The Oca river basin is the main part of the Urdaibai Biosphere Reserve, a protected area designated as such by UNESCO in 1984.

[1] Its naturalistic richness, especially ornithological, was a fundamental basis for this designation and for its declaration as a Special Protection Area for Birds (SPA) in 1994 and its integration into the Natura 2000 Network.

[4] The Oca crosses land in the municipalities of Mundaka, Sukarrieta, Busturia, Murueta, Forua, Guernica and Lumo, Ajangiz, Mendata, Arratzu, Kortezubi, Gautegiz Arteaga, Muxika and Ibarranguelu.

The streams flow in narrow, steep valleys that are covered with a riparian forest and surrounded by Monterey pine (Pinus insignis) plantations.

A few kilometres before Guernica the terrain becomes flat and the riverbed sinuous; from the Villa Foral it becomes a Ria, since the influence of the tide reaches there.

I command that you shall not owe any duty or thirty percent or price for any ship or Bagel, or any other merchandise, that comes or goes from this place of Guernica.

The sediments brought by the river are carried upstream by the tides and form a gently sloping estuarine plain (0.2 m/km from Guernica to Murueta).

Cretaceous limestones and Triassic ophites emerge among the sandy sediments along the entire length of the estuary, severely narrowing the valley in some stretches such as Bekoa Island.

Part of the marshes formed on both sides of the channel has been drained by structures called munas or polders to use the land thus reclaimed for agricultural work or pastures for livestock grazing.

The sediments of the river plain's slopes give way to the mud, and these, in a process of progressive mixing, to the sands that occupy the mouth of the estuary.

Closing the mouth of the estuary is the island of Izaro, which forms a small archipelago with the islets Hotzarri and Potorro-harri among others.

It abounds in meadows, oak groves, leafy forests and especially plantations of fast-growing conifers (Pinus radiata or insignis).

In areas of high marine influence, with salinity similar to that of the sea and sands or sandy silt soils, the nolti zostera develops.

Of special importance is the case of the European mink, a species in danger of extinction, but there are also the dormouse, polecat and wild cat, which are protected.

Among the projects carried out are the study of spatial ecology and habitat use, migratory movements, biology, population dynamics and other aspects of the birds of this protected environment.

According to the latest Ornithological Yearbook of the Gautegiz Arteaga wetland by the Urdaibai Bird Center, the most representative species include the grey heron, spoonbill, osprey, bittern, black-winged stilt, greenshank, little grebe, great egret, wigeons, Hobby, aquatic warbler and kingfisher.

On its banks are located different mills and forges such as those of Olazaharra and Uharka; the dam of the latter currently feeds a mini hydroelectric plant.

The limestone nature of the terrain has formed a karstic system that is striking for the numerous sinkholes that can be observed and the large number of caves.