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| The Columbia River Basin is the second largest drainage basin in the United States, and supports a diverse range of human and natural interests. However in many years the demands imposed on the Columbia River system account for more water than flows through the system, leaving managers especially vulnerable to low-flow years. In an analysis prepared by Dr. Ze'ev Gedalof (Department of Geography, University of Guelph), streamflow at The Dalles, Oregon, was reconstructed using a network of tree-ring-width chronologies. In the graph below left, the tree-ring sites are shown in red and The Dalles streamflow gage site is shown as the blue cross. The graph below right shows the reconstructed annual streamflow (red line) for 1750-1987 and The Dalles measured streamflow (cfs) (blue line) for 1931-1987 in the top half, and the precipitation for the hydrological year (October-September) over the Pacific Northwest for 1896-2003 (red line) in the bottom half. |
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As noted by Dr. Gedalof, several insights emerge from this perspective: first, severe sustained droughts have occurred with some regularity in the past. The drought of the "dustbowl" 1930s was clearly an intense drought, but was neither the driest nor the longest on record. The 1930s drought was not an anomaly in the longer context, and was probably a normal component of climatic variability in the Pacific Northwest. The most severe drought of the last 250 years likely occurred during the 1840s. An analysis of the extreme low flow years indicates that this severe drought episode lasted from ca. 1840 to 1855. The analysis also indicates that the distribution of single-year low-flow events is fairly constant over time (although there is a conspicuous cluster of low-flow years during the 1840s). Secondly, the period from 1950 to 1987 is anomalous with respect to the relative absence of multiyear drought events. This interval coincides with the development of most of the regulatory structures (both physical and institutional) within the basin, and suggests that the operating guidelines may be based on an unusual flow record. Lastly, the regression residual statistics exhibit an increasing trend over time, suggesting that the relationship between runoff and precipitation has changed over time. This result supports findings from empirical models, and is probably a consequence of deforestation in the upper Columbia basin.
Reference: Gedalof, Z., D.L. Peterson and N.J. Mantua (in review). "Columbia River Flow and Drought since 1750." Submitted to Journal of the American Water Resources Association. |
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