The Locks

Richmond lock and footbridge is described as a half tide lock. In each arch of the footbridge there is a shutter known as a curtain.
Two hours after high water the curtains are lowered into the river, creating a barrier to maintain the level of water above this point.
While the curtains are in the lowered position , vessels are obliged to use the lock, and transit time will vary depending the tide.
Two hours before the next high water the curtains are lifted, which allows vessels to navigate unimpeded through the bridge.

Teddington Lock, although generally a faster lock to pass through is probably the busiest lock on the Thames and in the summer months can influence timings.

A combination of unpredictable tidal conditions, bad luck at the locks and an unusually busy river can cause unforseen delays.