The Tallest Dam Spillway In America

At 770 feet (235 meters) tall, Oroville Dam in Northern California is the tallest dam in the United States, and its main spillway is an impressive feat of water-control engineering. Oroville’s main spillway has a vertical height of 500 feet (152 meters) and extends 3,055 feet (931 meters) along a concrete chute measuring 179 feet (55 meters) wide, making it the longest and highest concrete spillway in America. Its capacity is roughly 270,000 to 300,000 cubic feet per second. Built to secure California’s water supply, control flooding, and generate hydroelectric power, Oroville Dam is an important component of the state’s water infrastructure, though it has had its challenges as precipitation patterns in the region change.

A wide view of the Oroville dam. Construction of Oroville Dam began in 1961 as part of California’s State Water Project, an extensive network of reservoirs, aqueducts, pumping plants, and canals designed to capture and distribute water throughout the state. Completed in 1967, the dam spans the Feather River in Butte County, about 70 miles north of Sacramento. Its enormous earth-fill embankment contains roughly 80 million cubic yards of material and extends 1.3 miles (2.1 kilometers) along its crest.

The project addressed several of California’s needs at the time. The state’s growing cities and agricultural regions required reliable water supplies, while communities along the Feather River needed protection from seasonal flooding. The dam also provides hydroelectric generation at the Edward Hyatt Powerplant. Combined, these functions make Oroville Dam a cornerstone of California’s water management system.

The Oroville Spillway. Editorial credit: Chris Allan / Shutterstock.com Oroville Dam created Lake Oroville, one of California’s largest human-made reservoirs and a key storage facility. At full capacity, the reservoir holds about 3.5 million acre-feet of water and covers around 15,800 acres. In addition to supplying water and providing flood-control storage, the lake provides opportunities for boating, fishing, camping, and other recreational activities.

The spillways, however, are essential to the dam’s operation. When heavy rain and snowmelt send large volumes of water into the reservoir, operators must release excess water to prevent water levels from rising too high. As a result, Oroville has two principal spillways. These are a gated main spillway and an emergency spillway that provides an additional route for water when the reservoir rises beyond the main outlet’s capacity.

For the main spillway, eight large gates regulate the water released into the chute, which carries it away from the reservoir and into the Feather River. This controlled discharge helps operators manage inflows while maintaining the reservoir’s storage capacity at desired levels. The emergency spillway works differently. Water passes over a concrete weir and down the natural hillside below. Unlike the main spillway, which has a long, concrete-lined channel, the original emergency outlet relied on the hillside to carry the overflow. The difference between the two structures became a critical distinction during a crisis in 2017.

Water flowing from the eroded overflow spillway of Oroville Dam, CA. January and February 2017 brought unusually wet conditions to Northern California. The Feather River watershed above Lake Oroville received about 4.4 million acre-feet of runoff in just 50 days, equivalent to about one year’s average runoff. As the reservoir filled, operators increased releases through the main spillway to make room for the additional volume. On February 7, inspections revealed extensive damage to the main spillway’s concrete chute. Water had penetrated weaknesses in the structure, causing sections of concrete to fail while exposing underlying rock to erosion. Continued releases increased the damage to the concrete, complicating efforts to discharge the water safely.

With the main spillway compromised and more rain in the forecast, operators faced a mounting challenge to regulate the reservoir. On February 11, water began flowing over the emergency spillway for the first time in the dam’s history. The discharge rapidly eroded the hillside below the concrete weir, raising concerns that the erosion could undermine the entire spillway structure.

The Oroville Dam main spillway during phase 2 repairs. On February 12, authorities ordered the evacuation of around 188,000 people in downstream communities as a precaution. Although the dam’s earthen embankment structure had held, the possibility that rapid erosion could threaten the emergency spillway created an urgent safety concern. Operators made the decision to increase releases through the damaged main spillway to lower the reservoir and stop the flow over the emergency outlet. It worked as the main chute held, and two days later the evacuation order was reduced to an evacuation warning.

Investigations following the incident identified several contributing factors to the crisis, including weaknesses in the main spillway’s design, foundation conditions, construction quality, and inspection history. The incident demonstrated that the safety of a dam depends not only on its main structure but also on the reliability of the infrastructure used to control its water.

Reconstruction of the damaged spillway began later in 2017, and the main spillway was fully rebuilt in 2018. The emergency spillway also received significant improvements, including a concrete buttress supporting the weir and additional protective measures intended to limit erosion should water flow down the natural hillside again.

Oroville’s future challenges involve managing extreme runoff as well as maintaining and inspecting the infrastructure more frequently. Atmospheric rivers, which are expected to grow more intense with climate change, can transport enormous quantities of moisture toward California, producing intense rainfall events and thus more rapid runoff over short periods of time. The resulting demands on the reservoir depend on several key factors, including storm intensity, frequency, snowmelt, soil saturation, reservoir levels, and the accuracy of weather forecasts.

Climate change is expected to affect precipitation patterns not only in Northern California but across the country, adding uncertainty to long-term water planning. This doesn’t mean Oroville faces imminent failure. It does mean that regular inspections, reliable spillways, accurate runoff forecasting, and balancing flood protection with water-supply needs will become more important in the future for America’s tallest dam spillway.