It rises in Fuente la Nalona, in El Puertu (Puerto de Tarna) and flows into the river between San Esteban and L' Arena, forming the Pravia estuary, after irrigating the Peñaullán fertile plain and receiving the Narcea in Forcinas (Pravia).
Its main tributaries are on its left bank, most of them born in the Cantabrian divide between El Puertu (Puerto de Tarna) and El Puertu (Puerto de Leitariegos), being the Narcea the main one with 97 km.
- Caso, Muros de Nalón, Oviedo, Las Regueras, Langreo, Pravia, Ribera de Arriba, Candamo, San Martín del Rey Aurelio, Soto del Barco, Laviana and Sobrescobio.
One of its tributaries is the Caudal, 61 km long, which includes the drainage of a large area formed by the Rivers Aller and Lena.
The River Aller, 42 km long, has a catchment area of some 280 square km and an average annual flow of 10.88 cubic metres per second.
The River Lena, 32 km long, drains a catchment area of 232 square km and has an average annual flow of 6.48 cubic metres per second.
The Trubia River, 46 km long, drains a surface area of 483 square km and has an average annual flow of 18.08 cubic metres per second.
The River Cubia, 41 km long, has a catchment area of 210 square km and an average annual flow of 5.13 cubic metres per second.
The lowest tributary of some importance that the Nalón receives on its left bank is the Aranguín, in the town of Pravia.
On its right bank, however, the tributaries are much smaller, with only the River Nora standing out, which rises in the Loma de Sariego and, after a distance of 67 km, gives up its waters a little to the NW of the town of Nora. This tributary has a surface area of 377 square kilometres, with an average annual flow of 20.98 cubic metres and maximum and minimum daily flows of 7.15 and 0.78 cubic metres, respectively.