This paper presents results about potential effects of climate change on the hydrology and water resources in the Rio Conchos basin, with special emphasis on the transboundary water treaty signed between the US and Mexico in 1944. The Rio Conchos watershed, the main tributary of the lower portion of the bi-national Rio Grande/Bravo basin, is located in Chihuahua State in northern Mexico. The most relevant problem in the basin is the frequent occurrence of long drought periods, such as those that occurred in 1950s, 1960s, and the most recent from 1992 to 2003. Coupled with increased water demand and low irrigation efficiencies, the competition for water resources is high on both sides of the border. This paper reports on part of an assessment of expected climate change impacts in the basin. The methodology includes: 1) developing a model that represents the hydrological behavior of the basin; 2) analyzing downscaled climate data from five General Circulation Models (GCMs), for emission scenarios A2 and A1B; 3) simulating the basin under these climate change scenarios; 4) assessing the impacts of climate change on the hydrology and water resources in the basin; and 5) simulating and evaluating water management scenarios that could be used to adapt to the expected climate impacts in the coming decades. Some results from tasks (1) and (2) are reported here. These indicate a negative trend of precipitation in the area over the next few decades. Consequently, streamflow is projected to be lower by more than 18% at the basin outlet (Ojinaga station at the confluence with the Rio Grande) for scenario A2. Moreover, seasonal runoff analysis shows that winter and summer flows will be reduced more than 25% by the end of the century. Trend analysis also indicates negative trends over time. In addition, the change in runoff concentration and periodicity was explored in this research. © 2010 ASCE.