population growth. There are barriers to transferring water to municipalities from agricultural and other users, such as tribal groups. These barriers include direct and third-party effects, and limited physical facilities for storing and rerouting water among willing buyers and sellers. As agricultural supplies are diverted to urban uses, this last remaining substantial amount of water that could be made available for urban uses in the Colorado River region is, slowly but surely, being depleted.

Steadily rising population and urban water demands in the Colorado River region will inevitably result in increasingly costly, controversial, and unavoidable trade-off choices to be made by water managers, politicians, and their constituents. These increasing demands are also impeding the region’s ability to cope with droughts and water shortages.

The drought of the early 2000s brought climate-related concerns to the fore across the Colorado River region. Not only did the drought result in numerous, direct hydrologic impacts, it raised questions about what climate trends and future conditions across the region and the planet might portend for Colorado River flows. The early 21st century also saw a great interest in several climate and hydrologic studies of the Colorado River region, especially several long-term reconstructions of past Colorado River flows that were based on studies of the annual growth rings of coniferous trees. The following chapter discusses how features of the global climate system affect the Colorado River region, temperature and precipitation trends and projections across the region, the gaged record of Colorado River flows, and studies of annual growth rings of coniferous trees (dendrochronology) and what they imply for regional hydrology and climate.