effects of the dam. Siltation The Colorado River carries an estimated 45 million tons of sediment annually at Glen Canyon, and as Lake Powell is the farthest large reservoir upstream on the Colorado River mainstem, the sediment load is completely trapped in the reservoir as the Colorado discharges into it. This sediment is creating a steadily advancing "toe" of sediment, i.e. an underwater alluvial fan, that is advancing towards the dam. Some estimates by conservationists predict that the sediment will reach the dam base in about 80 to 100 years, while others made primarily by government agencies estimate it will take as long as 500 to 700 years. It will take still longer for the sediment to accumulate to the point where it could clog the outlet works, which are the lowest openings in the upstream face of the dam. Some critics predict that if the water level then drops, it may fall below the penstock openings, which are higher up on the dam face, which will cause the release capacity of the dam to drop to zero. This would dewater the Colorado River bed below the dam which would remain dry until the next major spring inflow. Only springs, seeps and tributaries such as the Paria, Little Colorado and Virgin River would supply the river flow during these times, perhaps causing unprecedented drops in levels of Lake Mead as well. The primary failure mode for Glen Canyon dam will likely be an overtopping one spring caused by insufficient storage capacity (either a huge inflow, bigger than 1983, or insufficient storage capacity because the lake was too full of either water or sediment). An example of the fast deposition rate of Colorado River sediment is found in Lake Mead itself, where, before the completion of Glen Canyon Dam, ten percent of its storage was already compromised by sediment. When Lake Powell levels drop, sediments deposited in its upper reaches are carried into the receding water by the Colorado and its tributaries. One large flash flood caused by heavy rainfall could move all or most of these sediments into Lake Powell, creating a sudden loss of storage capacity. One way to control sediment flow in the reservoir, without removing the dam outright, is by dredging. 1983–84 flooding In May 1983, three years after Lake Powell was first filled, an unusually long-lasting winter due to the 1982–83 El Niño event produced increased