Comprising the shallow western end of Threipmuir Reservoir on the northern slopes of the Pentland Hills, Bavelaw Marsh lies to the west of Redford Bridge and a half-mile (0.8 km) northwest of Bavelaw Castle. It comprises an area of open water with emergent vegetation which is a relatively rare habitat in the Lothians that is of importance for breeding and over-wintering wildfowl. It has an area of 38 ha (acres) but, together with the adjacent Red Moss of Balerno, it forms the Balerno Common Site of Special Scientific Interest (SSSI), which was designated in 1971 and extends to 62.7 ha / 154.9 acres. Management of the area passed from Lothian Regional Council to East of Scotland Water (now part of Scottish Water) in 1996.
This an important breeding ground for little grebe, great crested grebe, black-headed gull, mallard, tufted duck, teal, coot, oystercatcher, snipe, curlew and redshank. Over-wintering species include whooper swan, golden eye, teal, pink-footed and greylag geese. Several nationally-rare bryophyte species occur on the mud exposed at low water levels.