(1) The environmental conditions and seasonal changes of plankton in the three manmade reservoirs (two of which are hydro-electric while the Achim Reservoir is agricultural), located in the Northern Han River System, are described and compared. (2) The thermocline phenomena is seen the epilimnin and is revealed to be parallel to chemical stratification only in the months of July and August. The summer stagnation period is short in the hydro-electric reservoirs, while the period is longer in the agricultural reservoir. (3) Although the oxygen consumption and acidic phenomenon derived from the decomposition of detritus in the bottom substratum are not seen in the hydro-electric reservoirs, these phenomenon can be distinctly seen in the agricultural reservoir. (4) It is suggested that the decrease in water transparency in comparison to previous results may be caused by turbid substances in the water column. The increase of oxygen in the phytoplankton poor intermediate water layers of the hydro-electric reservoirs may be a result of water disturbance by mechanical factors. (5) Of the total plankton collected, there were 11 species of Protozoa, 15 of Rotifera, 9 of Cladocera and 5 species of Copepoda. The phytoplankton identification showed 3 species of Cyanophyta 13 of Chlorophyta and 19 of Diatomeae. (6) While the amounts periodically vary, the largest amounts of zooplankton are found in September and October, but phytoplankton is seen most abundantly in Spring and Summer. Phytoplankton shows a general bimodal pattern (i. e. they are revealed in abundance in the spring and summer seasons) in the hydro-electric reservoirs, whereas this does not hold true in the agricultural reservoir.