Winter snowpack, which slowly melts and releases water in spring and summer, is key to the Northwest’s hydrology and water supplies. Since around 1950, area-averaged snowpack on April 1 in the Cascade Mountains decreased about 20%, spring snowmelt occurred 0 to 30 days earlier, late winter/early spring streamflow increases ranged from 0% to greater than 20% as a fraction of annual flow, and summer flow decreased 0% to 15% as a fraction of annual flow. Under climate change, the largest changes are expected in basins that typically recieve lots of snow. Warming will increase winter stream flows and advance the timing of snowmelt as much as 3-4 weeks earlier by the middle of this century. Summer flows in these basins are projected to be substantially lower.