The effect of global warming on the snow cover and runoff in mountain basins can be evaluated by comparing present and future conditions modeled for a climate scenario. Rather than use a single historical year to represent today’s climate, an improved method is presented which consists of deriving a so-called normalized year. In the Rio Grande basin near Del Norte, Co (2414 km², 2432-4215 m a.s.l.) the effect of a hypothetical temperature increase of +4°C on the snow cover and runoff as experienced in hydrological year 1979 was evaluated. Because 1979 has relatively frequent temperatures below -4°C, snowmelt as well as conversion of snowfall to rainfall was limited during winter. Consequently, the redistribution of runoff from summer to winter was not as significant as evaluated in other basins. The data set of 1979 was modified by applying the so-called normalized temperatures for the period 1957-1994. The predicted future snow conditions in the normalized year show a complete disappearance of snow in the lowest elevation zone (2432 – 2929 m a.s.l.) on April 1, due to an increased snowmelt in the winter. The resulting winter runoff which increased from 7.6% to 12.8% of the annual total, in 1979, now increases from 11.7% to 24.2% of the normalized annual total. In evaluation of the impact of climate change in mountain basins, more realistic results can be expected if today’s climate is represented by normalized data of several decades instead of one single year.