future conditions. However, the prospect of changing states of atmospheric conditions and climate behavior, associated with anthropogenic emissions of greenhouse gases, calls this assumption into question. As a result, many water managers today are exploring ways of adjusting water planning and management strategies.

The study of climates that occurred before direct measurements of weather and climate data—paleoclimatology—can serve as part of the hydroclimatic information considered in water management decisions. This field of study draws upon indirect, or proxy, information about past climate conditions obtained from evidence contained in glacial ice, landscape features, sediment deposits in ancient lakes, pollen, species distributions, preserved organisms (e.g., mollusks), and middens. The science of dendrochronology, or the study of the sequences of annual growth layers (rings) of coniferous trees, is particularly relevant in the Colorado River basin. For several decades, cores from coniferous trees in the western United States have been analyzed to enhance understanding of past climate. Recent tree-ring analyses have incorporated updated chronologies and longer calibration periods to estimate annual Colorado River flows over the past several centuries. These new dendrochronological reconstructions have stimulated heightened interest in questions regarding the rarity and recurrence of drought conditions across the region.

This chapter discusses fundamental features and dynamics of Colorado River basin climate (including climate trends and future climate scenarios), the gaged record of Colorado River streamflow, and tree-ring studies of past Colorado River region streamflow. The concluding Commentary section discusses implications of this hydrologic and climatic information for water resources planning and decisions.


Precipitation Patterns and Sources

The Colorado River is primarily a snowmelt-driven system, with most precipitation in the basin falling as winter snowfall in higher

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