ring data. Because patterns of tree-ring growth of trees at lower elevations can reflect moisture availability, tree-ring data can be used to assemble records, or “reconstructions,” of past river flows. Using data from coniferous tree species with long life spans in the Colorado River region, flows dating back several centuries have been reconstructed. The first tree-ring-based flow reconstruction for the Colorado River at Lees Ferry, Arizona—the point at which the Colorado River basin is divided legally into its upper and lower basins—was assembled by Charles Stockton and Gordon Jacoby, Jr., in 1976. Additional reconstructions of Colorado River flows that date back to the 15th century, including several undertaken in the past few years, have enhanced scientific understanding of the region’s long-term hydrologic and climate patterns.

Tree-ring-based reconstructions became increasingly prominent topics of discussion in western water circles in the early 2000s. Because this period was exceptionally dry across much of the West, the tree-ring-based reconstructions prompted many questions and concerns about the possible extent and severity of future droughts. The water years 2002 and 2004 (as measured from October 1 through the following September 30), for example, were among the 10 driest years of record in the upper basin states of Colorado, New Mexico, Utah, and Wyoming. Significantly, flows into the basin’s reservoirs dropped sharply during this period; for example, 2002 water year flows into Lake Powell above Glen Canyon Dam were roughly 25 percent of mean values. These drought conditions stimulated increased interest in tree-ring-based flow reconstructions and long-term Colorado River flows and water availability.

Out of interest in these issues and their implications, in 2005 the National Research Council’s Water Science and Technology Board initiated a study to review hydrologic and climatic sciences of the Colorado River region. The Committee on the Scientific Bases of Colorado River Basin Water Management was appointed to assess the extant body of scientific studies regarding both Colorado River hydrology and hydroclimatic trends that might affect river flows. The committee also was asked to consider related topics, including hydrologic models, data, and methods; organizations for evaluating hydroclimatic data; and systems operations and water management practices (the full statement of task to this committee is listed in Chapter 1).

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