The river's source is at an elevation of about on the slopes of Monte Mars, in the heart of the Biellese Prealps. Having passed through various centres of population, it is joined from the right at Mongrando by its major tributary, the Viona. From here its broad pebbly shores follow the edge of the Bessa nature reserve, which lies to its right and is part of the Riserva naturale orientata delle Baragge.