The region designated as the Paci®c Coastal Mountains and Western Great Basin extends from southern Alaska (648N) to southern California (348N) and ranges in altitude from sea level to 6200 m. Orographic eects combine with moisture-laden frontal systems originating in the Paci®c Ocean to produce areas of very high precipitation on western slopes and dry basins of internal drainage on eastern ¯anks of the mountains. In the southern half of the region most of the runo occurs during winter or spring, while in the northern part most occurs in summer, especially in glaciated basins. Analyses of long-term climatic and hydrological records, combined with palaeoclimatic reconstructions and simulations of future climates, are used as the basis for likely scenarios of climatic variations. The predicted hydrological response in northern California to a climate with doubled CO 2 and higher temperatures is a decrease in the amount of precipitation falling as snow, and substantially increased runo during winter and less in late spring and summer. One consequence of the predicted earlier runo is higher salinity in summer and autumn in San Francisco Bay. In saline lakes, the incidence of meromixis and the associated reduction in nutrient supply and algal abundance is expected to vary signi®cantly as runo ¯uctuates. In subalpine lakes, global warming will probably will lead to increased productivity. Lacustrine productivity can also be altered by changes in wind regimes, drought-enhanced forest ®res and maximal or minimal snowpacks associated with atmospheric anomalies such as El NinÄ o±Southern Oscillation (ENSO) events. Reduced stream temperature from increased contributions of glacial meltwater and decreased channel stability from changed runo patterns and altered sediment loads has the potential to reduce the diversity of zoobenthic communities in predominately glacier-fed rivers. Climatic warming is likely to result in reduced growth and survival of sockeye salmon in freshwater, which would, in turn, increase marine mortality. Further research activities should include expanded studies at high elevations and of glacier mass balances and glacial runo, applications of remote sensing to monitor changes, further re®nement of regional climatic models to improve forecasts of future conditions and continued analyses of long-term physical, chemical and biological data to help understand responses to future climates.