snowfall over the entire multi-state Colorado River basin. Dam engineers anticipated a normal winter, and maintained reservoir levels at their usual levels through the winter. Inflow capacity Spring ended with a sudden influx of warm weather and then rain. The combination of rain and snowmelt eventually produced a combined inflow into Lake Powell of over per second. The average annual peak flow prior to 1963 was per second. The Bureau of Reclamation predicts that the probable maximum flood at Glen Canyon is per second, almost seven times the 1983 total. Outflow capacity Glen Canyon Dam has two tunnel spillways capable of allowing to bypass the dam's regular spillways. The spillway tunnels were excavated around both abutments of the dam, dropping steeply from their control gates on Lake Powell to connect with the lower reaches of the diversion tunnels, which were utilized as the lower ends of the dam's spillways. This measure saved cost, but introduced a weak point at the elbow where the two tunnels intersected. The upper ends of the diversion tunnels were then sealed with a solid concrete "plug". While this made the spillways more economical to construct, they had less capacity in part because engineers must maintain at least 30 percent clearance between the water level and the tunnel ceiling. in addition to the tunnel spillways, the dam has a set of river outlet works designed to release per second. The dam also releases water through the dam's power turbines, which are capable of releasing per second. The official spillway capacity is . Tunnel spillways damaged At first, as inflows exceeded normal levels, the US Bureau of Reclamation engineers opened penstocks to full release. When inflow rates continued to rise, they also opened the river outlet works. The reservoir continued to rise. Reclamation Bureau officials met in late June and agreed that the maximum water level the dam could handle was . At that level they feared they could not control the spillway gates. The engineers decided to raise the floodgates controlling the tunnel spillways. This became the first time that the spillways had ever been used for their intended purpose. Volume initially ran at per second per tunnel. After several days, noticeable vibrations were felt in the dam wall and surrounding rock. Water exiting the spillways contained noticeable debris, including sandstone, signaling severe erosion taking place within the tunnels. Reclamation responded by reducing releases by half,