It is 43 kilometres (27 mi) long from its source at Elorrio to the Nervión, and it passes by the towns of Durango and Amorebieta and joins the Nervión river at Basauri.

Both rivers run then together for a short length until they merge with the sea at Bilbao.

It is commonly agreed that it is the Nervión, but there are some who argue that the Ibaizabal carries more water.

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