Lock 21

Lock 21, built between 1759 and 1804, is a single lock with an oval basin that regulated the flow of water on the North Branch near Frómista. Its chamber, 25.15 meters long, 10.35 meters wide, and 4.20 meters high, is constructed of limestone masonry and retains the essential structure of the original work. The hydraulic system is completed by a diversion weir located on the right bank, which begins 13.50 meters upstream from the upper gate and returns the water 60 meters downstream, and by a fixed-lip spillway whose crest acts as a weir. Most of the flow, however, is diverted through a concrete spillway on the left bank, connected by an underground conduit that drains directly into the basin. Although its state of preservation is poor, the original hydraulic structure remains intact.

The complex includes several secondary elements. The bypass bridges, also built between 1759 and 1804, feature semicircular arches in limestone ashlar, with lateral wing walls and dimensions ranging between 3.20 and 5 meters in length.

At the junction are the ruins of the flour mill, an old 18th-century mill of which only foundation remains and some fragments of walls survive. It is in ruins, but retains original features that allow us to identify its function within the production system associated with the canal. Next to it are the ruins of the lock keeper’s house, also reduced to structural remains, which recall the permanent presence of the personnel responsible for operating and maintaining the lock.

A visit to Lock 21 offers a serene tour along a stretch where the agricultural landscape blends with industrial remains and stone bridges that still retain their original character.