Water is critical to life, and the effects of climate change on ecosystems are mediated through changes in hydrology. Changes in how snow accumulates and melts are one of the more consistently noted climate-induced changes to water in the western United States (Barnett et al. 2005; Service 2004), and these changes affect when water will be available for forests and fish alike. Changes in summer atmospheric circulation patterns may alter the ability of summer precipitation to allow midsummer respite from seasonal drought and dampening of wildfire spread (Intergovernmental Panel on Climate Change [IPCC] 2013; see chapter 8). Fish will be affected by both lower low flows with earlier snowmelt and higher midwinter floods caused by rain-on-snow events. Declining summer water supplies will likewise challenge municipal and agricultural water supplies. All of these meaningful effects can be traced to interactions between temperature and precipitation changes projected for the future and described in chapter 3. In this chapter, we describe mechanisms of hydrologic change, and provide maps illustrating variations in effects across the Forest Service, U.S. Department of Agriculture Northern Region and the Greater Yellowstone Area, hereafter called the Northern Rockies region. We also discuss some uncertainties relevant to these effects. Climate change effects on stream temperature in the region are discussed in chapter 5.