Grafham Water is an 806.3-hectare (1,992-acre) biological Site of Special Scientific Interest (SSSI) north of Perry, Huntingdonshire.

An area of 114 ha (280 acres) at the western end is a nature reserve managed by the Wildlife Trust for Bedfordshire, Cambridgeshire and Northamptonshire.

[7] The reservoir has nationally important numbers of wintering great crested grebes, tufted ducks and coots, and of moulting mute swans in late summer.

As of January 2011, it was the only site in England, and the first in the UK, to harbour the invasive killer shrimp (Dikerogammarus villosus).

[9][10] Grafham Water is popular for a range of leisure activities including sailing, fly fishing and cycling.