Oroville Dam: America's Tallest Dam and the 2017 Spillway Crisis

**Oroville Dam** on the Feather River in Butte County, California, is the tallest dam in the United States at **235 meters**, surpassing Hoover Dam by 14 meters. Completed in 1968 as a key component of California's State Water Project, the earth-fill dam creates **Lake Oroville**, which stores water for delivery to southern California and the Central Valley, serving 27 million people and over 300,000 hectares of farmland.

Introduction

The dam gained worldwide attention in February 2017 when its main concrete spillway developed a massive hole during heavy rains, and the emergency spillway — never before used — began eroding dangerously. The crisis forced the evacuation of nearly 188,000 people downstream and exposed critical infrastructure vulnerabilities in America's dam system.

Engineering & Structure

• Type: Earthfill dam
• Height: 235 meters (tallest in US)
• Crest Length: 2,110 meters
• Fill Volume: 59.6 million m³

Oroville Dam is a zoned earth-fill dam — the tallest of its type in the United States. It contains 59.6 million cubic meters of material, including a clay core, gravel and sand shells, and a rock face. Unlike concrete dams, earthfill dams cannot allow water to flow over their crest without catastrophic erosion, making robust spillway systems essential.

The dam includes a gated main spillway, an ungated emergency spillway, and an underground power station with three 240 MW turbines (plus a smaller 644 MW pumping-generating station added later). The 2017 spillway crisis revealed that the main spillway's concrete had deteriorated due to poor original construction and decades of deferred maintenance. The subsequent repair and reconstruction cost $1.1 billion.

Geographic Setting

• River: Feather River (Sacramento River tributary)
• Location: Butte County, California, United States
• Coordinates: 39.54°N, 121.48°W
• Lake Oroville Capacity: 4.36 km³

The Feather River drains the western slopes of the Sierra Nevada in northeastern California, collecting snowmelt and rainfall from a watershed of approximately 9,324 square kilometers. The dam site sits where the river exits the Sierra foothills and enters the flat Sacramento Valley. The surrounding landscape transitions from conifer forests and oak woodlands at higher elevations to grasslands and orchards in the valley below.

Lake Oroville extends approximately 25 kilometers into the Sierra foothills, with a surface area of about 64 square kilometers at full pool. The reservoir is a key storage facility in California's State Water Project, the largest state-built water conveyance system in the United States, which moves water from the wetter north to the drier south through 1,100 kilometers of canals, tunnels, and pipelines.

Hydroelectric Power

• Installed Capacity: 933 MW (Hyatt + Thermalito)
• Annual Output: ~2.2 TWh
• Hyatt Powerplant: 3 × 240 MW (underground)
• Thermalito: Pumped storage facility

The Edward Hyatt Powerplant, located in a cavern 300 meters inside the mountain beneath the dam, houses three 240 MW turbines that exploit the dam's exceptional 200-meter head. The Thermalito complex downstream includes additional generating and pumping capacity, bringing the total to 933 MW. The facilities generate approximately 2.2 terawatt-hours annually.

The Thermalito Afterbay and Forebay serve as regulating reservoirs, smoothing the pulsed releases from the main dam into a steady flow for downstream agriculture and the Feather River fish hatchery. The hydroelectric revenue helps fund the State Water Project's operations, partially offsetting the enormous cost of pumping water over the Tehachapi Mountains to reach southern California.

Environmental & Social Impact

• 2017 Evacuation: ~188,000 people evacuated
• Repair Cost: $1.1 billion for spillway reconstruction
• Salmon: Feather River salmon runs severely impacted by dam

The February 2017 spillway crisis was a landmark event in American dam safety. After heavy rains, a crater opened in the main spillway's concrete surface, rapidly expanding to a gaping chasm 90 meters wide and 15 meters deep. When operators reduced spillway flow to limit damage, the reservoir rose to the emergency spillway crest, and that hillside began eroding catastrophically. The resulting evacuation order for 188,000 people in downstream communities was the largest dam-related evacuation in US history.

The dam has permanently altered the Feather River's ecology. Cold, clear water releases from the depths of Lake Oroville replaced the river's natural warm, seasonal flows, affecting native fish populations. A fish barrier dam and hatchery maintain Chinook salmon runs, but the population is a fraction of its historical size. California's drought cycles periodically reduce Lake Oroville to alarming levels, revealing the "bathtub ring" that has become a media symbol of the state's water challenges.

Key Facts

• Oroville Dam is the tallest dam in the United States at 235 meters.
• The 2017 spillway crisis forced the evacuation of 188,000 people.
• Spillway repairs cost $1.1 billion and took until 2019 to complete.
• Lake Oroville is a key component of California's State Water Project.
• The dam's earthfill construction makes it vulnerable to overtopping — water cannot flow over its crest safely.

Fun Facts

• When Lake Oroville dropped to just 24% of capacity during California's 2021 drought, the Hyatt Powerplant was shut down for the first time in its history due to insufficient water levels.
• The emergency spillway used during the 2017 crisis was essentially an unlined hillside — it had no concrete surface, and engineers had warned about its vulnerability years earlier.
• Oroville Dam was built near the site of some of California's richest Gold Rush placer mining operations from the 1850s.

Final Thoughts

Oroville Dam is a critical piece of California's complex water infrastructure, delivering water and power to millions of people across the state. The 2017 spillway crisis served as a wake-up call about the aging condition of America's dams and the catastrophic risks of deferred maintenance on major infrastructure.