The water levels are kept high in the spring/summer period to encourage Great Crested Grebes to nest along the reservoir margins and support other bird life in the reed beds.
In the later summer levels are dropped to provide storage capacity after heavy rainfall, which reduces the risk of flooding downstream. The lower levels also expose rich marginal mud, ideal feed grounds for many species.
This is done with the support of the Environmental Agency. Managing the level in this way is also beneficial to wildlife, encouraging grebe nesting and the late summer migrants.