Water access in the western United States is controlled by property rights to use water. In most of the region’s watersheds, all of the water supply is legally claimed or is projected to be by 2030. In such locations, new water demands can frequently only be met through reallocation of existing water rights. For decades, water markets have helped the western U.S. voluntarily adapt water rights to new demands and changing supplies, providing water for growing cities, freshwater ecosystems and new farms and industries. However, many have questioned whether western U.S. water law provides sufficient flexibility to adapt to unprecedented water demand and a changing climate.