page_title: Glen Canyon Dam
section_title: The 1983 floods
hierarchical_section_title: Glen Canyon Dam / Later history / The 1983 floods
caption_attribution_description: English: Spillway gates at Glen Canyon Dam — during record floods of 1983. Note the "flashboards" installed atop the gates to prevent water from spilling over.
context_page_description: Glen Canyon Dam is a concrete arch-gravity dam on the Colorado River in northern Arizona, United States, near the town of Page. The 710-foot high dam was built by the U.S. Bureau of Reclamation from 1956 to 1966 and forms Lake Powell, one of the largest man-made reservoirs in the U.S. with a capacity of 27 million acre feet. The dam is named for Glen Canyon, a series of deep sandstone gorges now flooded by the reservoir; Lake Powell is named for John Wesley Powell, who in 1869 led the first expedition to traverse the Colorado's Grand Canyon by boat.
  A dam in Glen Canyon was studied as early as 1924, but these plans were initially dropped in favor of the Hoover Dam which was located in the Black Canyon. By the 1950s, due to rapid population growth in the seven U.S. and two Mexican states comprising the Colorado River Basin, the Bureau of Reclamation deemed the construction of additional reservoirs necessary. Contrary to popular belief, Lake Powell was not the result of negotiations over the controversial damming of the Green River within Dinosaur National Monument at Echo Park; the Echo Park Dam proposal was abandoned due to nationwide citizen pressure on Congress to do so.
context_section_description: 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 120,000 cubic feet per second (3,400 m³/s). 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.
  At the beginning of June, dam operators opened the gates on the left spillway, sending 10,000 cubic feet per second (280 m³/s), less than one-tenth of capacity, down the tunnel into the river below. After a few days, the entire dam suddenly began to shake violently. The spillway was closed down for inspections and workers discovered that the flow of water was causing cavitation – the explosive collapse of vacuum pockets in water moving at high speed – which was damaging the concrete lining and eroding the rock spillway tunnels from the upper ends of the diversion tunnels, which connect to the bottom of the reservoir. This was rapidly being destroyed by the cavitation and it was feared that a connection would be made to the bottom of Lake Powell, compromising the dam's foundation and causing the dam to fail.
  Meanwhile, snow continued melting in the Rockies and Lake Powell continued to rise rapidly. To delay having to use the spillways, the USBR installed plywood flashboards (later replaced by steel) atop the gates to increase the lake level. Even this additional capacity was exhausted; discharges through the left spillway reached 32,000 cubic feet per second (910 m³/s), and the right spillway was opened to 15,000 cubic feet per second (420 m³/s). At Lee's Ferry, the Colorado River peaked at 97,300 cubic feet per second (2,760 m³/s), which was and still is the highest water flow recorded there since the dam was built. On July 14, Lake Powell reached 3,708.34 feet (1,130.30 m) elevation, a level that has not been exceeded since. Just as it seemed inevitable that the dam would fail, inflows fell and the dam was saved. Upon inspection, it was found that cavitation had caused massive gouging damage to both spillways, carrying away thousands of tons of concrete, steel rebar and huge chunks of rock.
  Repairs to the spillways commenced as soon as possible and continued well into 1984. Air slots were installed at the bottom of each spillway to break up and absorb the shock of the bubbles formed by cavitation. In 1984, the Colorado River basin produced even more runoff than 1983, peaking at 148,000 cubic feet per second (4,200 m³/s) in early June. This time, the USBR had drawn down the reservoir enough that it absorbed most of the early high flows. Nevertheless, Lake Powell rapidly approached the top of the spillway gates and construction efforts were subsequently focused on the left spillway in order to get it in operation in time. On August 12, the left spillway gates were opened, releasing water at a rate of 50,000 cubic feet per second (1,400 m³/s). The spillway was undamaged, proving the worth of the re-engineering and suggesting that Glen Canyon Dam will also be able to hold against future floods with the magnitude of 1983.
caption: The right spillway gates during 1983 flood, showing flashboards that were installed to increase the water level
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