This is where boats entering the Grand Union from the Thames were gauged and paid their tolls. Below this lock, the canal joins the River Brent, which is semi-tidal until Thames locks take it into the fully-tidal Brentford Creek. Above the lock (behind the photographer) used to be a big depôt for transhipping goods between Thames lighters and canal boats or lorries. That area is now being redeveloped.
This photo was taken during a London IWA London Ring Cruise. Wendy is at the helm of Felis Catus II on the left, and Libby Bradshaw on Adrian Steinkamp's Chyna Too on the right. Beyond Chyna Too, is David & Elizabeth Wood's Anker No.5.