Sukhna Lake was created in 1958 by damming the Sukhna Choe, a seasonal torrent off the Shivaliks, to give the city a water body.
It silted up immediately, because a Shivalik choe carries an enormous sediment load. The response was a large catchment treatment programme in the hills above it: check dams, afforestation and grazing control across the Sukhna catchment.
That programme worked and is one of the more successful watershed management projects in India, and it is the reason the lake still exists.
The lake is also the city's principal public space, used at dawn by several thousand people walking.
It has run dry or near dry in bad monsoon years and its water level is a standing local preoccupation.
An artificial lake fed by a torrent that would fill it with silt, kept alive by planting trees on a hillside ten kilometres away, is a working demonstration of the relationship between the Shivaliks and the plain that the rest of Punjab has not applied.
Le Corbusier laid down that no vehicle should reach the water, and the promenade is the one provision of that plan which has held for seventy years without argument. The regatta channel, the rowing club and the golf course on the far bank all came later and were not in it.
A wildlife sanctuary was notified on the reserved forest above the lake, which is what has kept construction off the shoreline on that side. The lake takes migratory duck and waders in winter and is counted in the annual waterbird census.
The Sukhna Choe carries on below the dam, through the city's edge and out into Haryana, taking the storm water and a good deal else, and it is one of the streams that eventually feeds the Ghaggar. The lake is the visible three kilometres of a drainage line nobody looks at.