To assess whether the 100-year gage-based flow record for the Colorado River at Lees Ferry captures the full range of natural variability of the river system, we need a longer window onto past streamflow, which can be obtained from natural records of climate variability. In general, these natural records include tree rings, ice cores, fossil pollen, ocean and lake sediments, corals, and other organisms. The data from these natural records are called proxy data. Collectively, these proxy data make possible the study of past climate, or paleoclimatology.

The climate proxy best suited to reconstructing streamflow over past centuries is tree growth, as recorded in annual rings. Tree rings can be absolutely dated to the year of their formation, so the tree-ring records have annual resolution, like the gaged streamflow records. Tree growth is often very sensitive to moisture fluctuations, especially in the southwest US. And old, moisture-sensitive trees are widely distributed in the Colorado River basin, making possible the development of a rich archive of proxy streamflow data. Other types of climate proxies can be used to complement the tree-ring data and confirm the climate variability shown by the trees, and to extend the record further back in time, although with less detail.

The pages that follow explain how tree rings are used to extend streamflow records, and describe several major efforts to reconstruct streamflows for the Colorado River at Lees Ferry, starting with early exploratory work in the 1940s and ending with recent (2006-2007) reconstructions of Colorado River flow.