The Pinn is an ancient rivulet, which is one of the feeders of the Colne. It was at one time also known as Ruislip brook and is nearly twelve miles long. It rises on Harrow Weald Common and flows down through Headstone into Pinner. From there it runs into Old Eastcote, past Haydon Hall Lodge from where it flows into Ruislip , through the golf course into Ickenham. From there it crosses Uxbridge Common into Uxbridge, and then on through the RAF base to Brunel University. It then runs through Pield Heath to join the Frays at Yiewsley. The Frays joins the Colne southwest of Drayton Mill. The Colne in turn flows into the Thames near Isleworth.
Once this was a good fishing stream. In 1804, at the time of the Ruislip enclosure, there was a right of way three feet wide along the banks of the Pinn so that the lord of the manor and his lessees and servants could enjoy the fishing despite the allotments nearby. In those days there was far more water in the Pinn.
During the Second World War the Pinn was dammed at the bottom of Pinner High Street as a reserve water supply in case of fire from air-raids.
Work has been done in recent times to prevent the Pinn from flooding surrounding areas.