Turned out it was the Molteno reservoir (or Molteno dam), one with a colorful history.
Construction was completed in 1880, but then the dam stood empty through two unusually dry winters. When the drought finally broke, the dam overflowed. The eastern wall broke and sent a tsunami of water down through the city, destroying houses and uprooting trees. Yikes!
Another catastrophe occurred in June 1900, when a famous hot air balloonist called Isidore Michaels got in trouble with the wind, jumped from his balloon basket with a parachute, but ended up in the middle of the dam and drowned, enmeshed in his parachute strings. The dam had to be drained to retrieve his body.
After that, the dam served the young city of Cape Town for many decades to come. It is still in service, supplying the city center with water, alongside several other dams in the Western Cape Water Supply System that were brought on-line.
