A river runs under it

This is where the Hackney Brook ran. Sort of. Probably. 

There certainly was a Hackney Brook and it certainly ran from west of Holloway Road to east of Stoke Newington. It must have crossed Hornsey Road somewhere and just north of the Emirates is plausible enough.

I shouldn't have used the past tense. The brook's still there somewhere, babbling away underground. Like other London rivers it became an open sewer and was covered up. 

Much more here.