The creek is formed near the village of Tonno (comune of Valbrevenna) from the union of rio di Tonno (coming from Monte Antola) with rio dell'Orso, which collects the waters descending the southern slopes of Monte Buio. After a brief stretch heading south the Brevenna receives the rio Senarega from its left and then turns west. The following part of its course is rather meandering and reaches Molino Vecchio, the administrative centre of Valbrevenna. Downstream from the village, the Brevenna turns southeast and, flanked for a long stretch by the provincial road nr.11, it ends up at the Scrivia not far from the centre of Casella. The confluence elevation is 402 m.s.l..