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This artificial lake, which some other contributors have named as Wortley reservoir (it is between the two places), acts as a flood prevention measure by attenuating (taking up the peak of) flash floods on the Farnley Beck (a tributary of the River Aire) and the water level is therefore deliberately kept low to provide strategic storage. It was apparently built in response to a severe flood in 1967.

The earth dug out for the reservoir was used to build terraced playing fields nearby. In dry weather the reservoir contains about 12 million gallons [54 Megalitres] and is less than 1 metre deep. In severe weather the reservoir can hold over 47 million gallons [213 Ml] and be over 3 metres deep. (source: Link )

A video clip on youtube shows the balancing reservoir in action in times of high flow: Link