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The Chao Phraya River basin, the largest basin in Thailand, is located in the centre of the northern part of the country. This basin has two large-scale reservoirs: the Bhumibol Reservoir on the Ping River and the Sirikit Reservoir on the Nan River. A comparison of the annual and monthly flow regimes downstream from the reservoirs before and after reservoir development showed a constant increase in low flow and a drastic decrease in high flow. The spectrum of the daily discharge was analysed using the fast Fourier transform on data collected in the area of Nakhon Sawan and immediately downstream from the Bhumibol Reservoir after the reservoir was constructed. The flow at Nakhon Sawan had a periodic characteristic of 7 days. The water released from the Bhumibol Reservoir at hydrological station P.12 also had a periodic characteristic of 7 days. Reservoir operations have a significant impact on the hydrological cycles. The effect of human activities is evident in the spectrum analysis of recorded historical discharge data.

The predicted aquatic estrogen concentrations for the whole of England and Japan were determined and compared using population and flow data. The overall value for English surface waters was 0.9 ng/L estradiol equivalents (EEQ) compared to 0.1 ng/L overall for Japan. Available dilution of sewage effluent was considered to be more important than contraceptive pill usage in this relative risk. A national survey of Japanese rivers using the yeast estrogen assay (YES) gave a median value of 0.27 ng/L EEQ which, while higher than that predicted, confirmed an overall low endocrine disruption risk. Using local population and flow data for 27 separate catchments, the predicted EEQ and measured EEQ (YES) values compared well, confirming the national picture that endocrine disruption would not be a widespread phenomena in Japan. Simple predictions based on population and flow can give an appropriate "ball park" value for catchments and even nations for concentrations of polar organic contaminants which have a majority human origin.