Latest: High speed rail threat to Broadwater Lake Nature Reserve
Of the four lakes within the SSSI, Broadwater Lake is by far the biggest at around 80 hectares and is one of the largest expanses of open water in the Colne Valley. It supports a number of wooded islands. The other lakes are Harefield Moor, Korda Lake and the Long Pond. The nature reserve includes Korda, Long Pond, the River Colne and the western side of Broadwater Lake. Around the pits are remnants of the original alluvial grasslands and valley alderwoods. These grade into beech and hornbeam woodland and mixed scrub on the western slopes.
The Mid-Colne Valley SSSI has considerable ornithological importance, particularly for the diversity of breeding wetland birds and for the numbers of wintering waterbirds. The principal area of interest is Broadwater Lake which supports nationally important numbers of both wintering and summer moulting waterbirds. The smaller Korda Lake, Long Pond and River Colne all provide an important supporting role to the main lake.
Wintering birds include great crested grebe, cormorant, gadwall, shoveler, tufted duck, pochard, wigeon, teal, goldeneye, smew, ruddy duck and coot. The first five species have been present above the level of national importance in recent years, while gadwall have also exceeded the level of international significance.
Breeding birds include coot, greylag goose, little ringed plover, kingfisher, mute swan, grey heron and tufted duck. Gadwall and shoveler occasionally breed. A colony of cormorants has also become established on the islands of Broadwater Lake. The lake also supports nationally important moult gatherings of tufted duck. Over 500 were recorded in the summer of 1990 but counts in recent years have been lower.
Since excavation there has been a natural colonisation by typical wetland plants and animals. The River Colne is the most natural feature of the site. It still retains a meandering channel in parts with some pools and riffles. The banks of the gravel pits, although of relatively recent origin, support a variety of willows and fen plants such as water-plantain, yellow iris and gipsywort. In sheltered areas, where the banks are gently shelving, more extensive stands of tall swamp vegetation occur, comprising mainly common reed and reedmace.
The River Colne is known to be important for bats, with large numbers of Daubentons feeding along the river.