The Ankh is a river serpenting through the Sto Plains, reaching the Circle Sea by the town of Ankh-Morpork. By then, it can only be called a river because it moves faster than the banks. You can't drown in the Ankh, but it is really easy to suffocate.
Paradoxically, Morporkians are very proud of it. They say "A water that has been filtered by so many kidneys can only be very pure indeed" or "A water so full of life can't be bad for health". Fishes are known to dwell in the Ankh, but they can't be easily observed, because they tend to explode in clear water. Few of them are eaten, due to the "terrible gastric trouble" that would otherwise occur. (Raising Steam, p. 11)
It has been said that the Ankh is the only river in the multiverse where cops can draw a chalk outline when a corpse is found.
A certain member of the The Watch likes to spend his time guarding the Brass Bridge (lined by 8 hippos) that arches over this water. Why? because he likes to be one step ahead. You never know when thieves are going to get it in their minds to steal a brass bridge, and he'll be there, waiting for them.
There is a legend in Ankh-Morpork of an ancient drum in the palace that will bang itself if ever an enemy fleet is seen sailing up the Ankh, although the legend has died out in recent centuries, partly because it's the Age of Reason and also because no enemy fleet could sail up the Ankh without a gang of men with shovels going in front.