Plaque: New River - N4
The New River opened 1613 to bring fresh water from Hertfordshire to London.
As far as we know (and we have walked the full length) this is the only such plaque on the New River. Finsbury Park was created in 1869 so it's possible it was erected then, or possibly in 1913, to celebrate the 300th anniversary.
Site: New River - N4 (1 memorial)
N4, Finsbury Park
Our thanks to Mandy for adding scale to our photo.