In wet tropical regions, study of climate warming effects on water resources is important due to the socioeconomic and ecological implications. Observed changes in river flows of a major Indian river basin called the Mahanadi during the period 1926–1980 arepresented in relation to climatic changes that have occurred there. The Mahanadi River Basin is located in the monsoon region of India; its area is 141,600 km2 and its mean annual flow is 66,640 million m3 . An earlier study by the author revealed that the surface air temperature over the basin has increased at a rate of 1.1 C° per century, which is highly significant However there has been no considerable change in the precipitation regime, though a slight decreasing trend is observed. In this article, an analysis of the trends in the runoff of the upper catchment and the whole catchment gauged at Hirakud and Naraj is presented. The results show a steady decrease in the river flows at these locations during the 55 year period of the study; this is significant statistically at the 1 per cent level. In order to increase confidence in this result, the moisture indices for the catchment have been computed and examined. Time series of these indices also show a clear declining trend during the period 1901–80. The main result obtained here is that climate warming that occurred over the basin, without being offset by an increase in precipitation (in fact a slight decrease in precipitation was observed), has resulted in a gradual decrease of river flows of the upper catchment as well as of the entire basin during the period 1926–1980. As a backdrop to future climate change scenarios over the Indian region, studies related to its impacts on water resources in the basin are important. However General Circulation Models (GCMs) at present are not capable of simulating adequately at regional and subregional scales. Nevertheless, it is hoped that recent efforts toward development of nested regional climate models may soon make it possible to have acceptable climate simulations for regions and subregions of this scale. The results of this article may be useful at that stage in hydrological forecasting studies for the basin.