This paper describes geotechnical aspects of a reservoir enlargement project located in the headwaters of the Amazon Basin, in the Andean Cordillera of central Bolivia. This reservoir serves the Corani and Santa Isabel hydroelectric stations, which harness in sequence 1409 m of head below reservoir level. The project involved raising the existing Corani dam and constructing a diversion scheme to convey water from the adjacent Malaga River watershed into the enlarged reservoir. The Corani dam was raised 5 m by adding a zoned earth–rockfill section to the existing embankment dam and constructing a concrete overflow spillway that incorporated the existing masonry structure. The Malaga diversion scheme involved construction of four intakes, 1.7 km of canals, and 12.1 km of tunnels. The tunnels were driven through a wide variety or rock types ranging from good-quality quartzites to squeezing shales. The project was completed in 1984.Key words: dam raising, tunnel, grouting, shotcrete, canal, squeezing shales, underseepage, hydropower, spillway.