Kielder Dam on the River North Tyne, northwest of Hexham in Northumberland, retains Kielder Water, an artificial lake impounded in 1982. The earth embankment dam and its associated concrete structures are founded on both drift deposits and rocks of the Scremerston Coal Group, a very mixed sedimentary succession of Lower Carboniferous (Dinantian) age affected by moderate folding and faulting. Aspects discussed include the staged site investigation for determination of construction materials, the stratigraphy and structure, the groundwater regime before and after dam construction, the relief well testing and the constructional geology. The latter is described in the form of a ‘guided tour’ through the dam foundations and covers, mapping, the recognition and treatment of adverse conditions arising from tectonic and glacially related processes of the past, reservoir impounding and its effects.