River Gate

The River Gate, when closed, blocks the Ankh at the rimward wall of Ankh-Morpork. This originally provided defense against attack up the river, but its main use is in a fire emergency. Ankh-Morpork does not have a fire department, since an unfortunate experience some years ago, and if impromptu bucket brigades and a few Golems can't contain a conflagration the River Gate is shut and, very slowly, the Ankh fills the city with a viscous fluid referred to as "water" for simplicity. Despite its high hydrocarbon content, it's not very volatile and makes an excellent fire extinguisher. This technique preserved much of the city from the great Inn-Sewer-Ants fire recorded in The Colour of Magic.

There is also one official ferry across the Ankh which happens just inside the city near to the River Gate. As yet this has not inspired a song in its honour.