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., of climatic change on runoff, given a relatively short and variable streamflow record. Thus. it is likely that long-term changes in the hydrologic regime on the Colorado River attributable to climatic change would be interpreted as extreme events (e.g. as droughts) for some time and may delay adaptation as a result. Although all the scenarios studied alter the annual and monthly distribution of flows, annual variability is not strongly affected. This is as we expected, given that we did not alter the distribution of the model inputs, but merely transposed them. In addition, the differential effect of the scenarios on high- and low-flow years is relatively moderate. While the percent change in mean annual runoff with respect to the base case is higher for low-flow years than it is for high-flow years, in all cases these differences are within 10 percent. Of potentially greater concern is the increased frequency of extreme events; however, better information is needed from GCMs before changes in interannual variability can be properly evaluated (Mearns, et al., 1990). The analysis of seasonal impacts is constrained by the fact that changes in temperature and precipitation were applied uniformly to all daily data. Actually these annual changes would be distributed unevenly throughout the year. While GCM results provide some insights into seasonal changes, they are not definitive. The GISS and UKMO models suggest that absolute temperature increases in the Colorado River Basin are greater in winter, while the GFDL model indicates that temperature increases are greatest in the summer and fall months. All three GCMs are in agreement with respect to the prediction that percentage increases in precipitation are likely to be greatest in the winter and spring. Because these are the seasons with the greatest precipitation under current conditions and because there is likely to be a considerable loss of snowmelt storage due to higher temperatures, a relative increase in winter and spring precipitation could substantially increase the probability of flooding, particularly if operational procedures are not rapidly adjusted. 38