Above-normal rainfall since mid-January has raised the water level of Lake Mead more than a foot at Hoover Dam.
Only a fraction of that has occurred because of surface runoff from watersheds between lakes Powell and Mead, the Bureau of Reclamation said.
Bureau spokesman Bob Walsh said the rising water level is more a product of the rain reducing Colorado River water demand for agricultural use in Southern California and Southern Arizona.
He said releases from Hoover Dam have been scaled back, leaving more water in the reservoir.
Eighty-five percent of the water that feeds the Colorado River water system comes from the snowpack that accumulates on the western slope of the Rocky Mountains of Wyoming, Utah, New Mexico and Colorado.
Despite the series of rainstorms that have saturated lower parts of the basin, the upper basin has not seen a corresponding abundance of snowfall.
"Historically, El Niño weather patterns bring more precipitation to the southern basin, but not the northern basin," Walsh said, referring to a periodic warming of water in the tropical Pacific Ocean that can affect weather around the world.
"The current basin snowpack as of Feb. 8 was about 83 percent of average, so we’re below normal," he said.
Walsh said above-normal accumulations have occurred three times in the previous 10 water years, which run from Oct. 1 to Sept. 30. Snowpack was 109 percent of normal in the 1999 water year, 105 percent in 2005 and 103 percent in 2008.
And some snowpack accumulations in the other years were significantly below normal. The snowpack was 25 percent of normal in 2002, 52 percent in 2003 and 88 percent in 2009.
Despite a prolonged drought that has depleted the water supply in the Colorado River system and dropped Lake Mead some 120 feet below Hoover Dam, experts think recharge will occur in the future.
"Right now we’re in a quite extended drought, and we’ve seen a loss of storage in the reservoir system," Walsh said. "But we think we’re going to get that back. Can’t tell you when, can’t tell you how much, but given the history of Colorado River flows, the likelihood of us seeing a full system again is very good."