It rises at the foot of Mounts Trebulani, at 86 meters above sea level, in the territory of Calvi Risorta.

With a surface of just over a square kilometer, the enchanting Lake of Letino was created at the beginning of the twentieth century in order to feed the hydroelectric power station of Prata Sannita.

It starts in the area of Frosinone from the confluence between the Liri, which first crosses Abruzzo and Lazio, and the Gari, which rises at Cassino and is often called Liri-Garigliano.