The source of the river Lea is in Oiz mountain, and it ends in the marshes of the old sea mill called Maria Errota. On its journey it wets lands of great natural beauty and a low population density. Apart from being one of the cleanest rivers in Biscay, its water is drinkable in the whole of its length.
The valley that the river Lea forms has an area of 81,48 Km². Compared to other rivers in Bizkaia, we may say that it has a medium size. It is 24 kms. long, and including all its affluents it has an extension of 125 kms. It has a perimeter of 46 kms. The straight line from its source to the sea is 17,5 kms. long, and it goes down a 1.024 m slope, that is, a slope of 19,7 degrees.
The source of the river is in the north side of Oiz mountain, and it flows with a north-northeast direction running through Munitibar, Aulesti, Gizaburuaga and Amoroto and reaches the sea in the estuary of Mendexa and Lekeitio. The river is called Oiz at the beginning of its journey, and takes the name Lea when the river Nabarriz flows into it in Aulesti. In this town, between the water reservoirs of Beko-Errota and Errotabarri, there is a trout and salmon fishing reserve.
The course of the river Lea is far from uniform. In its first kilometres it goes down the steep Oiz mountain, till it reaches the bottom of the valley; and thus, in a very short distance it goes down a 800 meter slope. From there onwards it flows through a wide valley, but then the river narrows. After a few kilometres, as it approaches the estuary of Lekeitio, it broadens again.
The flora surrounding the river changes as the river flows; for instance, on the first stretch there are beautiful woods, and the rest of the flora in this area is made up of replanted conifers and grazing fields. However, this richness of flora decreases as we approach the river´s mouth. The river is said to have been navigable once.