The main purpose of introducing the selective intake to the reservoir of the Ogochi dam was to cape with cold water downstream of the dam. However, since 1992 when the selective intake was commenced, increasing levels of water pollution and water quality problems have reported. This paper investigates how the selective intake influences vertical distribution of planktons and their discharge from the reservoir, which in turn directly influence water quality. Based on around 20 years of examination of the relationship between vertical distribution and surface water discharge due to the selective uptake, it has become evident that surface water discharge affects the number of planktons in the reservoir production layer to decrease, while producing a relative increase of planktons in reservoir effluent.