Stantec UK carried out a built heritage assessment prior to the infilling of the old spillway at Colt Crag Reservoir in Thockrington, Northumberland. The old spillway is contemporary with the rest of the reservoir complex, which was built in the late 19th century. However, at some point between the early 1950s and early 2000s, the feature was replaced by the current spillway located directly to the north-east; it was subsequently blocked and is no longer in use. The above ground section of the old spillway has been sealed off from the adjoining tunnel, which is situated below ground; this assessment focuses on the above ground assets only, although a consideration of the tunnel is also included, based off earlier surveys. A full Historic England Level 2 survey of the old spillway was carried out. A complete archive of digital photographs from the survey will be deposited with the appropriate repository (ADS). This report, accompanied by the archive, is considered to be a complete record of the old spillway, and no further work is recommended prior to or during the proposed works.