A pure-limestone Asón tributary rising in the karst cave of La Gándara (Soba) and falling fast to join the Asón near Ramales de la Victoria. Short — perhaps fifteen kilometres of fishable water — but exceptional in character: gin-clear pools, white-water steps and overhanging beech and oak. Wild brown trout only, sometimes neurotically educated; the kind of water that pays back patient observation and punishes a long line. Best fished in the upper meadows below the spring and the shaded gorges down to Ramales. Limestone buffering keeps clarity high through most spates, but the river drops back in under four hours after rain, so the spate window is short.

• Limestone