Lower Arkansas River, CO

The Arkansas River is a major tributary of the Mississippi River. The Arkansas generally flows to the east and southeast as it traverses the U.S. states of Colorado, Kansas, Oklahoma, and Arkansas.  The river's initial basin starts in Colorado, specifically the Arkansas River Valley where the headwaters derive from the snowpack in the Collegiate Peaks.  It runs as a steep mountain torrent through the Rockies in its narrow valley, dropping 4,600 feet (1.4 km) in 120 miles (193 km). This section (including The Numbers near Granite, Colorado, Brown's Canyon, and the Royal Gorge) sees extensive whitewater rafting in the spring and summer. 

At 1,469 miles (2,364 km), it is the sixth longest river in the United States, the second-longest tributary in the Mississippi-Missouri system, and the 45th longest river in the world.  Its origin is in the Rocky Mountains in Lake County, Colorado, near Leadville, and its mouth is at Napoleon, Arkansas.  The Arkansas River drainage basin covers nearly 195,000 sq mi (505,000 km²).  In terms of volume, the river is much smaller than both the Missouri and Ohio Rivers, with a mean discharge of 8,460 ft³/s (240 m³/s).