Mont-rebei is the only large-scale canyon in Catalonia that is still largely unspoiled, uninterrupted by roads, railways or electricity cables.

The canyon is formed by the River Noguera Ribagorçana on its course through Montsec Mountains, and can only be traversed by a bridle path partially carved out of the rock, affording spectacular views. A large number of hiking and kayak options are available.

The canyon walls have vertical drops of over 500 metres, and minimum widths of a mere 20 metres. The ecological interest includes native fauna (especially birds of prey), rich vegetation, and the beauty of a rugged landscape. Most of the canyon is in Montsec Natural Space.