Paleohydrology

Overview

Paleohydrology refers to information about past hydrology that covers the period prior to observed records, most often sourced from natural 'proxy' records. In the Colorado River Basin, the primary source of paleohydrology is the ring-widths of moisture-sensitive trees. Tree-ring reconstructions of streamflows have been developed for several dozen gages within the basin, including Lees Ferry, extending the perspective of hydrologic variability back up to 1200 years. While some information about sub-annual features of climate and hydrology can be extracted from tree rings, the streamflow reconstructions in the basin are almost exclusively for annual (water-year) flows.

A consistent finding from these tree-ring reconstructions for the basin is that the period prior to 1900 contains a broader range of variability than that seen in the observed records, notably drought events worse that are more severe than the worst-case droughts since 1900. The most severe and sustained of these paleo-droughts have been termed "megadroughts." The tree-ring reconstructions also show that the 20th century had overall more favorable hydrology than the preceding 3-10 centuries, with two multi-decadal pluvials or wet periods, from 1905-1930 and from 1982-1999. The earlier of those two periods may have been the wettest multi-decadal period in the past 500 years.

Application of the Science

Tree-ring reconstructions of streamflow have been used by many water management entities using Colorado River water in their long-range planning, to provide system stress tests that are more extreme than the observed hydrology, but are still physically plausible as they have already occurred in the past.