It is the longest river in Asturias, measuring 140.8 km (87.5 mi).

Its many tributaries include the Narcea, which is itself the second largest river in Asturias, and the Cubia.

The river is home to multiple endangered species, including the Iberian desman and the Cantabrian brown bear.

Strabo made the first known historical reference to the river, calling it "Melsos."

This, in turn, would later morph into "Nilonis" and "Nalonis" in documents from the medieval Kingdom of Asturias, leading to its current denomination.