America’s longest river is the Missouri. It originates in Montana’s high Rocky Mountains close to the Continental Divide. It drains the Upper Great Plains from the State of Montana to the State of Missouri. Its upriver dams were constructed in mountainous hard-rock canyons. Its very large downstream dams were constructed on gentle rolling-prairie canyons. The later structures are enormous in storage capacity and in hydroelectric capacity. These dams block the natural silting process that feeds the Mississippi River and its delta. The Missouri River Watershed has shifted toward concentrated animal feeding operations (CAFOs) that have substantially drained the Ogallala Aquifer of the Central Plains and the Platte River, one of the Missouri Rivers two most significant tributaries. The other significant tributary is the Yellowstone River, an iconic American river that has a lengthy history of organized opposition to dam constructions along its length, a resistance led by environmental coalitions that continues to today.