The Caen Hill flight of 29 locks was designed by John Rennie, and has a rise of 237 feet (72m) in 2 miles (3.2km). The locks are set in three groups. The top 6 locks and the bottom 7 locks are fairly normal sets, but the central 16 are so close together that there is not enough water in the intervening pound to allow the locks to operate. These locks have large side-ponds to store water. Due to the large amount of water used, a back-pumping system was installed in 1996 which can return around 7 million gallons of water to the top of the flight each day.