is the capital of Loreto Region and Maynas Province.Located on the Amazon River, it is only above sea level, although it is more than from the mouth of the Amazon at Belém on the Atlantic Ocean...

, making it an island. Other nearby settlements on the river include the villages of Santo Tomás, Padre Cocha, and Santa Clara. During periods when the river is low, the many beaches along the Nanay are popular destinations. The Nanay belongs entirely to the lowlands, and is very crooked, has a slow current and divides into many canos and strings of lagoon

Lagoon

A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...

s which flood the flat, low areas of country on either side. It is simply the drainage ditch

Ditch

A ditch is usually defined as a small to moderate depression created to channel water.In Anglo-Saxon, the word dïc already existed and was pronounced 'deek' in northern England and 'deetch' in the south. The origins of the word lie in digging a trench and forming the upcast soil into a bank...

of districts which are extensively overflowed in the rainy season. Captain Butt ascended it 195 miles, to near its source.