Nestling in a deep valley of stunning cliffs and forests, this vital source of water has sunk so low it's exposed the eerie sight of a medieval village that was flooded when the reservoir was opened in the 1960s.
The huddle of ancient stone buildings, including a church with its spire, has now re-emerged into the light and stands as a potent symbol of the severity of this water crisis.
The emergence of the village has brought sightseers to the reservoir
In a year that so far ranks as Spain's driest since records began 60 years ago, the reservoir is currently holding as little as 18% of its capacity - at a time of year when winter rains would usually have provided an essential boost by now.
Rainfall figures show a consistent series of shortfalls in recent years - just as Barcelona's population has expanded to more than five million and the region's booming agribusinesses demand ever more irrigation.
For residents here, the arrival of water by ship is a profound shock - normally it's the drier areas further South that are notoriously parched.
Already they are living with restrictions on the use of hosepipes and the filling of swimming pools.
Shipments of water are now needed to sustain the great city
Now the Barcelona authorities are having to take the unprecedented step for any major European city of topping up supplies by the highly visible means of giant tankers arriving in relays, each bringing 28 million litres, up to a dozen ships coming over the next month.
The shipments won't be enough to restore the reservoirs - or make the ancient village vanish again.
But they may buy time for a highly controversial pipeline to be completed by the end of the year. That should bring more reliable supplies from a neighbouring region but at a high political cost.
And it may also remind people of the forecasts from climate scientists of still drier conditions to come in the approaching decades.
BBC environment correspondent, Barcelona