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New Streamflow Records of Upper Colorado River Tributaries
Gray, S.T., Lukas, J.J. and Woodhouse, C.A. 2011. Millennial-length records of streamflow from three major Upper Colorado River tributaries. Journal of the American Water Resources Association 47: 702-712.

The authors write that "over the past decade severe drought conditions in the western United States have driven a growing interest in the range of natural hydrologic variability that has occurred over past centuries to millennia," as have "concerns related to the detection and prediction of anthropogenic climate-change impacts," for in order to know how unusual or unprecedented certain aspects of climate may have been recently, one has to know how they varied over past centuries to millennia, when man's influence on them was minimal to non-existent.

What was done
Gray et al. derived millennial-length records of water year (October-September) streamflow for three key Upper Colorado River tributaries -- the White, Yampa and Little Snake Rivers -- based on tree-ring data they obtained from seventy-five preexistent chronologies for a number of sites scattered throughout the region, where each chronology was derived from average annual ring-widths of at least 15 and as many as 80 trees per site.

What was learned
The three U.S. researchers report that "as in previous studies focused on the Upper Colorado River system as a whole (e.g., Meko et al., 2007)," their sub-basin reconstructions "show severe drought years and extended dry periods well outside the range of observed flows." Although they note that 1902 and 2002 "were among the most severe in the last ~1,000 years," they state that "pre-instrumental dry events often lasted a decade or longer with some extended low-flow regimes persisting for 30 years or more." In addition, they indicate that their research "shows anomalous wetness in the 20th century, a finding that has been well documented in the Colorado River basin and surrounding areas (Gray et al., 2004, 2007; Woodhouse et al., 2006; Watson et al., 2009)."

What it means
First of all, with respect to climate-alarmist claims of recent extreme climate/weather phenomena being either unusual, unnatural or unprecedented -- and, therefore, human-induced -- it is clear that that claim with respect to drought in the Upper Colorado River Basin just doesn't hold water. However, noting that their reconstructions "point to the unusual wetness of the gage period," Gray et al. write that there is a "potential for recent observations to paint an overly optimistic picture of regional water supplies," which is indeed true. Hence, we have the unusual situation where both climate alarmists and climate skeptics would likely agree that wisdom would dictate that plans be made for a return to much drier conditions than those of the present, although those plans would likely greatly differ from each other in certain important aspects.

Gray, S.T., Graumlich, L.J. and Betancourt, J.L. 2007. Annual precipitation in the Yellowstone National Park region since CE 1173. Quaternary Research 68: 18-27.

Gray, S.T., Jackson, S.T. and Betancourt, J.L. 2004. Tree-ring based reconstructions of interannual to decadal scale precipitation variability for northeastern Utah since 1226 A.D. Journal of the American Water Resources Association 40: 947-960.

Meko, D.M., Woodhouse, C.A., Baisan, C.H., Knight, T., Lukas, J.J., Hughes, M.K. and Salzer, M.W. 2007. Medieval drought in the Upper Colorado River basin. Geophysical Research Letters 34: 10.1029/2007GL029988.

Watson, T.A., Barnett, F.A., Gray, S.T. and Tootle, G.A. 2009. Reconstructed stream flows for the headwaters of the Wind River, Wyoming, USA. Journal of the American Water Resources Association 45: 224-236.

Woodhouse, C.A., Gray, S.T. and Meko, D.M. 2006. Updated streamflow reconstructions for the Upper Colorado River Basin. Water Resources Research 42: 10.1029/2005WR004455.

Reviewed 28 September 2011