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be a major Impact of climate change in hydrologic basins dependent on snowfall and snowmelt. One scenario was run to study the effects of shifts in the seasonality of runoff. The results suggest that an increase in temperature of only 20C would shift peak runoff one month earlier, to May, in the Upper Basin. Under current operating conditions, such a shift in timing reduces the overall efficiency with which the system is operated, reducing effective storage and deliveries, and increasing the average annual salinity. We recommend that changes in operations to account for changes in the timing of runoff should be evaluated. Summary and Discussion The results of this assessment suggest that violations of the Colorado River Compact are likely to occur under all scenarios of decreased runoff, assuming that no changes in the operating parameters of the system occur. For instance, storage strategies and targets work extremely well In the base case scenarios but are substantially less effective under alternative scenarios. Thus, violations of the Compact would potentially occur even if runoff dropped only 5 percent. The sensitivity of storage to changes in runoff reflect how carefully the current system is operated and how little room there is for forecast error If water supply is to be maximized without resulting in damaging flood-control releases or uncontrolled spills. As might be expected, the reservoir simulation results presented here suggest that many of the procedures and inputs used In the Bureau of Reclamation model are closely tuned to the historic hydrologic record. While It is likely that many of the severe impacts noted here could be avoided under different operating conditions and rules, we were constrained in the current study from evaluating any alternative operating criteria. The problem of planning water management in the face of a high degree of climate and hydrological uncertainty cannot be easily resolved; nonetheless, it may be possible to increase flexibility In water management. This flexibility will need to be reflected In technical and operational decisions, as well as in the legal and economic Institutions that govern water use In the basin. The problem of planning Is compounded by the fact that we cannot say with certainty whether runoff in the basin will Increase or decrease. Most people with an Interest In the basin have focused on the prospect of long-term decreases In runoff and the shortages that would result, which Is a logical reflection of the region's preoccupation with drought. The fact that average temperatures In the region will almost certainly Increase suggests that, If we assume no knowledge about changes In precipitation, we would expect runoff to decrease as a result of increases In evaporation and vegetative water use. This may be reason enough to plan for supply shortages; but Increased water storage must be traded off against the need for flood-control space. The greatest risk of climatic change Is the potential for streamflow variability to Increase substantially, Increasing the frequency of both sustained drought events and high-flow events. xiii