Not many people know there's a giant reservoir in the middle of Clifton. I found a fascinating tidbit in 'To Keep Open and Unenclosed':
The Management of Durdham down since 1861, by Gerry Nichols, that also explains why the toilets and changing rooms on Durdham down are where they are (right next to the giant water tower):

Bristol Water Works Company (BWW) was incorporated by Act of Parliament in 1846 and its first engineering works included three service reservoirs: Bedminster Down for the area south of the River Avon; Victoria Reservoir at Oakfield Road for the lower areas north of the Avon; and the Durdham Down Reservoir for the higher districts. The water flowed from the Barrow Reservoir to Oakfield Road by gravity where there was a pump driven by a steam engine to raise the water to the Downs level. Land was purchased from the Lords of the Manor of Henbury for the Durdham Down Reservoir free of any covenants or restrictions on building. As noted above, the requirement in the 1861 Act for the Downs to be 'open and unenclosed' has always been interpreted as a ban on permanent buildings on the Downs. Thus BWW land has been used for public toilets (1893) and Dressing Rooms (1932 and 1994) to avoid challenges under the 1861 Act.