Ulley Reservoir is a reservoir located a few hundred metres to the west and downhill of the village of Ulley, 4 miles south of Rotherham, South Yorkshire, England. It is 2.5 miles away from junction 33 of the M1 motorway.

Ecology
On 25 June 2007, cracks in the reservoir walls were found after heavy rains and flooding in the area, causing concerns that the reservoir might burst, prompting the evacuation of the three nearest villages Whiston, Treeton and Catcliffe. Several other areas as far away as Canklow were put on emergency flood alert. The M1 motorway was also closed between junctions 32 and 36 amid fears over safety. are actually piezometers installed to measure sub-surface water levels within the dam, as discussed at the 1970 conference.
It was feared that, if the reservoir's dam was breached, the sudden flow of water could bring down high voltage power lines, and flood the nearby M1 and a crucial regional electricity substation.