Forestmill Weir, Forestmill

Rare, striking horse-shoe plan weir of early design, created to direct water from River Black Devon (flowing E to W) through sluice into Forestmill Lade (see Notes) and return floodwater back over top of weir into river.

Built as part of John Erskine, Earl of Mar's water management system commencing at Forest Mill and incorporating sluices controlling water flow to 2.5km lade leading to Gartmorn Dam, the oldest reservoir in Scotland.

FURTHER DESCRIPTION: ashlar weir formed from raised platform of large slabs set around curve of squared stonework encompassing deep basin apparently set on bedrock. Weir sides at W slope down to approximately 4' in height.

The fine weir at Forest Mill is part of a hugely ambitious water management system visualized by Sir John Erksine in the closing years of the 17th century. Sir John had inherited the family estates in 1689 and needed an efficient and reliable water supply to operate new hydraulic machinery in his collieries at Alloa and Sauchie, as well as ensuring reliable drainage. The system demanded expertise beyond the scope of local engineers and Derbyshire based engineer George Sorocold, who was recognised by two of his contemporaries, Hatton and Thoresby, as the 'Great English Engineer' was commissioned to solve the problem. The resultant hydraulic scheme extends from the weir and sluices at Forestmill, together with a nearby smaller weir which has fallen into disrepair, along a lade a little under 2 miles in length which joins Gartmorn Dam to the west. Gartmorn Dam was created in 1713, and is the oldest reservoir in Scotland. It was substantially strengthened by Sorocold's scheme, and the lade which feeds it and which significantly raised the water level, by some 10' to 16' (accounts vary), runs alongside the River Black Devon which falls into a deep gorge just west of Forestmill.

The weir and lade, referred to as a dam-head and aqueduct, are described in the New Statistical Account: "nearly 140 years ago, the celebrated John, Earl of Mar, ... caused a strong dam-head to be thrown across the Black Devon, at Forest-Mill, ... by which he raised the bed of this river, 16 feet higher. From the top of this dam-head, he carried an aqueduct westward, about four miles, which carried the water into Gartmorn dam" (p8). The distance of four miles is inaccurate as the actual distance is less than two miles. In 1835 the Forestmill weir was rebuilt at a cost of £248, the account was raised by Alloa Colliery (NAS GD 124/17/594).

There are still three sluices near the weir, one is modern, but two significantly earlier examples are of timber and ironwork construction. (Historic Scotland)

February 2010: Inspection finds of the sluices; timber elements are damp, covered in plants and rotting. Metal elements are rusty but appear to remain functional. Stone elements are damp and overgrown. The weir itself is becoming overgrown. Substantial trees on top of it threaten its integrity; at least one stone has already been dislodged. The structure does not appear to be functioning as a weir.

15 April 2013: External inspection finds no significant change from the previous site visit. Heavy grass growths remain on the top of the weir, though trees appear to have collapsed. Outline Planning Permission for the creation of a new villae near Forestmill was conditionally granted Jul 2011 ref: 06/00283/OUT. One of the conditions noted is for a detailed and comprehensive scheme of repair and interpretion for Forestmill Weir and associated lade.

11 April 2019: Inspection finds the weir structure remains in much the same condition as seen previously. Footpaths and the surrounding greenspace are overgrown. Water flow appears to be still managed with sluices.