Thief Valley Dam is a reinforced concrete, slab and buttress (Ambursen) structure, 73 feet high and 390 feet long. The original reported (1932) total capacity of Thief Valley Reservoir was 17,600 acre-feet (active 17,400 acre-feet) and covered an area of 740 acres. A sedimentation survey completed in 1992 estimated the total capacity at 13,307 acre-feet (active 13,300 acre-feet) and the surface area at 685 acres.
Thief Valley Dam is founded on closely fractured, metamorphosed basaltic rock. The basalt is hard, brittle and only slightly altered. Principal orthogonal fractures are spaced 12 to 18 inches apart and are oriented parallel and perpendicular to the canyon axis. Subsidiary jointing is spaced less than 2 inches apart and oriented obliquely to the principal joint-set. There appears to be no change in fracture orientation between the right and left abutments inferring that there are no faults of significance within the canyon.
Two shear zones are visible in the rock of the right abutment. These zones are about one foot wide, the width varying in both directions. A third shear zone was reported to pass between No.6 and No. 9 buttresses within the stilling basin area. The shear zone material, which consisted of alterered rock, was excavated to a depth of about 6-feet below sound rock and to a sufficient width to allow for the placement of a 6-foot concrete footing where buttress 7 crosses. A cutoff trench was excavated on the upstream toe of the dam, and curtain grouting was performed along the trench through grout holes which were drilled on about 5-foot centers and to a depth of about 10 to 20 feet.