The geological and geotechnical characteristics of the proposed site for General Mugica multipurpose dam (Mexico) strongly conditioned its basic and executive design. The presence of a paleo depressed relief wrought in volcanic rocks (andesites and volcanic breccias) covered by deposits of fluviovolcanic origin (lahars), from the quaternary age, on the right margin of the project, imposed restrictions in terms of the load capacity of the foundation, deformability and permeability. This condition on the right margin forced to modify the initial concept of a comprehensive RCC dam about 90 m height, for an RCC mixed dam on the left margin and river bed, and clay core rockfill on the right margin. The permeability problems of the lahar and the implications of groundwater flow (hydraulic gradients and uplift pressure) in downstream slope stability were resolved by a drainage gallery excavated in the lahars deposit downstream of the dam axis.