ABSTRACT : Between the years 1963 and 1976, very detailed surface and subsurface investigations have been made in order to solve the complex geological and geotectnical problems of the Qymapınar project which is located on the Manavgat River in the Southern part of Turkey, There is no doubt that the west side leakage problem of the reservoir area is the most interesting among the other engineering geological problems In order to solve this problem 52 core borings for a total length 11x130 meters 495geo - elm Meal soundings for a total length IßS 000 meters have been made. Besides regional and loca l dye tests have been realized, Am of the surface and subsurface data confirm the favourable position and adequate thicknesses of the Karadere for mation and Sinanhoca formation (flysch) from the reservoir leakage point of view, Karadere fm has an extension between Çamdağ anticline and Eynif Polje which is located at 35 km. Northwest of the damsite. As a result; the Tilkiler reservoir grout curtain having the active karstic route in the scale of crab cavity at Keban project and whioh is a doubtful project from the technical and economical feasbilitiy point of wiew, has been abondv billion Turkish liras to the project.