The impact of climate change on river ?ow in the Chao Phraya River basin in Thailand is analyzed by feeding future runo¤ projection data into a distributed ?ow routing model. The projection data used consists of daily runo¤ generation, which is downscaled into hourly data, by assuming the temporal pattern is proportional to GCM generated hourly precipitation. The GCM dataset used is a 20 km spatial resolution general circulation model (MRI-AGCM3.1S) developed by the Meteorological Research Institute, Japan Meteorological Agency, for the present climate experiment (1979–2003), the near future climate experiment (2015–2039), and the future climate experiment (2075–2099). The main ?ndings of the river discharge projections are as follows: 1) clear changes in hourly ?ood peak discharge, daily drought discharge, and monthly discharge were detected; 2) for each discharge, the degree of change di¤ered by location; 3) the changes appeared in the near future climate experiment and became clearer in the future climate experiment; and 4) a signi?cant decrease in discharge was detected at the Pasak River basin in October.