UNITED STATES

The damaged California dam is in the spotlight now, but it’s not alone among the facilities needing upgrades. (Nat Geo News)

Why do we have dams? Use our resources to better understand.

Teachers, scroll down for a quick list of key resources in our Teachers Toolkit, including today’s MapMaker Interactive map.

Discussion Ideas

• Nat Geo News reports that there are more than 80,000 dams across the country. Why does the U.S. have so many dams? For perspective, take a look at this “picture of practice” to see what students think.
  • Dams provide three major benefits to communities:
    • flood prevention. Dams help communities control the flow of water from rivers and lakes, reducing the threats from flooding.
    • water reservoirs. Water is stored in artificial lakes (reservoirs) created by dams. This water is used for drinking, hygiene, irrigation, industry, and recreation.
    • hydroelectricity. Many dams generate electricity by harnessing the energy of moving water.

 

• Where is the Oroville Dam? Take a look at today’s MapMaker Interactive map for some help.
  • The Oroville Dam is in the western foothills of the Sierra Nevada mountain range in Northern California. Lake Oroville, the reservoir created by the dam, is fed by the Feather River and snowmelt from the Sierra Nevada.
  • At 235 meters (770 feet) high, the Oroville Dam is the tallest dam in the U.S. and holds the second-largest reservoir in California. (Only Oroville’s Northern California neighbor Lake Shasta is larger.)

 

• What industries and communities does the Oroville Dam serve? Take a look at today’s MapMaker Interactive map for some help.
  • The Oroville Dam supplies all three benefits associated with dams to local and regional communities:
    • flood prevention. Flood threats from the Feather River are compounded at its confluence with the Yuba River, which in turn threatens the urban areas surrounding Sacramento (California’s capital city) and the Sacramento-San Joaquin Delta downriver. The Oroville Dam controls the flow of the river, allowing communities and agricultural fields to flourish along its banks.
    • water reservoirs. Lake Oroville supplies water not only to local regions, but to coastal communities hundreds of miles away. The flow of the Feather River is diverted by the California Aqueduct to supply communities and irrigation to the San Joaquin Valley in Southern California.
    • hydroelectricity. The Oroville Dam is a hydroelectric complex, supplying billions of kilowatt-hours to Californians every year.

 

• Why does the American Society of Civil Engineers give Oroville and other U.S. dams a “D” grade?
  • U.S. dams are “aging and underfunded.” In addition to wear-and-tear, “designers of older dams didn’t know as much as we do now about dams and how they perform when stressed, which can lead to design deficiencies.”

 

• The crisis at the Oroville Dam was caused by damage to both the main spillway and the emergency spillway. What is a spillway? Take a look at our short resource on hydroelectric energy for some help.
  • A spillway is a structure that allows water to flow directly into the river or other body of water below the dam, bypassing all tunnels, turbines, and generators. Spillways prevent the dam and the community from being damaged. Spillways, which look like long ramps, are empty and dry most of the time.
  • An auxiliary spillway is simply an additional spillway where engineers can divert water when the water level is high. Emergency spillways can be concrete, like main spillways, or simply bare earth.

 

• What happened to the spillways at Oroville Dam?
  • A large sinkhole developed in the concrete of the main spillway. To prevent further damage, engineers decreased flow to the main spillway and used the emergency spillway for the first time since the dam was constructed almost 50 years ago.
  • Engineers did not anticipate the erosion caused to the hillside beneath the wall of the emergency spillway. Failure of that wall would send a 9-meter (30-foot) flood of water into the Feather River basin, threatening more than 100,000 people.

 

• Are there any other outlets for water from Lake Oroville, the reservoir created by the dam?
  • Yes, but neither one does much good.
    • The dam’s mighty hydroelectric generators regularly release water, but this is tightly controlled and the release is relatively small.
    • The Feather River, which feeds the lake, has a bypass valve that would reduce the flow of water to the lake. That was damaged in 2009 and hasn’t been used since.

 

• More than 100,000 people were evacuated after the spillways were damaged. Why do engineers and urban planners think similar mass evacuations may be possible if other dams are damaged?
  • “The number of high hazard potential dams is increasing because many of them were built in rural areas where the risks to nearby residents didn’t need to be considered at the time. But as the population increases, many dams are subject to what’s known as ‘hazard creep.’ … Rural and urban population sprawl has created new development downstream of many existing dams … putting more people at risk should a dam fail.”

 

• How are engineers working to combat the hazards posed by damage to the Oroville Dam?
  • Helicopters have dropped rocks on the emergency spillway to mitigate further erosion.
  • Engineers have increased the amount of water released to prepare for ongoing storms.

 

TEACHERS TOOLKIT

Nat Geo: The Oroville Dam Crisis Could Happen Elsewhere

Nat Geo: Where is the Oroville Dam? map

Nat Geo: What is hydroelectric energy?

Nat Geo: Why do we build dams? picture of practice video

American Society of Civil Engineers: Dams