Oiartzun (river)

Oiartzun
River Oiartzun on its way through Errenteria
Origin	Pyrenées
Mouth	Cantabrian Sea (Bay of Biscay)
Basin countries	Spain
Length	18.5 km
Source elevation	700-800 m
Basin area	84.75 km²

The Oiartzun River (pronounced [oiaɾts̻un]) is a short river in the Basque Country in the north of the Iberian Peninsula. It's the smallest river of Gipuzkoa flowing into the Bay of Biscay (Atlantic basin), with the river joining the sea at the Bay of Pasaia. The river rises in the Aiako Harria massif at an altitude higher than 700 m. The river flows down through steep slopes and falls, with an average drop to its mouth of 5.49% (600 m drop in the first 5 km).

The river meanders through the valley and several hamlets of the town of the same name. On its last stage crosses squeezed in a canal the A-8 motorway and Errenteria before joining the Bay of Pasaia. The river underwent on past decades severe environmental problems on its last 10 km, especially on account of the paper mill located in Errenteria, while the issue has been redressed to a large extent.