The river isn't new. It isn't even a river. It was a channel dug in the early 1600s to bring clean water into the City through pipes made of elm. These days the elm has been replaced, but the river still brings water into central London.
This year is the New River's 400th anniversary and Islington Council have organised a series of events in celebration (23rd until 29th September) which include three free lunchtime guided walks led by the wonderfully informative Peter Bertoud, a fully qualified Westminster Guide, who has plenty more interesting tours on his site.
Spaces for his New River walking tours are limited so if you do not manage to secure a space don't panic because the whole 45km route can be walked at your own leisure all the way from its source in New Gauge, Herts, to Sadlers Wells, The New River Head, where the water goes underground. The paths are well-maintained and there are information boards along the way.
Ooh look... I just found this interesting commemorative edition Canonbury newsletter which has even more information about the area.
And The Dugdale Centre within Enfield Museum has a free exhibition about the New River that is on until 5th January 2014. (Thinks... a walk north along the river to Enfield one day soon would be nice.)