Paleoflood Hydrology of the Colorado River System: Implication for Climate Changes
Although future climate simulations vary in the exact timing and magnitude of projected changes, considerable agreement exists amongst all models that hydrologic changes will be paramount in semi-arid regions of southwestern North America [Seager, 2007]. Hydrologic studies show differing results regarding the characteristics of extreme flood regimes and how they relate to climate change. In 2007, the Intergovernmental Panel on Climate Change (IPCC) stated that global warming would increase winter flooding in the western U.S. In 2013, the IPCC stated "There continues to be a lack of evidence and thus low confidence regarding the sign of trend in the magnitude and/or frequency of floods on a global scale over the instrumental record" (p. 112, Stocker et al., 2013) and that there is only medium confidence that modern floods have been larger than historical floods in central North America (Stocker et al., 2013).
Our understanding of flood hazard is based upon flood magnitude-frequency curves derived from a short observational period, limiting our knowledge of the potential for extreme flooding. In the southwestern U.S., the gaged and historical record rarely exceeds one hundred years; therefore, large floods are statistically under-represented (Thornycraft et al., 2003). Paleoflood studies produce flood chronologies that can be used to improve flood-frequency analysis (e.g. Benito and Thornycraft, 2005; Harden, et al., 2015) and maximum flood discharge-drainage area relationships (Enzel et al., 1993).
This research poses the following questions for the Upper Colorado River basin: (1) What is the magnitude and frequency of extreme paleofloods? (2) How does the addition of new paleoflood information alter flood frequency curves and confidence intervals at low annual exceedance probabilities (AEPs)? (3) How do paleoflood data alter the temporal context of modern and historical extreme floods with regard to their magnitude and frequency?
Need and Benefit
The proposed research addresses Priority Area 4.03 in the long term user needs document (Brekke et al., 2011). This was identified as a high priority research area within the Natural Systems Response step. With over 60 dams in the Upper Colorado River watershed and major infrastructure on the mainstem Colorado River, this work will help to inform flood risk for a broad range of infrastructure.
Despite the powerful application of dendrochronology in the Colorado River basin as an excellent proxy for average annual streamflow, it provides little or no information for reconstructing extreme flood events because floodwaters are conveyed too rapidly across the landscape to allow for significant increases in soil moisture (a primary driver for tree growth). The lack of understanding of flood potential can lead to ineffective management of reservoirs during flood events, which may result in costly flood damage downstream and the lost opportunity for water storage. By working cooperatively with local universities and other federal agencies, this project will reduce the cost of obtaining this information by greater than 50% when accounting for cost share and lower daily rates for university staff.
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