Missouri River: Three Forks Headwaters, Fort Peck Dam & 2,341-Mile Course

## The Lifeline of the American Northern Plains

The **Missouri River** is the longest river in the United States and North America, spanning **2,341 miles (3,767 kilometers)** from the glaciated valleys of southwestern Montana to its confluence with the Mississippi River near St. Louis, Missouri. Often referred to by historical cartographers and Indigenous tribes as the *\'Big Muddy\'*, the Missouri drains approximately one-sixth of the contiguous United States, encompassing 529,350 square miles (1,371,000 km²).

### Three Forks Headwaters & Tectonic Setting
The Missouri River officially originates at Three Forks, Montana, within Gallatin County, at an elevation of 4,045 feet (1,233 meters). Here, three distinct Rocky Mountain rivers unite:
1. **The Jefferson River:** Originating furthest upstream near Brower\'s Spring on the Continental Divide.
2. **The Madison River:** Flowing from northern Yellowstone National Park through scenic basalt canyons.
3. **The Gallatin River:** Sourced in the glaciated Gallatin Range.

### Pick-Sloan Program & Mainstem Reservoirs
Following catastrophic flooding throughout the 1940s, the US Congress enacted the Flood Control Act of 1944, creating the **Pick-Sloan Missouri Basin Program**. The US Army Corps of Engineers constructed six colossal rolled-earth embankment dams across the mainstem:

| Dam Name | State Location | Impounded Reservoir | Normal Storage Capacity | Completed Year |
| :--- | :--- | :--- | :--- | :--- |
| **Fort Peck Dam** | Montana | Fort Peck Lake | 18,700,000 acre-feet | 1940 |
| **Garrison Dam** | North Dakota | Lake Sakakawea | 23,800,000 acre-feet | 1953 |
| **Oahe Dam** | South Dakota | Lake Oahe | 23,100,000 acre-feet | 1962 |
| **Big Bend Dam** | South Dakota | Lake Sharpe | 1,900,000 acre-feet | 1966 |
| **Fort Randall Dam** | South Dakota | Lake Francis Case | 5,700,000 acre-feet | 1956 |
| **Gavins Point Dam** | Nebraska/SD border | Lewis and Clark Lake | 470,000 acre-feet | 1957 |

### Fluvial Hydrology and Sediment Depletion
Historically, the natural Missouri transported more than **320 million tons of suspended silt and sand** annually into the Mississippi system. The Pick-Sloan dams have trapped over 90% of this fluvial sediment load, clear-watering the downstream reaches, armoring channel bedload, and depriving the Louisiana coastal delta of essential building sediment.

*Data Source: United States Geological Survey (USGS) Water-Data Report & US Army Corps of Engineers Northwestern Division.*