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Folsom Dam
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Vital Statistics:
Location: Folsom, California, USA
Completion Date: 1956
Cost: $81.5 million
Reservoir Capacity: 43.9 billion cubic feet
Type: Gravity
Purpose: Hydroelectric power, irrigation, recreation
Reservoir: Folsom Lake
Materials: Concrete
Engineer(s): U.S. Army Corps of Engineers

On July 17, 1995, a spillway gate on the Folsom Dam broke open as it was being raised, causing an uncontrolled five-story cascade to gush down the face of the dam. Nearly 40 percent of Folsom Lake drained out past the broken gate before it could be repaired. Normally, when a reservoir becomes too full -- like after a heavy rainstorm -- engineers open spillway gates, which allow the excess water to drain out of the reservoir at a controlled rate of speed. When these gates open suddenly and engineers lose the ability to control the flow, disaster can result.

Folsom Dam
Click photo
for larger image.

Luckily, no major flooding occurred as a result of the failure, and the Folsom Dam was fully repaired at a cost of $20 million. After a year of thorough investigation, the United States Bureau of Reclamation blamed the disaster on a design flaw. Some spillway gates, like the ones at Folsom Dam, roll up and down on giant brass and steel pins, like a big garage door. As the gate opens and closes, the pins can become hot with friction. Friction reduced the strength of the pins in the Folsom Dam spillway gate and caused the gate to break.

Today, engineers trained in rope-climbing techniques can inspect these difficult-to-reach spillway gates and help prevent similar disasters from happening.

Here's how this dam stacks up against some of the biggest dams in the world.
(reservoir capacity, in cubic feet)

Chart showing the relative size of the biggest dams in the world Folsom Dam
43.9 billion cubic feet

Fast Facts:
  • When the spillway gate broke, more than 40,000 cubic feet per second of water gushed through the dam.
  • So much freshwater poured into the San Francisco Bay when the dam burst that salmon and striped bass were fooled into thinking that fall had arrived. The fish began their annual fall migrations way ahead of schedule.
  • The Folsom Dam came close to flooding half a million people in 1986, when engineers were forced to open the spillway gates after heavy rains.

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