The western Sierra Madre is the main water-providing area of northern Mexico. However, most of this mountain range has suffered a progressive degradation of soils and vegetation due to overgrazing and deforestation, for four or five decades. The objective of this study is to determine the impacts of these changes on water balance and hydrodynamic basin behaviour. The hydrological data of two basins (the Sextin basin, 4660 km 2 and the Ramos basin, 7130 km 2) of this area were analysed. Annual runoff coefficients have not changed. Therefore, other indices were used to determine changes in the streamflow regime: ž an index of irregularity of daily discharge; ž the separation of flood flow and base flow; ž the lag time of the watersheds; ž the baseflow recession index; ž the two-day recession index. Some changes were noticeable at the basin scale in the water balance of the catchments: ž the ratio of the flood runoff coefficient to the base runoff coefficient increased from the 1970s to the 1990s; ž the basin lag time decreased 2Ð1% in the Ramos basin and 6Ð1% in the Sextin basin; ž the two-day recession index and the baseflow recession index increased, reflecting a more rapid decrease of streamflow after the peak flow; ž the mean annual runoff coefficient and the irregularity as estimated by the index used here showed no significant evolution. The convergence of such observations confirms that dramatic changes in the evolution of water resources in the near future in this area are to be expected, if the current land use is not strongly modified. Water management and dam operation would also be seriously affected.