Fort Peck Dam: America's Largest Hydraulic-Fill Dam

**Fort Peck Dam** on the Missouri River in northeastern Montana is the largest hydraulic-fill earth dam in the United States and the second-largest earth dam by volume in the country, containing approximately **96 million cubic meters** of material. Built between 1933 and 1940 as a Depression-era public works project, it creates **Fort Peck Lake**, a reservoir stretching **218 kilometers** upstream with 2,500 kilometers of shoreline — more than the entire California coast.

Introduction

The dam was one of the first major structures in the Pick-Sloan Missouri Basin Program, which eventually placed six massive dams on the main stem of the Missouri River. It was featured on the cover of the very first issue of *Life* magazine in 1936, and its construction employed up to 11,000 workers during the worst years of the Great Depression, fundamentally reshaping the economy and society of eastern Montana.

Engineering & Structure

• Type: Hydraulic-fill earth dam
• Height: 76 meters
• Crest Length: 6,409 meters
• Fill Volume: 96 million m³

Fort Peck Dam was built using the hydraulic-fill method, in which water jets dislodge soil and gravel from borrow pits and the resulting slurry is pumped through pipelines to the dam site, where the material settles and compacts. This technique, while efficient for moving enormous volumes of material, creates a dam core of fine sediment that can be vulnerable to liquefaction during earthquakes — a risk that prompted a major safety review in the 1990s.

The dam stretches 6,409 meters across the Missouri River valley, making it one of the longest dams in the world. Its relatively low height of 76 meters belies the enormous volume of material involved. The dam includes four 42.5 MW generators, with a total installed capacity of 185 MW — modest by modern standards but significant in 1940s Montana.

Geographic Setting

• River: Missouri River
• Location: Glasgow, Valley County, Montana, United States
• Coordinates: 48.00°N, 106.42°W
• Fort Peck Lake Capacity: 23.1 km³

The Missouri River at Fort Peck flows through the vast, arid prairies of northeastern Montana, a landscape of rolling grasslands, badlands, and cottonwood-lined river bottoms. This is the "Missouri Breaks" country, where the river has carved a wide valley through layers of sedimentary rock that hold some of the richest dinosaur fossil beds in North America.

Fort Peck Lake's 2,500 kilometers of shoreline wind through a landscape so sparsely populated that the reservoir's eastern arms extend into the Charles M. Russell National Wildlife Refuge, one of the largest wildlife refuges in the lower 48 states. The refuge and reservoir together protect habitat for elk, mule deer, bighorn sheep, prairie dogs, and over 200 bird species in an increasingly fragmented Great Plains ecosystem.

Hydroelectric Power

• Installed Capacity: 185 MW
• Annual Output: ~1.2 TWh
• Turbines: 4 × 42.5 MW + 1 × 15 MW
• Grid: Western Area Power Administration

Fort Peck's hydroelectric capacity of 185 MW is modest compared to the giant dams of the Columbia and Colorado basins, reflecting the Missouri's lower gradient in this reach. The five generators produce approximately 1.2 terawatt-hours annually, serving eastern Montana and the Dakotas through the Western Area Power Administration.

The dam's primary purposes are flood control, navigation support for the downstream Missouri, and water storage rather than power generation. Fort Peck Lake stores spring snowmelt and releases it gradually through the summer and fall, moderating the Missouri's once-violent flood cycles that historically devastated downstream cities like Omaha, Kansas City, and St. Louis.

Environmental & Social Impact

• Workers Employed: Up to 11,000 during Depression
• Tribal Lands: Fort Peck Indian Reservation partially affected
• Downstream Impact: Missouri River ecosystem fundamentally altered

Fort Peck Dam was a transformative Depression-era project that brought employment and economic activity to one of the poorest regions in Montana. At its peak, the construction effort employed 11,000 workers and created temporary towns that served as boomtowns with all their attendant social challenges — saloons, dance halls, and improvised housing sprawled along the muddy streets of places like Wheeler, New Deal, and Park Grove.

The dam, along with the five other main-stem Missouri dams built between the 1940s and 1960s, fundamentally transformed the Missouri River's ecology. The once-braided, flood-prone river became a series of stable reservoirs connected by channelized reaches. Native species like the pallid sturgeon, which depended on the river's seasonal flood pulses, declined dramatically and are now federally endangered. The dam also affected the Fort Peck Indian Reservation of the Assiniboine and Sioux peoples, whose lands border the reservoir.

Key Facts

• Fort Peck Dam is the largest hydraulic-fill dam in the United States.
• Fort Peck Lake has over 2,500 km of shoreline — more than the California coast.
• The dam was featured on the cover of the first issue of Life magazine in 1936.
• It employed up to 11,000 workers during the Great Depression.
• The Charles M. Russell National Wildlife Refuge borders much of the reservoir.

Fun Facts

• A massive landslide during construction in 1938 carried away a section of the dam, killing eight workers and delaying completion by over a year.
• Fort Peck Lake is one of the best walleye fishing lakes in North America, attracting anglers from across the continent.
• The dam's spillway tunnels were designed large enough to drive trucks through, and the entire structure is so large that the dam crest serves as a highway for vehicular traffic.

Final Thoughts

Fort Peck Dam is both a monument to Depression-era public works ambition and a sobering example of how mega-dams reshape entire river ecosystems. Its vast reservoir and remote prairie setting make it one of the most impressive and least-visited great dams in the United States.