Paleoflood Hydrology of the Colorado River System: Implications for Climate Change
Climate models vary in estimating the specific magnitude and spatial variation of anthropogenic
climate change; however, considerable agreement exists amongst all models that hydrologic
changes will be paramount in semi-arid regions of southwestern North America [Seager, 2007].
Limited knowledge exists 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
short gaged and historical records, limiting our knowledge of the risks associated with 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 floods prior to the historical period and do they
cluster into distinct time periods?
2. What are the climatic conditions during extreme paleofloods based on paleoclimate
3. What implications do the results have for extreme floods and/or flood hazard given future
Need and Benefit
The proposed research addresses Priority Area 4.03 described in the long-term user needs
document (LTDOC). Because conventional engineering approaches to flood-risk analysis rely upon
flood-frequency procedures, the expected changes in flood magnitudes from global climate
change pose a major concern for the evaluation of future flood risk. Clearly such changes violate
the stationarity assumption that underpins engineering design decisions based on mathematical
extrapolations from flood-frequency distributions. As an alternative consideration to these
procedures, it is clear that the largest paleofloods are very real manifestations of the most
extreme kinds of flooding that can be produced by meteorological phenomena. Global warming is
indeed producing climate change, i.e., it will result in shifts in the flood-producing storm patterns
of the future. Nevertheless, global warming is not going to create entirely new kinds of
meteorological phenomena that have never before occurred on the planet.
Evidence for the kinds of extreme floods likely to be associated with future climate projections is
available from the natural archives of paleofloods. Despite the powerful application of
dendrochronology 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). By improving our long-term view of extreme floods that have occurred over a broader
range of climate variability than the historical period, this work will complement
dendrochronologic data and research on historical flood extremes, and will reduce uncertainty in
flood frequency analyses and subsequent risk analyses for Reclamation facilities. 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. This work will
complement other ongoing work in cooperation with NOAA/CIRES on historical extremes,
including analysis of extreme precipitation, flood frequency, storm trajectories, and atmospheric
By increasing our knowledge of extreme events, this work will provide watershed managers, policy
makers and other decisionmakers with improved knowledge to make well-informed decisions
concerning flood-associated hazards in the context of future climate change.
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