The most popular walk in the Cazorla natural park follows the crystal-clear Río Borosa upstream to its source through scenery that progresses from the pretty to the majestic, via a gorge, two tunnels and a mountain lake. The walk is about 11km each way, with an ascent of about 600m, and takes six to seven hours there and back.
To reach the start, turn east off the A319 at the "Sendero Río Borosa" sign opposite the Centro de Visitantes Torre del Vinagre, and go 1.7km. The first section of the walk criss-crosses the tumbling, beautiful Río Borosa on a couple of bridges. After just over 3km, where the main track starts climbing to the left, take a path forking right, clearly signposted for "Cerrada de Elías." This leads through a lovely 1.5km section where the valley narrows to a gorge, the Cerrada de Elías, and the path becomes a wooden walkway cantilevered out over the river. You re-emerge on the dirt road and continue 4km to the Central Eléctrica, a small hydroelectric station.
Beyond the power station, the path crosses a footbridge, after which a "Nacimiento Aguas Negras, Laguna Valdeazores" sign directs you onward and upward. The path winds its way up the valley, getting gradually steeper as it climbs through increasingly dramatic scenery past a series of waterfalls. After about an hour, you enter the first of two tunnels cut through the rock for water flowing to the power station. It takes about five minutes to walk the narrow path through the first tunnel (the path is separated from the watercourse by a metal handrail), then there’s a short section in the open air before a second tunnel, which takes about one minute to get through. You emerge just below the dam of the Embalse de los Órganos, a small reservoir surrounded by forested hills. From the top of the dam, follow the trail along the reservoir's left bank and in five minutes you reach the Nacimiento de Aguas Negras, where the Río Borosa begins life welling out from under a rock. Enjoy your picnic beneath the spreading boughs of a large tree here, then head back down the way you came.
Due to its popularity, it's preferable to do this walk on a weekday. Do carry a water bottle: all the trackside springs are good and drinkable but the last is at the Central Eléctrica. A torch (headlamp) is comforting, if not absolutely essential, for the tunnels.
