It's not New and it's not a River. But The New River is 400 years old this weekend. From Ware to Islington, get out there and celebrate by walking a bit of it or attending one of the events. 

http://diamondgeezer.blogspot.co.uk/2013/09/new-river-path.html
http://londonist.com/2013/09/400-years-of-the-new-river.php
http://newriver.org.uk/nr/index.php
http://www.amwell.org.uk/newriver400th.html