Glen Canyon Dam, a concrete arch dam on the Colorado River in the American state of Arizona, is viewed as carrying a large amount of risk, most notably due to siltation. The Colorado and San Juan rivers deposit large volumes of silt into Lake Powell, slowly decreasing its capacity. The sediment will eventually build up against the dam and could affect its safe operation and lead to its failure.

The dam, anchored in unstable Navajo sandstone (sometimes said to be "solidified sand dunes"), nearly failed in 1983 as the result of a flood on the upper Colorado River that led to extended use of its tunnel spillways. The spillways, designed for short-term use, soon underwent cavitation and began to fail. Emergency installation of flashboards and other efforts narrowly averted total failure.

Siltation, concrete degradation, spillway operational problems, and unstable dam abutments are all key factors that affect the safe operation of the dam. It is estimated that a breach of the dam would produce a floodwave that would overtop the Hoover Dam. The dam's useful lifespan has been estimated by some to be 85 to 100 years, and was described as "America's most regretted environmental mistake" by David Brower, then-head of the Sierra Club. Different estimates by the Bureau of Reclamation and others suggest a lifespan of between 500–700 years.

Overview

Glen Canyon Dam is a concrete arch dam on the Colorado River in northern Arizona in the United States, just north of Page. The dam was built to provide hydroelectricity and regulate the flow of the river from the upper Colorado River Basin into the lower. The Lake Powell reservoir is the second largest artificial lake in the country, extending upriver well into Utah. The dam is named for Glen Canyon, a colorful series of gorges, most of which now are buried under the reservoir.

Construction of Glen Canyon Dam began in 1956 and was finished in 1966. When the reservoir filled, the dam began to deliver a steady, regulated flow of water downstream and generate a cheap, plentiful supply of electricity. In 1983, major floods nearly destroyed the two tunnel spillways that could have led to the dam's collapse, but disaster was averted by a close margin. By taming floods that were characteristic of the Colorado River, the dam has led to major physical and ecological changes in the lower river. Controversy continues over the positive and negative 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.

Downstream sediment transport The construction of Glen Canyon Dam was significantly changed by the natural sediment transport along the Colorado river by trapping large amounts of sand and silt in Lake Powell. This trapping of sediments altered downstream geomorphic processes that previously maintained depositional features along the river corridor. Which caused a reduction in the amount of available sand that would be used to rebuild depositional features in the Grand Canyon corridor. Before the dam regulations, seasonal flooding would transport large amounts of sediments downstream, which helped maintain sandbars that supported riparian vegetation, archaeological site protection, and aquatic habitat complexity through regular sediment redistribution during natural flood cycles. Following the dam closure, the reduction of sediment supply led to a widespread sandbar erosion and long-term geomorphic adjustments of the river channel that changed how sediment is stored and redistributed downstream.

Since natural sediment transport processes were disrupted, controlled high-flow releases were implemented as an adaptive measurement strategy to rebuild sandbars using tributary-supplied sediment as part of experimental restoration flow programs designed to partially mimic historical flood conditions. These experimental floods can increase the sandbar volume temporarily, but model studies show that the sandbars erode between flood events because sediments remain limited under dam operations. Because Glen Canyon Dam traps most upstream sediments in Lake Powell, the sand that's available for rebuilding the sandbars downstream now comes from tributaries  which are smaller rivers that flow into the main Colorado River. A major remaining source is the Paria River, which delivers bursts of sand during seasonal runoff events that temporarily increase sediment availability within the Grand Canyon corridor. The long-term effectiveness of restoration flows depend on the frequency of the controlled releases and the availability of sand delivered from the tributaries, like the Paria River, which provides a small fraction of the river's historical sediment supply. If controlled floods happen when little tributary sediments are available, the sandbars that were rebuilt during the restoration releases will erode quickly after, reducing the long-term stability of restoration-created depositional features. Which limits their ability to support the ecological, cultural, and recreational functions they were historically able to provide along the river corridor. The reduction of sediment availability also constrains broader ecological restoration efforts downstream of the dam by altering channel morphology and riparian vegetation patterns that support native species habitat and influence how restoration strategies are implemented across the river landscape.

Researchers have emphasized that restoration outcomes depend on balancing hydropower operations with ecological objectives, which limits the ability of management strategies to fully restore pre-dam geomorphic conditions.  As a result, restoration success depends on not only the sediment supply but also on flow regulation, and ecosystem-based management strategies increasingly rely on coordinated reservoir operations together with tributary sediment inputs to maintain sandbar habitat within the Grand Canyon corridor as part of ongoing adaptive management approaches used to support downstream restoration goals.

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 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 states that the highest inflow the Glen Canyon Dam can withstand 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".Powell, pp. 12–13

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, however, the rumblings continued. The noise was so noticeable that a worker in the employee dining room, located near the power plant, reported that it "sounded like the barrages that he had experienced in Vietnam". The engineers soon closed the spillways for an inspection.

Cavitation damage found

Inspection crews were lowered down the spillway tunnels in a small cart to assess damage. They found that cavitation, a known risk associated with tunnel spillways, had severely damaged and eroded the concrete tunnel lining. In some locations the cavitation had exposed the soft sandstone.

The tunnels could not be kept closed as more rain fell in the Colorado River Basin and the reservoir continued to rise. Reclamation reopened the left spillway, allowing per second; the right, which had suffered worse damage, was only permitted to carry per second.

Emergency flashboards installed

As the reservoir level increased, plywood flashboards were installed on top of the spillway gates. They allowed reservoir water to rise without increasing releases.

Tunnels damaged further