Mid Colne Valley is a 132 hectare biological Site of Special Scientific Interest in Harefield in the London Borough of Hillingdon and Denham in South Buckinghamshire.

Its main importance lies in its extensive diversity of birdlife in lakes in former gravel pits.

The great majority is in an area of four lakes bounded on the west by the River Colne and on the east by the Grand Union Canal.

[1][2] The main area of the site is called Broadwater Lake after its principal body of water, and it is part of the Colne Valley regional park.

[10] The SSSI includes a 6 hectare field east of the Grand Union Canal described in the Natural England citation as Coppermill Down, although it is not known by that name locally.

Natural England commented that there would be a serious loss of ancient woodland and a potentially damaging indirect impact on Broadwater Lake.