In the border between Navia de Suarna and Ibias, the River Navia draws a sharp meander very similar to Montefurado in Quiroga (Lugo). An enormous tunnel was opened in the rock along the meander neck, in a place where the river was forcefully encased in rock. This gap created a canal to divert the river flow away from the curve, thereby liberating the silted placer deposit which had lain at the river bottom. This silt was then panned, on both sides of the Navia River, both at the inside and outside bends.
The tunnel is approximately 2.5 m wide and 6 m high, and it prompts a jump in the river of 3 m. There are no remains of the lock which necessarily would have diverted the river, or part of it, into the tunnel. This engineering feat dried 950 m of alluvial placer.