Background
The authors introduce their study by noting that climate models "predict the western U.S. will experience reduced snowpack, increased temperatures, and more severe and longer duration droughts," citing Barnett et al. (2004), Cook et al. (2004) and Barnett and Pierce (2009)," and they add that the models predict the warming will "intensify the effects of droughts and their economic impact," citing Rauscher et al. (2008). But are there any real-world data by which to evaluate these claims? In the case of northern Utah, fortunately, there are!

What was done
Allen et al., as they describe it, "created six new tree-ring chronologies in northern Utah, which were used with pre-existing chronologies from Utah and western Wyoming to reconstruct mean annual flow for the Logan River, the largest tributary of the regionally important Bear River," which efforts resulted in what they say is "the first extended record of streamflow in northern Utah," covering the 400-year period of AD 1605-2005.

What was learned
The six scientists say their work reveals that "the Logan River has experienced highly variable streamflow over the last four centuries," adding that this variability "is only partly apparent when considering only the instrumental record." And in this regard they further note that "the instrumental record does not capture the full range of natural variability," as they say has been found to be the case "in studies in surrounding basins and across the western U.S.," citing Graumlich et al. (2003), Woodhouse et al. (2006), Timilsena et al. (2007), Watson et al. (2009) Barnett et al. (2010) and Wise (2001). More specifically, they indicate that their reconstructions of Logan River flow suggest that "overall flows were more variable at times preceding the instrumental period," and that "it is likely that past droughts and wet periods [were] more extreme than the models indicate, thereby implying the possibility that water supplies may have been more volatile in the past."

What it means
Rather than causing droughts and floods to become both more frequent and severe than they have been in the past, as climate models are prone to predict for a CO2-warmed world (such as is claimed we now reside in by many of the world's climate alarmists), it would appear that in the case of northern Utah and western Wyoming - and possibly much more of the intermountain U.S. west - just the opposite appears to have been the case when lengthy real-world data sets have been obtained and analyzed.

References
Barnett, T., Malone, R., Pennell, W., Stammer, D., Semtner, B. and Washington, W. 2004. The effects of climate change on water resources in the West: Introduction and overview. Climatic Change 62: 1-11.

Barnett, T.P. and Pierce, D.W. 2009. Sustainable water deliveries from the Colorado River in a changing climate. Proceedings of the National Academy of Sciences USA 106: 7334-7338.

Cook, E.R., Woodhouse, C., Eakin, C.M., Meko, D.M. and Stahle, D.W. 2004. Long-term aridity changes in the western United States. Science 306: 1015-1018.

Graumlich, L.J., Pisaric, M.F.J., Waggoner, L.A., Littell, J.S. and King, J.C. 2003. Upper Yellowstone River flow and teleconnections with Pacific Basin climate variability during the past three centuries. Climatic Change 59: 245-262.

Rauscher, A.A., Pal, J.S., Diffenbaugh, N.S. and Benedetti, M.M. 2008. Future changes in snowmelt-driven runoff timing over the western U.S. Geophysical Research Letters 35: 10.1029/2008GL034424.

Timilsena, J., Piechota, T.C., Hidalgo, H. and Tootle, G. 2007. Five hundred years of drought in the upper Colorado River basin. Journal of the American Water Resources Association 43: 798-812.

Watson, T.A., Barnett, F.A., Gray, S.T. and Tootle, G.A. 2009. Reconstructed streamflows for the headwaters of the Wind River, Wyoming, United States. Journal of the American Water Resources Association 45: 1536-1554.

Wise, E. 2010. Tree Ring record of streamflow and drought in the upper Snake River. Water Resources Research 46: 10.1029/2010WR009282.

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.