Long-term changes in water quality in and upstream/downstream of four reservoirs (Uokiri, Tarutoko, Tateiwa and Ohdomari reservoirs) were statistically investigated using an approximately 20 years monitoring data. Increasing trend of TN/TP in the reservoir water was commonly found in the four reservoirs. At Uokiri Reservoir where complete data set was available, it was clear that both nitrogen and phosphorus were significantly trapped in the reservoir. Furthermore, addition of nitrogen and phosphorus in the downstream due to human activity was also obvious. The amounts of trapped dissolved inorganic nitrogen (DIN) and phosphorus (DIP) at the Uokiri Reservoir were estimated to be 6.9 tNy-1 and 0.63 tPy-1. Assuming the same trapping mechanism is working at 15 reservoirs on the Ohta River, it is estimated that 30 % of DIN and 35 % of DIP which should be loaded to the estuary could be reduced by their trapping mechanisms.