During the El Niño winter of 1982–1983, the Bureau of Reclamation predicted an average runoff for the Colorado River basin based on snowpack measurements in the Rocky Mountains. However, snowfall during April and May was exceptionally heavy; this combined with a sudden rise in temperatures and unusual rainstorms in June to produce major flooding across the western United States. With Lake Powell nearly full, the USBR did not have enough time to draw down the reservoir to accommodate extra runoff. By mid-June, water was pouring into Lake Powell at over . Even with the power plant and river outlet works running at full capacity, Lake Powell continued to rise to the point where the spillways had to be opened. Other than a brief test in 1980, this was the only time the spillways had ever been used.