The basin of the river Barbadún or Flea market has an approximate surface of 136 km2, spreading in direction south-west - north to end in the sea Cantabrico. The river bed principal, of approximately 15 km long, forms at a height of Flea market (Sopuerta) where they join the river Goritza, proceeding of mount Kolitza (374 m), one of the peaks more representative of Ordunte mounts, and several creeks that have his origin in high places of the areas of Abellaneda (Urreztieta) and Bezi.

Later, the principal river bed gets for both different margins flowing, between which they stand out for the right margin Galdames proceeding from the Eretza (873 m) and the Cotorrio, proceeding of the Triano mounts. Streamwise from Muskiz the estuary begins, where the river in the proximities of his mouth in the beach of The Sand circulates bordered along salt marshes and dunes. The Triano mounts separate this basin of that of the Galindo, and those of Ordunte, the Eretza and they separate the mountain range Sasiburu from that of the Kadagua.

It is a basin truchera, with preserve in Sopuerta and it forbids perm in Goritza. There is also foxino and eel, in addition to plaice, corcón and loina