Southwest Hydrology: The Resource of Semi-Arid Hydrology Volume 4 Number 2

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How Low CC"& It Go? Terry Fulp, Ph.D. — U.S. Bureau of Reclamation, Boulder Canyon Operations Office Lake Mead's delta area, 1999 (left) and 2004 (right). Photos from the U.S. Bureau of Reclamation. he Colorado River Basin is in the midst of a multi -year drought. Water year 2004, which ended on Sept. 30, 2004, marked the fifth consecutive year with below - average inflow into the major Colorado River reservoirs. Inflow into Lake Powell adjusted for the effect of upstream reservoirs was 62, 59, 25, 51, and 51 percent of average in water years 2000 through 2004, respectively. Inflow in 2002 was the lowest on record, and without doubt this drought represents the worst five -year period in the past 100 years. Although we are unable to definitively determine where in the drought cycle we may be, the drought is clearly impacting many aspects of the system. 16 • March /April 2005 • Southwest Hydrology Effect on Reservoir Storage The effect of the drought is immediately evident in reservoir storage. In 1999, reservoirs on the Colorado River collectively were more than 90 percent full. Today the system -wide storage is about 50 percent, a decrease in volume of some 25 million acre -feet of water. Although the situation is very serious, the reservoir system is clearly doing its job, as about 30 million acre -feet of water remain in storage, or nearly two full years of average inflow into the system. Currently, the two largest reservoirs, Lake Powell and Lake Mead, stand at 35 percent and 59 percent full respectively (U.S. Bureau of Reclamation, 2005). On Feb. 10, the water surface elevation at Lake Powell was 3,561 feet above mean sea level (amsl), more than 130 feet below full. Lake Mead's elevation is 1,1338 feet amsl, some 82 feet from the top of the spillway gates. More likely, storage will continue to decline in the near -term and the system will take longer to recover than it did after previous droughts... Have these lakes been this low in the past? As shown in the charts above right, Lake