Abstract The proposed Miner Flat dam will be across the North Fork of the White River just below the Mogollon Rim on the White Mountain Apache Indian Reservation, Arizona. The dam will be approximately 46 m high with a crest length of about 80 m and will be either a gravity concrete or roll-crete dam. The dam will fill a canyon cut in Tertiary/Quaternary basalt. The basalt, in a series of flows, filled an ancestral channel of the White River. The ancestral channel had alluvium in the bottom and colluvium along the walls, which, in part, were incorporated into the base of the basalt flows. The bedrock below the basalt and the paleocolluvium/paleoalluvium is a sandstone of the Upper Limestone and Siltstone Member of the Supai Formation of Pennsylvanian/Permian age. The sandstone bedrock is almost flat-lying. The basalt flows, in general, dip gently downstream. The sandstone is grossly jointed and the upper part of the basalt flows are columnar. No faults occur in the area. Geotechnical investigations included drilling for core, digging test pits in unconsolidated material and geophysical surveys. The core was logged, point load tests were made on the core and packer tests and Goodman jack tests were made in the drill holes. Based on the point load tests, the strength of the basalt ranged from about 6.60 to 290 MPa, the sandstone of the Supai Formation about 0.1–96 MPa and the siltstone averaged from 1.6 to 96.4 MPa. The packer tests gave hydraulic conductivities in the basalt of1.15 × 10⁻⁴−5.20 × 10⁻⁸cm/sec, in the paleocolluvium/ paleoalluvium of1.06 × 10⁻⁴−9.56 × 10⁻⁶cm/sec, and in the siltstone and sandstone of the Supai Formation of1.73 × 10⁻³to3.33 × 10⁻⁷cm/sec. Youngs modulus (E), as determined, from Goodman jack tests, ranged from 0.34 to 2.76 in the basalt and 0.07–53.91 in the paleocolluvium/paleoalluvium. The test pits, geophysical surveys and laboratory testing indicate that there are about 150,000 m³ of good quality aggregate available for construction. The geological and geotechnical investigation, of the proposed dam and reservoir site established that there were no fatal flaws to the construction of the dam and reservoir. Of particular concern was the possibility of lava tubes or zones of high permeability between the lava flows at the abutments and in the reservoir area; none were identified. Also of concern was the permeability of the paleocolluvium/alluvium beneath the basalt in the reservoir area. The investigations establish that this would not be a problem. Additional geotechnical investigations will be required for final design.