An eastern Cantabrian coastal river rising in the Montes de Ordunte and falling north through Guriezo to the Bay of Biscay between Castro-Urdiales and Laredo. The Agüera is best known as a small sea-trout river in modern terms — wild brown trout in the middle reaches, with a fragile reo (sea-trout) population running into the lower river in summer. Mixed limestone-and-sandstone geology gives reasonable clarity but the river drops back quickly after rain. Best fished in May and June for trout, with sea trout interest from July onwards after summer rain.
- Mixed