ESTIMATING PALEOFLOOD VOLUME IN SNOWMELT DOMINATED SYSTEMS: CASE STUDY, COLORADO RIVER AT LEES FERRY

The length and severity of recent drought have highlighted the importance of understanding Holocene climatic variability in the interior western United States. Although water supply shortages are likely to be the most severe problem in the coming decades, the focus on drought has lessened consideration of the potential for extreme wet events. This study estimates the volume of water produced during extreme snowmelt events of the late Holocene in a well-studied system. The modern streamflow record as well as the paleorecord of high flows on the Colorado River at Lees Ferry (~ 10 miles below Glen Canyon Dam) were used as an example case.

The Colorado River at Lees Ferry is a snowmelt-dominated river with 43 years of streamflow data collected by the U.S. Geological Survey before the completion of Glen Canyon Dam. Using these data, a least-squares regression (r² = .90) between annual peak instantaneous discharge and associated 60-day event volume was used to provide an estimate of historic and paleoflood event volumes at that site. This information is useful because although paleoflood magnitudes are regularly estimated, the value calculated is almost always instantaneous discharge rather than total event volume. Knowing the total volume of water in a snowmelt event would be useful to engineers considering design parameters for new reservoir construction and for planning the operations of existing reservoirs.

Recently, the instantaneous discharge of a 1,000-year flood near Lees Ferry was estimated by the U.S. Geological Survey to be 300,000 cubic feet per second using a method that incorporated both modern flood and paleoflood records. The least-squares regression model predicts the 60-day event volume of this 1,000-year flood would be 22.1 million acre-feet. The most recent publicly available Bureau of Reclamation study to assess probable maximum flood magnitudes on the Colorado River at Glen Canyon Dam has a maximum 60-day event volume of 16.5 million acre-feet.

Predictions that the southwestern United States is expected to experience drier conditions during the coming decades should not be confused with predictions for the Colorado River water supply at Lees Ferry, which comes predominantly from snowmelt originating in the Rocky Mountains.