Snowmelt runoff comprises a surprisingly large part of the world’s water supply, especially in mountainous regions where it is depended upon for irrigation and hydropower. The Snowmelt Runoff Model (SRM) was used on three mountain basins in North America under conditions of climate change in order to simulate the hydrological response to expect in the future. The focus was on increases in temperature and decreases in snow cover, although the effects of changes in precipitation and transpiration were considered briefly. It was found that in response to a 4-5°C warming, the beginning of the snowmelt season advances by about a month, snow cover disappears from the basin as much as a month earlier, and runoff is shifted from the summer half year to the winter half year with winter runoff sometimes doubling. It was further found that runoff in April and May (when water demands are low) is increased greatly at the expense of large decreases in June and July (when water demands are high). In extreme years the hydrological effects of climate change are intensified. The effects on water management will include changes in reservoir operating rules, increased maintenance for existing water control structures and replacement or construction of new facilities, reviews of water law in relation to climate change, and reassessment of existing interstate and international water compacts. The continually mounting demand for water will increase the need to know the climate change effects on water supply and the appropriate water management responses.