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. Copyright © 2012 John Wiley & Sons, Ltd.