The Lea river begins on the northern slope of the Oiz Mountains and at Lekeitio flows into the Cantabrian Sea. It is a short river, but its hydropower has been used since the 14th century for the operation of forges and mills. Even today, the remains of buildings, barrages and inflow trenches of these pre- and early industrial plants and former shipyard facilities just before the mouth of the river are visible in the landscape. Along the river, a 23 km trail was established, which opens up the industrial history of the Lea river valley.
Close to its mouth the Marierrota tide mill, built in the 16th century and demolished in 1980, was situated. From the historic site only the dam remained, behind which the water collected at high tide and again flowed out at low tide and thus drove the mill. The mill building was rebuilt and set up with an information centre, which currently is closed.