Transition between aerial images of the river Thames between West Thurrock and Dartford, taken in the middle of the 20th century and in 2012. The Queen Elizabeth II Bridge was opened in 1991, carrying southbound traffic on the M25 motorway over the river Thames, between Thurrock in Essex and Dartford in Kent. It is the busiest estuary bridge in the UK. In the mid-20th century, the area was largely unused marshland.