A final word

In February 2007, a committee of scientists convened by the National Research Council released a report called "Colorado River Basin Water Management: Evaluating and Adjusting to Hydroclimatic Variability". One of the main purposes of the report was to review the available data describing the variability of the flow of the Colorado River, including the gage-based "natural flow" record and the tree-ring reconstructions described in this Paleo Perspective.

The report's Summary conveys the critical role of the tree-ring data in providing a more complete perspective of the Colorado River and its long-term variability:

For many years, scientific understanding of Colorado River flows was based primarily on gaged streamflow records that covered several decades. Recent studies based on tree-ring data, covering hundreds of years, have transformed the paradigm governing understanding of the river’s long-term behavior and mean flows. These studies affirm year-to-year variations in the gaged records. They also demonstrate that the river’s mean annual flow—over multi-decadal and centennial time scales, as shown in multiple and independent reconstructions of Colorado River flows—is itself subject to fluctuations. Given both natural and human-induced climate changes, fluctuations in Colorado River mean flows over long-range time scales are likely to continue into the future. The paleoclimate record reveals several past periods in which Colorado River flows were considerably lower than flows reflected in the Lees Ferry gaged record, and that were assumed in the 1922 Colorado River Compact allocations (p.4).

Furthermore, the Summary points out, the tree-ring data have shown that severe drought is a "normal" feature of the Colorado River basin:

Multi-century, tree-ring based reconstructions of Colorado River flow indicate that extended drought episodes are a recurrent and integral feature of the basin’s climate. Moreover, the range of natural variability present in the streamflow reconstructions reveals greater hydrologic variability than that reflected in the gaged record, particularly with regard to drought. These reconstructions, along with temperature trends and projections for the region, suggest that future droughts will recur and that they may exceed the severity of droughts of historical experience, such as the drought of the late 1990s and early 2000s (p.4).

That last sentence is especially important, as it points the importance of considering both past variability and current and future trend with respect to drought. The phrase "temperature trends and projections" refers to the general increase in average temperature in the Colorado River basin since 1900--particularly since 1970--and the consensus of climate models that future climate in the basin will progressively warm, which would lead to reduced snowpacks, earlier snowmelt, greater evapotranspiration, and lower streamflows. The effects of this likely warming will be superimposed on the natural variability described by the tree-ring data, worsening the impacts of future drought.

The tree-ring reconstructions for Colorado River flow described in these pages can play an important role in preparing for this uncertain but very likely warmer future. Together with information about climate change, the reconstructions can guide expectations for future water yield from the Colorado River basin and help provide a basis for sustainable water management.