After leaving the confines of the Black Canyon, the river emerges from the Colorado Plateau into the Lower Colorado River Valley (LCRV), a desert region dependent on irrigation agriculture and tourism and also home to several major Indian reservations.

Here, the San Juan River, carrying runoff from the southern slope of Colorado's San Juan Mountains, joins the Colorado from the east.

The Colorado then enters northern Arizona, where since the 1960s Glen Canyon Dam near Page has flooded the Glen Canyon reach of the river, forming Lake Powell for water supply and hydroelectricity generation. Downstream, the river enters Marble Canyon, the beginning of the Grand Canyon, passing under the Navajo Bridges on a now southward course.

In Arizona, the river passes Lee's Ferry, an important crossing for early explorers and settlers and since the early 20th century the principal point where Colorado River flows are measured for apportionment to the seven U. Below the confluence with the Little Colorado River, the river swings west into Granite Gorge, the most dramatic portion of the Grand Canyon, where the river cuts up to one mile (1.6 km) into the Colorado Plateau, exposing some of the oldest visible rocks on Earth, dating as long ago as 2 billion years.

The 277 miles (446 km) of the river that flow through the Grand Canyon are largely encompassed by Grand Canyon National Park and are known for their difficult whitewater, separated by pools that reach up to 110 feet (34 m) in depth.

At the lower end of Grand Canyon, the Colorado widens into Lake Mead, the largest reservoir in the continental United States, formed by Hoover Dam on the border of Arizona and Nevada.

Situated southeast of metropolitan Las Vegas, the dam is an integral component for management of the Colorado River, controlling floods and storing water for farms and cities in the lower Colorado River basin.

Beginning with small bands of nomadic hunter-gatherers, Native Americans have inhabited the Colorado River basin for at least 8,000 years.

Between 2,000 and 1,000 years ago, the river and its tributaries fostered large agricultural civilizations – some of the most sophisticated indigenous cultures in North America – which eventually faded due to a combination of severe drought and poor land use practices.

Large engineering works began around the start of the 20th century, with major guidelines established in a series of international and U. interstate treaties known as the "Law of the River". Most of the major dams were built between 19; the system keystone, Hoover Dam, was completed in 1935.