Water is a critical resource in dry forest and rangeland environments of western North America, largely determining the distribution of plant and animal species across a broad range of elevations and ecosystems. Water is also essential for human endeavors, directly affecting where and how human communities and local economies have developed. The Blue Mountains of northeast Oregon and southeast Washington are an important source of water for forest ecosystems and human uses. Surrounding communities rely on water from national forest lands in the Blue Mountains for drinking water, industrial uses, irrigation, livestock watering, and recreation, among other uses. Climate change affects water supply by changing the amount, timing, and distribution of precipitation and runoff. These changes have the potential to affect water supply, roads and other infrastructure, and access to national forest lands in the Blue Mountains region. Reduced or less reliable water supply affects local economic activities, planning, and resource management. Damage to roads, bridges, and culverts creates safety hazards, affects aquatic resources, and incurs high repair costs. Reduced access to public lands reduces the ability of land managers to preserve, protect, and restore resources and to provide for public use of resources. Understanding vulnerabilities and the processes through which climate change affects hydrology will help U.S. Forest Service land managers identify adaptation strategies that maintain ecosystem function, a sustainable water supply, and a sustainable road system.