Most dams and reservoirs are located at or near the bases of river valleys. The topography and geological situation at every site will have developed by the interaction of many geological and related processes during vast periods of time. The resulting geological structure at dam and reservoir sites can be complex and no two sites will be the same. Some of the processes which formed the site may still be active or may be reactivated by the project and capable of influencing the feasibility of its construction or operation. It is vital therefore, that sites are investigated using all appropriate knowledge and methods of classical geology. These are essential for good engineering geology (Baynes, 1999; Fookes, 1997; Fookes et al., 2000; Hutchinson, 2001).