Mort Lake is a body of water on the Ankh side of the river. It is bisected by Water Street, which links the Fronts of the University via Water Bridge to Pallant Street. Judging by the artwork of The Streets of Ankh-Morpork, while suffering the unique fate of all water-crossing features of having had houses built on both sides, Water Street is built on a causeway rather than a bridge.
Interestingly enough, The Streets of Ankh-Morpork shows the line of a very old aqueduct (explicitly named a such on the Mapp) which runs both parallel to and right alongside Water Street. The line of the aqueduct may be traced back along Hopesprings to The Tump, and then it emerges again in direct line on the widdershins side of the Tump to disappear out into New Ankh. In the opposite direction, the aqueduct appears to cross the river at Water Bridge, but is then lost without trace on the Morpork side. In The Compleat Ankh-Morpork, this is, interestingly, described as the viaduct and not the aqueduct.
This suggests that many many years ago, under the old kings, clean water was directed into the City from an unknown external source, suggesting that even then people had learnt not to drink the river water. The Discworld Mapp does not show an obvious source of fresh water on this side of the city, but then the scale and detail of The Discworld Mapp is not great, even allowing for a few suspected gross inaccuracies.
The Compleat Ankh-Morpork tells us that the Lake is popular as a venue for fishing and boating: leisure facilities are provided here including boats for hire, family picnic sites and playgrounds for the kids. Areas where it is perfectly safe to swim are clearly signposted, and the guidebook adds the caveat that for the keen swimmer and the fisherman, Mort Lake is possibly the only city facility where this is permitted or indeed recommended. Swimming in the River Ankh is described as an activity for which the Watch will return a verdict of suicide, and it is noted that fishing in the River is forbidden, as it constitutes "waterborne salvage", for which Sir Harry King has the sole monopoly. Should you escape chastisement by Harry King's boys, be warned: the Ankh is the only river known where the catch has enough attitude to throw you back.
It is entirely possible that Mort Lake acted as a secondary reservoir for fresh water to serve the city's needs. This is despite the observed geographical fact, supported by the alignment of the lake to the river, that Mort is an ox-bow lake, created when the loop of a river lazily meandering across the plain is cut off at both its turning points by silting and deposited mud. Over the course of years, the river then takes the shortest possible journey across the base of the loop, leaving its former course behind as an isolated curved lake. Generally speaking, water in an ox-bow lake, no longer flowing, can become stagnant and unappealing, but in the case of Mort Lake, what is the betting that it might actually be fresher (comparatively speaking) than the semi-solid Ankh?
London's Mortlake (in the London borough of Richmond-upon-Thames) is most famous for having been the home and workplace of Doctor John Dee, Elizabeth 1st's court astrologer and ritual magician. Dee is rumoured to have conjured angels and demons, was responsible for the Enochian Alphabet used in the Discworld novels as the private language of golems, was an influence on the life and career of Aleister Crowley, and of course makes a personal appearance in The Science of Discworld II: the Globe as reluctant host of Discworld's wizards on a visit to Earth.