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<br />WOODHOUSE ET AL.: UPDATED COLORADO RIVER RECONSTRUCTIONS
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<br />Figure 6. Multitaper method spectral analyses [Mann and
<br />Lees, 1996] of reconstructed flows for (a) the Colorado at
<br />Lees Ferry, Arizona, (b) the Green River at Green River,
<br />Utah, (c) the Colorado near Cisco, Utah, and (d) the San
<br />Juan near Bluff, Utah. All spectra cover the common period
<br />from 1569 to 1997. Peaks are shown versus the 95%
<br />confidence level (dotted line). These analyses were
<br />performed using a 3 x 2 pi taper under red noise
<br />assumptions.
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<br />periods (lowest 15% of flows) in the Lees Ferry reconstruc-
<br />tion, none ranked above the driest tercile in the Colorado-
<br />Cisco reconstruction, which accounts for the greatest
<br />proportion of Lees Ferry flow. Two of driest Lees Ferry
<br />flow periods ranked in the middle tercile in the Green River
<br />reconstruction record (1728-1732, 1628-1632). There
<br />were nine periods in the San Juan reconstruction that fell
<br />within the middle tercile that were dry periods in the Lees
<br />Ferry record. Four of these periods occurred in the 1580s
<br />and 1590s, which is known regionally as an extreme
<br />drought throughout the western United States [e.g., Stahle
<br />et al., 2000]. While there were some extremely dry years in
<br />the San Juan reconstruction over this period (e.g., 1590),
<br />this period was also marked by several wet years (e.g.,
<br />1589, IS95, 1599).
<br />
<br />[33] Important regional variations do exist within extreme
<br />dry periods (Table 7). Rankings of S-year averages show
<br />that the driest 5-year period in the Lees Ferry record, 1844-
<br />1848, was extremely dry in the Green and Colorado-Cisco
<br />records (driest and third driest, respectively), but was
<br />somewhat less extreme in the San Juan (17th driest). The
<br />second most extreme 5-year low-flow period in the Lees
<br />record, 1622-1626, was similarly dry in the Colorado-
<br />Cisco and San Juan records (second driest and driest,
<br />respectively), but to a much lesser extent in the Green
<br />(63rd driest). Regional variability in extreme low flows is
<br />also evident over longer timescales. The period 1622-1631
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<br />was the driest lO-year period in the Lees Ferry reconstruc-
<br />tion. As in the S-year periods, low flows in 1620s are less
<br />extreme in the Green River record, but are markedly low in
<br />both the San Juan and Colorado-Cisco records (ranks 71st,
<br />third, and sixth, respectively). In contrast, the Green River
<br />appears to be most strongly impacted by decadal-scale
<br />droughts in the 1870s and 1880s. As suggested above, the
<br />San Juan appears to be less sensitive to the low flows in
<br />the 1580s and 1590s, and this is evident at both to-year and
<br />20-year timescales. The 20-year period ending in 1592 is
<br />the driest such period in the Lees Ferry and Colorado-Cisco
<br />reconstructions, and the sixth driest in the Green recon-
<br />struction, but it was the 48th driest period in the San Juan
<br />reconstruction.
<br />[34] Regional drought variability was also examined in
<br />the context of its impacts on Lees Ferry flows. Rankings for
<br />lO-year moving averages of flow in tlle three subbasins
<br />were divided into terciles. Periods when the value for one
<br />basin fell in the dry tercile while flow in another basin fell
<br />into the wet tercile, were tabulated (Table 8). Again,
<br />droughts tend be widespread, affecting, to some degree,
<br />all three subbasins simultaneously. However, in 15 of these
<br />10-year periods, contrasting conditions exist between two
<br />basins. Most commonly (eight periods), high flows in the
<br />San Juan reconstruction coincide with low flows in the
<br />Green reconstruction. Dry conditions in the San Juan and
<br />wet in the Green are far less common (three periods). In two
<br />periods, the Green is dry while the Colorado-Cisco is wet,
<br />and there is one case each when the San Juan is wet and
<br />Colorado-Cisco dry and vice versa. The contrasting con-
<br />ditions in the pairs of subbasins appear to balance each other
<br />with respect to Lees Ferry flow for the most pmi, with Lees
<br />Ferry flow for these periods most often falling in the middle
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<br />Green River vs. Colorado at Cisco
<br />______ Green River vs. San Juan
<br />Colorado at Cisco VS. San Juan
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<br />Figure 7. Multitaper method cross-spectral analysis
<br />[Mann and Lees, 1996] of reconstructed flows at the major
<br />subbasin gauges on the Green River at Green River, Utah;
<br />Colorado River near Cisco, Utah; and San Juan River near
<br />Bluff, Utah. (top) Coherency spectra plotted against the
<br />95% confidence interval (dotted line). (bottom) Phasing of
<br />spectral peaks.
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